Symposium Organizers
Bernard Bewlay General Electric Company
Martin Palm Max-Planck Institut fuer Eisenforschung GmbH
Sharvan Kumar Brown University
Kyosuke Yoshimi Tohoku University
N1: Intermetallic Shape Memory Allloys
Session Chairs
Monday PM, November 29, 2010
Room 203 (Hynes)
10:00 AM - N1.2
Micromechanical Testing and Modeling of Shape Memory Behavior in Ni-Ti Based Alloys.
Matthew Bowers 1 , John Carpenter 1 , Michael Mills 1 , Peter Anderson 1 , Sivom Manchiraju 1
1 Materials Science and Engineering, The Ohio State University, Columbus, Ohio, United States
Show AbstractMicromechanical testing of single crystal Ni-Ti based shape memory alloys is being performed in an effort to understand the effects of specimen size and crystal orientation on martensite-induced pseudoelastic and shape memory response. The primary goal in this investigation is to determine the means by which the matrix accommodates the large strain associated with the martensitic transformation. This information is critical in extending the working life of components under cyclic loading and/or heating. It is theorized that the accommodation may take place by matrix plasticity and/or by inducing additional transformation variants, however no experimental verification exists. FIB-machined, micron-scale pillars of several crystallographic orientations chosen on the basis of Schmid factor calculations have been tested in compression and stress-strain data has been collected. Post-mortem TEM analysis has been done to analyze the resulting microstructure and dislocation substructure, including the presence of any residual martensite. Crystallographic theory of martensite and micromechanics-based stress field calculations are being used to explain the results. We demonstrate through this method the study of isolated martensitic transformations using direct mechanical response measurements for individual variants. Also, we investigate whether the shape memory response may be enhanced given select specimen dimensions and crystallographic orientations.
10:15 AM - N1.3
The Origins of Microstructural Instability in NiTi Shape Memory Alloy Actuators.
Nicholas Jones 1 , David Dye 1
1 Dept of Materials, Imperial College London, London United Kingdom
Show AbstractShape memory alloy (SMA) based actuators are currently of great interest to the aerospace industry as they offer the potential for dynamic components, which will be beneficial in the drive to reduce emissions. For these components to be structurally useful, they require the application of significant bias loads on the SMA. However, despite successful demonstrator components, the cyclic instability of the martensitic transformation continues to prevent the transfer of this technology into full scale production components. Here we present in situ synchrotron X-ray diffraction results investigating the microstructural evolution of a near equiatomic NiTi SMA over several thermal cycles using different uniaxially applied loads. Despite being cycled under a nominally elastic load, an accumulation of strain is observed. Certain crystallographic orientations are found to align with the sample tensile axis and we observe that the martensitic microstructure evolves during cycling. It would appear that this alteration is driven by an attempt to minimise the effect of the externally applied load. The reasons for this are explored, based on the fundamentals of the transformation process.
10:30 AM - N1.4
Characterization and Properties of a Near Stoichiometric NiTiPt High Temperature Shape Memory Alloy.
Fan Yang 1 , Libor Kovarik 1 3 , Ronald Noebe 2 , Anita Garg 2 , Michael Mills 1
1 , The Ohio State University, Columbus, Ohio, United States, 3 , Pacific Northwest National Laboratory, Richland, Washington, United States, 2 , NASA Glenn Research Center, Richmond, Virginia, United States
Show AbstractAging of the high-temperature shape memory alloy Ti50Ni30Pt20(at.%) results in precipitation of a previously unidentified phase. The precipitate phase has been analyzed with electron diffraction, high-resolution STEM HAADF imaging and 3-D atom probe tomography (L. Kovarik, F. Yang, A. Garg, D. Diercks, M. Kaufman, R.D. Noebe, M.J. Mills, Acta Materialia 2010, article in press). It is shown that the precipitates have a unique structure due to their non-periodic character along one of the primary crystallographic directions. It will be shown that the structure can be explained in terms of random stacking of three variants of a monoclinic crystal that is closely related to the structure of high temperature cubic B2 phase; the departure of the structure from the B2 phase can be attributed to the ordering of Pt atoms on the Ni sublattice and relaxation of the atoms (shuffle displacements) from the B2 sites. The shuffle displacements and the overall structural refinement were deduced from ab initio calculations. The effects of aging on mechanical and shape memory properties were investigated, with an emphasis on transformation strain, work output and dimensional stability by means of cyclic load biased tests. The precipitate has been shown to play a key role in achieving overall optimal shape memory properties.
10:45 AM - N1.5
Self-assembled NiTi Nanowires.
Xu Huang 1 , Yuriy Chumlyakov 2 , Ainissa Ramirez 1
1 Mechanical Engineering, Yale University, New Haven, Connecticut, United States, 2 Physics of Plasticity and Strength Materials Laboratory, Siberian Physical and Technical Institute, Tomsk Russian Federation
Show AbstractSelf-assembled NiTi nanowires were fabricated by electro-polishing single crystal NiTi alloys. The resulting NiTi nanowires were approximately 480 nm by 480 nm in cross section, with lengths of 50 µm, which were found to lengthen with increasing etching time. Most of the nanowires grew perpendicularly along the <110> and <100> planes of the single crystal substrate, while a few of the wires presented an inclined at a specific angle. The nanowires composition was determined using microprobe, and the electrical properties were investigated using four-point probe measurements. Their phase transformation properties were evaluated by testing the resistivity as a function of temperature using a sample heating stage. These methods for producing NiTi nanowires could provide new routes for smart nano-structures.
11:00 AM - N1.6
Computational Studies of the NiTi Alloy System: Bulk, Supercell, and Surface Calculations.
Amanda Stott 1 2 , David Dixon 1 , Chris DellaCorte 2 , Stephen Pepper 2 , Phillip Abel 2
1 The Deparment of Chemistry, The University of Alabama, Tuscaloosa, Alabama, United States, 2 Tribology and Mechanical Components Branch, NASA Glenn Research Center, Cleveland, Ohio, United States
Show AbstractTailoring the microstructure of the NiTi alloy by increasing the Ni atomic to ~ 55% leads to alloys with increased dimensional stability which can be used in bearing applications while maintaining tribological performance equal to that of traditional tool steels in friction tests. Unlike traditional tool steels, which are magnetic and may corrode in harsh environments, Ni55Ti45 is non-corrosive and non-magnetic, both of which are desirable properties currently not found in any bearing material. A density functional theory study of the constituent phases NiTi, Ni4Ti3, and Ni3Ti in the NiTi alloy system was performed for the bulk phase, supercell, and surface structures using the Vienna Ab-Initio Simulation Package (VASP 5.2). From these calculations, a detailed theoretical analysis of the microstructure, site-projected band structure, and density of states (DOS) was performed. The NiTi surface heats of formation are comparable to bulk-phase values. The heat of formation of NiTi is predicted to be lower in the bulk than in the supercell. In contrast, the supercell heats of formation for Ni4Ti3 and Ni3Ti are lower than in the bulk. The combined experimental and computational results suggest that Ni4Ti3 and Ni3Ti act to stabilize the parent NiTi phase. The predicted DOS shows a significant contribution of Ti states at higher energies (less stable) and Ni at lower energies (more stable). A pseudogap exists in the NiTi DOS, as often observed in ceramic materials. This pseudogap is due to NiTi phase stabilization from the partial occupancies between d-d or d-p hybridization orbital interactions. In Ni4Ti3 no pseudogap is present, and the Ni and Ti contributions occur at approximately the same energy range, although the peak apex of the Ti d-states occurs at a higher energy than the peak apex of the Ni d-states, indicating delocalized metallic bonding. In Ni3Ti, a narrow gap exists at the Fermi level, with Ni and Ti contributions occurring over this energy range. The Ni and Ti in the Ni3Ti d-states are similar to those in Ni4Ti3, consistent with delocalized metallic bonding. The band structure and charge density results show the d-orbital character of Ni4Ti3 and Ni3Ti to be highly symmetric compared to NiTi, also indicating these phases act to stabilize the parent NiTi matrix. The mixed phase composition of the bearing surface and the Ni-rich composition of the alloy are both factors in the positive lubrication behavior of Ni55Ti45. The delocalized metallic bonding found in the DOS calculations and the negative charge accumulation at Ni lattice sites could be responsible for this behavior.
11:15 AM - N1 : SMA
BREAK
11:30 AM - **N1.7
Pseudoelasticity of D03-Type Fe3Al and Fe3Ga-based Alloys.
Hiroyuki Yasuda 1 , Yukichi Umakoshi 1
1 Division of Materials and Manufacturing Science, Osaka University, Osaka Japan
Show AbstractThe pseudoelastic behavior of Fe3Al and Fe3Ga alloys with the D03 structure is reviewed. In general, pseudoelasticity which shape memory alloys exhibit is based on a thermoelastic martensitic transformation. However, pseudoelasticity regardless of the martensitic transformation is found to take place in D03-ordered Fe3Al and Fe3Ga alloys. For instance, a 1/4〈111〉 superpartial dislocation in Fe3Al alloys moves independently dragging an antiphase boundary (APB). During unloading, the APB pulls back the superpartial to decrease its energy resulting in pseudoelasticity, which is called “APB pseudoelasticity”. Moreover, three types of pseudoelasticity based on martensitic transformation, twinning and dislocation motion appear in Fe3Ga alloys depending on degree of D03 order, loading axis and stress sense. The mechanism of the pseudoelasticities in the D03-type intermetallics is discussed based on some in situ observations.
12:00 PM - N1.8
Thin Films of AuCuAl Shape Memory Alloy for Use in Plasmonic Nano-actuators.
Vijay Bhatia 1 , Annette Dowd 1 , Michael Cortie 1 , Gordon Thorogood 2
1 Institute for Nanoscale Technology, University of Technology Sydney, Broadway, New South Wales, Australia, 2 , Institute of Materials Engineering, Australian Nuclear Science and Technology Organisation, Menai, New South Wales, Australia
Show AbstractThe beta-phase shape memory alloy (SMA) with a composition in the vicinity of Au7Cu5Al4 has been shown to be relatively resistant to aging and martensite stabilization compared to copper-based SMAs. However, although Au7Cu5Al4 has been studied in the bulk form, there has been no attempt yet to prepare thin film actuators of it. In contrast, films of the better known TiNi SMA have been extensively studied for use in thin film actuators, however their use in nano-sized actuators has been limited due to the oxidation of films of less than 100 nm thickness. The Au7Cu5Al4 SMA is relatively resistant to oxidation due to its high gold content and may therefore be a better candidate for use in nanoscale SMA actuators. Another advantage of this alloy is that its dielectric properties suggest that it can support a surface plasmon in the visible spectrum. This has the potential to enable a range of interesting new functionalities in which the shape memory effect and plasmonics are combined. Here we describe the synthesis and characterisation of films of Au7Cu5Al4 and related alloys produced by magnetron sputtering. The microstructure of the films was controlled by varying the Al content, while keeping the Au:Cu ratio fixed. In this way, the microstructure could be controlled to produce alpha, beta or gamma phases according to position on the pseudobinary transect, however it is only the beta structured intermetallic phase that has the SMA property. The films were characterised by XRD, SEM, TEM, resistance measurements, x-ray reflectometry and SPM. These techniques showed that films of correct crystal structure and composition were produced and that they exhibited the reversible austenite to martensite phase transition required of a SMA. Attainment of of this property is the key prerequisite for the development of a SMA opto-mechanical nano-actuator.
12:15 PM - N1.9
Surface Modification of Ni-Ti for Biomedical Applications by Plasma-immersion Ion Implantation.
Rui M. Martins 1 2 , Nuno Barradas 1 2 , Eduardo Alves 1 2 , Dietmar Henke 3 , Helfried Reuther 3 , Maria Carmezim 4 5 , Teresa Silva 4 6 , Joao C. Fernandes 4
1 , Instituto Tecnológico e Nuclear (ITN), Sacavém Portugal, 2 , Centro de Física Nuclear da Universidade de Lisboa (CFNUL), Lisboa Portugal, 3 Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden- Rossendorf, Dresden Germany, 4 ICEMS/DEQB/IST, TULisbon, Lisboa Portugal, 5 ESTSetúbal, Instituto Politécnico de Setúbal, Setúbal Portugal, 6 ISEL, Instituto Politécnico de Lisboa, Lisboa Portugal
Show AbstractThe enormous elasticity of Ni-Ti is becoming integral to the design of a variety of new medical products. The wide spectrum of application in implantology imposes special requirements on the biocompatibility of Ni-Ti. The biological response to implant materials is a property directly related to their surface conditions and an optimum surface layer is thus desired. The plasma-immersion ion implantation (PIII) technique was used to modify and improve the surface of a Ni-Ti alloy (~ 50.2 at.% Ni, superelastic at body temperature) for biomedical applications. The main goal has been the formation of a Ni-depleted surface, which should serve as a barrier to out-diffusion of Ni ions from the bulk material. Ion implantation of oxygen was carried out. The depth profiles of the elemental distribution in the alloy surface region, obtained by Auger electron spectroscopy (AES), confirm the formation of a Ti-rich oxide layer. The working plan also comprised ion implantation of nitrogen. In this case, the formation of titanium oxynitride (TiNxOy) was observed. The AES depth profiles clearly show a Ni-depleted fraction for experiments performed with 40 keV.The deposition of a coating by a PVD technique would have disadvantages due to the interface between the coating and the bulk (lower adhesion). PIII creates a graded interface between the modified surface and the bulk. Techniques like thermal oxidation and nitriding could also lead to an improved corrosion resistance and Ni-depleted Ni-Ti surface. However, the high temperature necessary for the experimental procedure would lead to modification of the phase transformation characteristics and loss of specific mechanical properties of the alloy. Heat treatments tests performed at temperatures above 350C led to a shift of the transformation temperatures of the Ni-Ti alloy used in this work. Moreover, the R-phase is then present at body temperature, which is not the case for Ni-Ti samples modified by the PIII technique. This technique only changes the properties of the Ni-Ti alloy top layer.
12:30 PM - N1.10
Correlation of Specific Resistance and Phase Structure in the 3D Porous Nitinol after Laser Assisted Manufacturing.
Igor Shishkovsky 1 , Vladimir Sherbakov 1
1 Laboratory of Technological lasers, Lebedev Physics Institute of Russian Academy of Sciences, Samara branch, Samara Russian Federation
Show AbstractThe main goal of this study is a Shape Memory Effect (SME) in a porous titanium nickelide (- NiTi intermetallic phase referred to as nitinol) fabricated via the Selective Laser Sintering/Melting (SLS/SLM) process. Phase and structural transformation behavior of the intermetallide is characterized by scanning electron microscopy, EDX and X-ray analysis. The influence of the laser sintering parameters and additional heating on the phase content and the thermally dependence of the electrical resistivity are discussed. Peaks and humps of the electrical resistivity are found to correspond to the start and finish temperature of the austenite-to-martensite-and-back phase transitions and that knowledge can be used for the SME estimation and optimization. Advantages and drawbacks of the application of this porous material as the bio-MEMS are considered.Keywords: Selective laser sintering/melting (SLS/SLM), nitinol, shape memory effect (SME), micro-electro-mechanical systems (MEMS).
12:45 PM - N1.11
Simulating the Thermal Cycling Response of Stressed Polycrystalline NiTi.
Sivom Manchiraju 1 , Peter Anderson 1 , Darrell Gaydosh 2 , Ronald Noebe 2
1 , The Ohio State University, Columbus, Ohio, United States, 2 , N. A. S. A. Glenn Research Center, Cleveland, Ohio, United States
Show AbstractA challenge in modeling of shape memory alloys is to capture the load biased thermal cycling response. Experiments show that the transformation strain is a strong function of the bias stress. It is observed that the transformation strain is small under small bias stress, increases steadily with increasing bias stress and ultimately starts to decrease at large bias stress due to intervention of plastic deformation. Though this dependence has been simulated by phenomenological macroscopic models, detailed microstructure based models which track the volume fraction of individual martensite variants have been unable to capture the experimental observations. This work discusses three key modifications to a microstructure based FEM model in order to capture the load biased thermal cycling. First, the model couples deformation due to martensitic phase transformation and plastic deformation in austenite [1]. The model captures the grain-to-grain redistribution of stress caused by both plasticity and phase transformation, thereby allowing each mechanism to affect the driving force for the other. This leads to a decrease in transformation strain at large bias stress. Second, the martensite plate interaction energy approximation proposed in [2] is modified to include a plate “self-hardening” term. This enables formation of multiple martensite plate types in the grains of polycrystal upon thermal cycling under small bias stress. Thus, a less textured martensite and a small transformation strain is predicted at small bias stress, similar to experimental observations. Third, the elastic anisotropy effects are rigorously captured using crystallographic theory of martensite in conjunction with the FEM model by using the recent first principle calculations of monoclinic martensite stiffness.The model is calibrated and validated rigorously with macroscopic load-bias and pseudoelastic experiments for solutionized polycrystalline NiTi (55wt% Ni). Preliminary results show that the transformation strain and texture predicted by the model is in good agreement with the experiments. Despite these improvements, the model predicts the onset of plastic strain at bias stress levels much higher than that observed in the experiments. Reasons for this discrepancy will be discussed.References:1.S. Manchiraju and P. M. Anderson, 2010, International Journal of Plasticity, doi:10.1016/j.ijplas.2010.01.009 2.E. Patoor, A. Eberhardt, M. Berveiller, 1996, J. de Physique, 6, 277.
N2: Iron Aluminides
Session Chairs
Monday PM, November 29, 2010
Room 203 (Hynes)
2:30 PM - **N2.1
Processing Iron Aluminides by Heavy Deformation for Improved Room Temperature Strength-ductility and for High Temperature Creep Strength.
David Morris 1 , Maria Munoz-Morris 1
1 Physical Metallurgy, CENIM, CSIC, Madrid Spain
Show AbstractIron aluminides show many interesting properties, but still show relatively poor ductility at room temperature and only moderate creep resistance at temperatures above about 600C. Processes of severe plastic deformation have been investigated for a wide range of ductile alloys over the past decade, but have hardly been considered for intermetallics. This presentation discusses processing of a Fe3Al alloy by heavy cold rolling, followed by recovery-recrystallization anneals with an objective of refining grain size to improve strength at the same time as ductility. The high strength and poor ductility of the work hardened material leads to a danger of cracking during rolling. Such rolling, combined with some recovery annealing, can, nevertheless, lead to strong materials with some plastic ductility. A different technique of multidirectional, high-strain and high-temperature forging applied to a boride-containing Fe3Al alloy produces a material with large grain size and refined dispersion of boride particles. These lead to a considerable increase in creep strength under conditions of moderate stresses at temperatures around 700C. This high-strain forging technique can be seen as an intermediate processing method between conventional wrought metallurgy and mechanical-alloying powder metallurgy.
3:00 PM - N2.2
Fatigue Resistance and Elastic Properties of Fe3Al-Based Alloys.
Manja Krueger 1 , Florian Gang 1 , Alexandra Laskowsky 1 , Heike Ruehe 1 , Joachim Schneibel 1 , Martin Heilmaier 2
1 Mechanical Engineering, Institute for Materials and Joining Technology, Otto-von-Guericke University Magdeburg, Magdeburg Germany, 2 Materials Science, Technical University Darmstadt, Darmstadt Germany
Show AbstractIron aluminide alloys based on Fe3Al are of interest in applications where excellent corrosion and oxidation resistance, adequate strength at elevated temperatures and a low density are required. While fatigue can become an important issue in many structural applications of iron aluminides, it has not received much attention to date: publications on this topic deal either with high cycle fatigue (HCF) or fatigue crack growth behaviour.In this presentation the low cycle fatigue (LCF) behaviour of two cast as well as two hot extruded Fe3Al alloys (with and without Cr) is investigated at room temperature and 300°C in air. The LCF tests were carried out in strain control with strain amplitudes in the range of εa = 0.1-0.7% strain with frequencies of 1 or 3 Hz. In all cases strong cyclic hardening, much stronger than the hardening in a normal tensile test, was observed. Surprisingly, the addition of Cr did not improve the tensile and LCF properties of iron aluminide. For cycle numbers less than about 10000, the LCF fatigue resistance at 300°C was higher than that at room temperature. It is shown that the LCF behaviour of Fe3Al at 300°C is significantly better than that of Al-Si casting alloys. This feature makes Fe3Al competitive with Al alloys at temperatures of 300°C and above.
3:15 PM - N2.3
The Effect of Fine M2C (M: Mo, Cr, Fe) Particles on the High Temperature Strength and Recrystallization Temperature of Warm Rolled Fe3Al Base Alloys.
Satoru Kobayashi 1 , Takayuki Takasugi 2 1
1 , Tohoku university, Sakai Japan, 2 , Osaka prefecture university, Sakai Japan
Show AbstractThe effect of fine M2C carbide particles on the high temperature strength and recrystallization temperature of warm rolled Fe3Al base alloys containing Cr, Mo and C was investigated. Fe-27Al-1.2C-2Cr-xMo (x: 0, 0.3, 0.6, 0.9 at.%) alloys were arc melted, warm rolled and annealed. The 0.2% proof stress at 600 degree C increased from 280 MPa to 390 MPa with increasing Mo content from 0 to 0.9. TEM observations have revealed the presence of finely dispersed M2C particles in the alloys with the Mo contents higher than 0.6%, indicating that both Mo solid solution and the particles contribute to the strengthening. The recrystallization temperature also increased from 730 degree C to 810 degree C with increasing Mo content from 0.3 to 0.9. TEM observations have revealed that the M2C particles pin subgrain boundaries formed during recovery process, indicating that the particles stabilize the deformed structure at high temperatures. The stability of M2C particles against long term exposures at high temperatures will also be presented.
3:30 PM - N2.4
In-situ TEM Straining Study of the Yield Anomaly in L21-ordered Fe2MnAl Single Crystal.
Yifeng Liao 2 , Ian Baker 1
2 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States
Show AbstractFe2MnAl adopts the L21 structure (ordered B2) at lower temperatures and disorders to the B2 structure (ordered b.c.c.) at 890 K. It shows a positive temperature dependence of its yield strength at intermediate temperatures with a peak at 700 K. In order to unravel the reason for the anomalous yield strength behavior, TEM in-situ straining of Fe2MnAl single crystals was performed at room temperature, 700-800 K (peak temperature) and 900 K (above the peak temperature). Four-fold dissociated superdislocations with a/4<111> Burgers vectors were observed to glide during room temperature straining. The separation between the outer partials was ~5 nm, whereas the separation of the inner partials was ~20 nm. The two outer pairs were found to had separated at 700 K-800 K due to the decrease in the degree of L21 order. The motion a dislocation with a total slip vector of 1/2<111> must trail a 1/2<111>-type anti-phase boundary (APB) that increases the energy of the matrix. Above 900 K the motion of two-fold paired dislocations does not produce any APBs as the matrix disorders to the B2 structure. The increasing number of separated superdislocations at elevated temperatures below 700 K could be one of the reasons for the anomalous yield strength. This research was supported by National Science Foundation Grant DMR 0552380.
3:45 PM - N2.5
Attempts at Obtaining Fine-grained Ordered Fe-Al Alloys.
Anna Fraczkiewicz 1 , Jerzy Bystrzycki 2 , Sanaa Najjar 1 , Izabela Kunce 2 , Radoslaw Lyszkowski 2 , Frank Montheillet 1
1 Centre SMS, UMR CNRS 5146, Ecole des Mines de St-Etienne, St-Etienne France, 2 Faculty of Advanced Technology and Chemistry, Military University of Technology, Warsaw Poland
Show AbstractMain limitation of the so-sought industrial development of ordered Fe-Al alloys is nowadays the cost of their processing. Obtaining of fine-grained materials, with a micron-range grain size, has been shown to be the only possible way to avoid the material’s room temperature brittle fracture. Moreover, the presence of a second phase (mainly oxides) intergranular precipitates is necessary to prevent grain growth. Unfortunately, this kind of microstructure can only be obtained through powder-metallurgy based methods, too expensive if not leading to a net-to-shape processing.In this work, we show the results of different attempts of obtaining fine-grained structures in different FeAl alloys through severe plastic deformation processing. Ternary Fe-40Al (B2), alloyed with Mn (2%) and doped with B (100 ppm) has been laboratory deformed in severe compression tests, on a wide range of temperatures (800 to 1100°C) and strain (up to epsilon=2). Binary Fe-28Al has been tested in multiaxis compression (MaxStrain system). Present work deals with the use of the latter system to process Fe-40Al alloys. Evolution of deformed microstructures has been studied as a function of severe deformation conditions. Only extreme conditions of available deformation, giving the maximal “charge” of the material, allow obtaining the sought fine-grained structure. At lower “charges” (temperature and strain), different features are progressively observed: the microstructure transformation starts with a localized deformation, than, a local (“collar”) recrystallisation takes place; finally, an uniform microstructure can be obtained.
4:00 PM - N2 : Fe Al
BREAK
4:30 PM - N2.6
Microstructure and Mechanical Behavior of Two-phase Fe30Ni20Mn20Al30 Alloy.
Xiaolan Wu 1 , Ian Baker 1 , Michael Miller 2 , Karren More 2 , Ai Serizawa 2
1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe microstructure and mechanical properties of Fe30Ni20Mn20Al30, which has a room-temperature yield strength of 1200 MPa, are described. Transmission electron microscopy (TEM) and atom probe tomography (APT) were used to characterize the microstructures of the alloy in both as-cast (AS) and annealed conditions. Upon casting, the alloy had a fine (phase width ~5 nm) two-phase (B2/L21) microstructure, possibly formed by spinodal decomposition. After annealing for 72 h at 823 K the microstructure had coarsened substantially so that the phase widths were ~25 nm. Energy dispersive X-ray spectroscopy (EDS) and APT results showed that the B2 phase is rich in Fe and Mn, whereas the L21 phase is rich in Ni and Al. Room temperature hardness tests on the alloy annealed for different times at 823 K showed only a small change from the 515 VPN measured on the AS alloy to 540 VPN after annealing. Compression tests performed at several temperatures up to 873 K at a variety of strain rates showed a brittle ductile transition temperature for the AS alloy of ~ 650 K at a strain rate of 5 × 10-4 s-1. Research at the Oak Ridge National Laboratory SHaRE User Facility was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) at Dartmouth College through DOE grant DE-FG02-07ER46392.
4:45 PM - N2.7
Fabrication and Magnetic Properties of Fe-based Nano-crystalline Powders.
Sangmin Lee 1 , Yan Zhang 1 , Akihiro Makino 1 , Akihisa Inoue 1
1 , Tohoku University, Sendai Japan
Show AbstractHighly Fe-concentrated amorphous alloys were prepared by melt-spinning method and then heat-treated to get nano-crystallized powders for electromagnetic interference (EMI) / electromagnetic comparability (EMC) as an electromagnetic wave absorbing material. Fe-based nano-crystallized powders show a lower power loss on the high frequency band, a higher performance with a thinner layer due to a higher saturation magnetization as well as a higher permeability than that of the conventional electromagnetic wave absorbing materials. Furthermore, sub-micro grain sized nano-crystalline powders provide us high thermal stability and complicated shape of a product. Fragility of nano-crystallized alloys facilitated to obtain sub-micro grain sized powders in a short time by milling. Powder morphology and grain size distribution were investigated by scanning electron microscopy (SEM). Structural analysis and magnetic properties were explored by X-ray diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), vibrating sample analysis (VSM), and network analyzer. As a result, XRD, DSC and TEM profiles indicated that the as melt-spun ribbons were heat-treated to be nano-crystalline. Nano-crystallized melt-spun ribbons were pulverized to be sub-micro grain sized powders, and grain size of which decreased with an increase of milling time. Details will be demonstrated in the conference.
5:00 PM - N2.8
Alternative Current Magnetic Permeability of Fe81Ga19 Alloy with Bias Magnetic Field.
Hongping Zhang 1
1 Research Institute of Functional Materials, Central Iron & Steel Research Institute, Beijing China
Show AbstractThe Fe-Ga alloy (Galfenol) is a promising new magnetostrictive material with both relatively high magnetostrictive values and mechanical robustness. The alloy’s saturation magnetostriction is larger than 300 ppm under the magnetizing fields of several hundred oersteds. The mechanical properties and ductility of Fe-Ga alloys are better than that of the Tb-Dy-Fe (Terfenol-D) alloys, and the tensile strength of Fe-Ga alloys are over 440MPa. The property of the magnetic permeability of the Fe-Ga alloy is one of the important parameters in transducer design besides the magnetostriction property, especially the alternative current (AC) magnetic permeability in external magnetic field. In this study, the AC magnetic permeability of polycrystalline Fe81Ga19 alloy without crystal orientation under both the bias applied fields and frequencies were investigated by the method of measuring inductance of ring specimens. The tested results showed that the AC permeability of the alloy can reach more than 160 (Gs/Oe) under the conditions of low frequencies or quasi-static state. The permeability decreased with the increase of frequency. When the frequency was higher than 6 kHz, the permeability decreased slowly, and gradually stabilized with the increase of frequency. Under a parallel bias magnetic field, the permeability decreases obviously with the increase of frequency. And under a perpendicular bias magnetic fields, however, the permeability of the only initial point of the measuring frequency decreases a little compare to applying the under zero bias field.
5:15 PM - N2.9
Evaluation of Chemical Interaction Energies Between Metals and Interstitials in E21 (Fe,Co,Ni)3Al(C,O)1-x.
Hideki Hosoda 1 , Tomonari Inamura 1
1 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan
Show AbstractE21 type intermetallic compound M3AlC1-x (M = transition metals) is composed of M atoms at the face center sites, Al at the corner sites and C at the body center sites. This compound is regarded as L12 M3Al which is stabilized by the interstitial carbon atoms. Then, E21 intermetallic compounds often exhibit similar mechanical properties to L12 compounds: E21 Co3AlC exhibits the positive temperature dependence of strength, similar to L12 Ni3Al. However, when the atomic size of carbon is larger than the size of the L12 octahedral interstitial sites, the addition of carbon simultaneously destabilizes the L12 lattice due to generation of lattice strain energy. The M3Al lattice is expanded by the carbon atoms. Then, the appearance and the phase region of E21 phase is considered to be determined by the balance of chemical stabilization effect and elastic destabilization effect of interstitial atoms. This paper focuses on the chemical interaction energies between metals (Fe, Co, Ni and Al) and interstitial atoms (C and O) in E21 (Fe,Co,Ni)3Al(C,O)1-x, and a new evaluation method based on the least square method is presented using the experimental data for the solid solubility of interstitials, the probabilities of site occupancy and the bulk modulus (173GPa for L12 Ni3Al). The interaction energies between metals and interstitials evaluated are -74kJ/mol for Fe-C, -32kJ/mol for Co-C, and +3.5kJ/mol for Ni-C, for example. Based on the interaction energies between the metals and the interstitials the phase stability of E21 M3AlC1-x is also discussed.
5:30 PM - N2.10
Structural Characterization of Magnetostrictive Fe-based Binary Alloys.
Qingfeng Xing 1 , Thomas Lograsso 1
1 , Ames Laboratroy, Ames, Iowa, United States
Show AbstractMagnetostrictive Fe-Ga alloys are a novel class of magnetostrictive materials with a good combination of magnetostriction and mechanical properties. This combination of properties stimulated extensive research efforts over the past decade to characterize the structure of the alloys for exploring the magnetostriction origin. However, inconsistent results in structural characterization as well as in the origin of magnetostriction are present in the published work. In addition to presenting new research results, this talk will also provide an overview of the structural characterization results and discuss their link to the magnetostrictive behavior of the alloys. Other Fe-based binary alloys which show similar phase dependence of magnetostriction to Fe-Ga alloys are also discussed.The Fe-Ga alloys of interest are composed of four composition-magnetostriction regimes, as shown by transmission electron microscopy (TEM) results. The regime boundaries are dependent on thermal history of the alloys. The magnetostrictive behavior is phase dependent. Alloys of Regime I are single phase A2 with a bcc structure and show a monotonic increase in magnetostriction with Ga content. Both synchrotron x-ray diffraction in transmission geometry and lab x-ray diffraction in Bragg-Brentano geometry reveal the presence of short-range order (SRO) in this regime. It had been a debate whether SRO resulted in magnetostriction. Recently it had been found that SRO had little influence on magnetostriction. Alloys in Regime II have a phase mixture of A2 + D03 and show a monotonic decrease in magnetostriction with Ga content. Alloys of Regime III are of single phase D03 or phase mixture of A2 + B2 + D03 depending on sample thermal history and show a monotonic increase in magnetostriction with Ga content. In the regime IV, precipitates in a transition state of phase transformation form at lower Ga contents and change into a likely monoclinic phase at higher Ga contents. The precipitation is responsible for the magnetostriction decrease with Ga content. Observation of the phase dependence is failed in some investigations, which is because of improper operation of microscopy techniques and inappropriate data analysis.Lorentz microscopy work reveals that there is no connection between the underlying microstructure and magnetic domains in the Fe-Ga alloys unless the precipitates are pronounced and large in size. This is consistent with a recent magnetic force microscopy (MFM) work which shows that the frequent literature reports of complex magnetic domain structures are due to improper sample preparation and are not associated with structural inhomogeneities. TEM and transmission x-ray diffraction shows spot elongation in the diffraction patterns from many Fe-Ga alloys. The elongation has two major sources: thermal diffusing scattering and structural modulation.
5:45 PM - N2.11
Nanocharacterization of Magnetic Fe-Pt Bimetallic Alloy Derived from a Novel Feasible Pulse Electrodeposition.
Cuixia Li 1 2 , Zhihong Li 2 , Xueyan Du 1 3 , Jingbo Liu 3
1 School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou, Gansu, China, 2 College of Materials Science and Engineering, Tian University , Tianjin China, 3 Chemistry, Texas A&M University-Kingsville, Kingsville, Texas, United States
Show AbstractA new accomplishment of this research is to create a novel feasible approach to fabricate Fe-Pt system nanoparticles (NPs). The overarching goals are to: (1) provide guidelines for design of new nanoscaled magnetic materials with an electrodeposition method; (2) promote Fe-Pt bimetallic NPs’ preparation using environmentally friendly and cost effective techniques; and (3) integrate advanced instrumentation into nano-characterization and magnetic properties. Ordered FePt NPs of equal-stoichiometric Fe and Pt have recently attracted interest because of their diversified applications. The FePt NPs also display excellent intrinsic magnetic properties (μoMs = 1.43 T, K1 = 6.6 MJ/m3) and are very promising candidates for magnetic recording media with larger capacity than 1 Tbit/in2. However, the requirement that materials are controlled at the nanoscale to ensure high density records and good magnetic properties, has posed significant preparation challenges. Traditionally, the synthesis of Fe-Pt NPs by wet-chemical process was mainly focused on two methods. The first one was related to thermal-decomposition and the second utilizing a polyol-reduction method. The former caused environmental contamination due to the emission CO gas, and the later was operated at high temperature resulting in costly preparation. To overcome these drawbacks, a novel and feasible method was developed for fabrication of bimetallic FePtx nano-alloy using pulse electrochemical method (PEM) to provide a green-synthesis atmosphere. A single-bath was generated to achieve simplicity in experimental setup and efficiency compared with traditionally-used double-bath method. Subsequent annealing converts the face-centered cubic (FCC) NPs into FePt (L10) - FePt3 (L12) nanocomposites with less than 5 mol % of L10 of FePt. The PEM-derived and annealed FePtx NPs were characterized by several advanced techniques including X-ray and electron microscopy. The X-ray powder diffraction (XRD) analysis indicated that a FCC structure of FePtx NPs was obtained and its crystalline phase was transformed from FCC to L12 type structure when the NPs were annealed at 700 °C for 30 min under N2 atmosphere. The transmission electron microscopy (TEM) equipped with X-ray energy dispersive spectroscopy (EDX) results revealed that the FePtx particle sizes were on average 2.5 nm and their composition was identified to be Fe33.1Pt66.9. The vibrating sample magnetometer (VSM) was used to determine the coercivities of PEM-derived and annealed FePtx NPs, which significantly changed from 0 to 275 Oe due to the formation of limited amount of FePt with ordered phase of L10. Precise control of the stoichiometry of Pt and Fe and prevention of L12 phase formation will lead to a new direction in our further investigation.
Symposium Organizers
Bernard Bewlay General Electric Company
Martin Palm Max-Planck Institut fuer Eisenforschung GmbH
Sharvan Kumar Brown University
Kyosuke Yoshimi Tohoku University
N5: Poster Session
Session Chairs
Bernard Bewlay
Sharvan Kumar
Martin Palm
Kyosuke Yoshimi
Tuesday PM, November 30, 2010
Exhibition Hall D (Hynes)
N3: Titanium Alumninides I - Physical Metallurgy, Processing, and Properties
Session Chairs
Bernard Bewlay
Masao Takeyama
Tuesday PM, November 30, 2010
Room 203 (Hynes)
9:30 AM - **N3.1
Factors Influencing the Ductility of Cast TiAl-based Alloys.
Michael Loretto 1
1 Metallurgy and Materials, University of Birmingham, Birmingham United Kingdom
Show AbstractThe ductilities of microstructurally refined cast Ti46Al8Nb and Ti46Al8Ta differ significantly with the ductility of the Nb-containing alloy being typically 0.5% whereas that of the Ta-containing alloy is over 1%. TEM has been carried out on samples failed in tension and the observations suggest that the alpha 2 is much more difficult to deform in the Nb-containing alloy than in the Ta-containing alloy and the level of internal stress in the samples also differs significantly. These observations, and observations of the reduction in ductility which occurs on exposure in air at 700C for times as small as 2h, will be discussed in terms of the factors that limit ductility.
10:00 AM - N3.2
Formation of Phases, Phase Stability and Evolution of the Microstructure in Al-rich Ti-Al Alloys.
Martin Palm 1 , Nico Engberding 1 , Frank Stein 1 , Klemens Kelm 2 , Stephan Irsen 3
1 , Max-Planck Institut für Eisenforschung GmbH, Düsseldorf Germany, 2 Institut für Werkstoff-Forschung, DLR Deutsches Zentrum für Luft- und Raumfahrt e.V., Köln Germany, 3 , Research center caesar, Bonn Germany
Show AbstractTiAl-based alloys currently mature into application. Sufficient strength at high temperatures and ductility at ambient temperatures are crucial issues for these novel light-weight materials which can be solved by generating two-phase lamellar TiAl + Ti3Al microstructures. However, oxidation resistance at high temperatures is still a concern, which could be solved by increasing the Al content. Therefore Al-rich TiAl alloys have come into focus recently. In addition to a better oxidation resistance, these alloys have an even lower density (about 3.2 g/cm3) and it has been shown that their mechanical properties may improve by generating two-phase lamellar TiAl + TiAl2 microstructures. However, phase equilibria in the Al-rich part of the Ti-Al system are rather complicated because two metastable phases, i.e. h-Al2Ti and Al5Ti3, exist, whose formation and subsequent transformation into the stable phases severely influences the resulting microstructures.In order to clarify the formation of these metastable phases, their transformations, and the resulting microstructures, alloys with Al contents of 59 – 61 at.% Al have been produced by different melting and casting techniques to obtain a variety of initial microstructures. After heat treating the differently processed alloys at 800, 900 and 1000 °C for 1, 10, 100, and 1000 h, microstructures were studied by light-optical and scanning electron microscopy (LOM, SEM) and phases were characterised by electron probe microanalysis (EPMA). Kinetics of phase transformations were studied by differential thermal analysis (DTA) and transmission electron microscopy (TEM).The results show that the evolution of the microstructure with time and temperature is crucially influenced by prior processing of the alloys. Also the formation and transformation of the metastable phases Al5Ti3 and h-Al2Ti seems now to be fully understood and will be discussed.
10:15 AM - N3.3
Physical Metallurgy and Properties of β-solidifying TiAl Based Alloys.
Helmut Clemens 1 , Thomas Schmoelzer 1 , Svea Mayer 1
1 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria
Show AbstractNew structural materials have to be “stronger and lighter” to withstand the extremely demanding conditions in the next generation of automotive and aircraft engines, which are targeted to exhibit higher efficiency leading to reduced fuel consumption as well as significantly decreased CO2 emissions. Intermetallic γ-TiAl based alloys possess numerous attractive properties, which meet these demands. During the last years a promising family of high-strength γ-TiAl-based alloys has been developed. These so-called TNM alloys contain Nb and Mo additions in the range of 3 - 7 atomic percent as well as small additions of B and C. For the definition of the alloy composition thermodynamic calculations using the CALPHAD method were conducted. The predicted phase transformation and ordering temperatures were verified by differential scanning calorimetry and in-situ high-energy X-ray diffraction. TNM alloys solidify via the β-phase and exhibit an adjustable β-phase volume fraction at temperatures, where hot-working processes are performed. Due to the high volume fraction of β-phase these alloys can be processed isothermally as well as under near conventional conditions. In order to study the occurring deformation and recrystallization processes during hot-working, in-situ diffraction experiments were conducted during compression tests at elevated temperatures. With subsequent heat-treatments a significant reduction of the β-phase is achieved. These outstanding features of TNM alloys distinguish them from other TiAl alloys which must exclusively be processed under isothermal conditions and/or which always exhibit a high fraction of β-phase at service temperature. After hot-working and heat-treating, these alloys show yield strength levels > 800 MPa at room temperature and also good creep resistance at elevated temperatures. Finally, the development status and perspectives of TNM alloys as innovative automotive and aero-engine materials are presented.
10:30 AM - N3.4
Thermodynamic Modeling and Solidification Simulation of Ti-Al-Cr-Nb Alloys.
Ying Yang 1 , Bernard Bewlay 2 , Fan Zhang 1 , Y. Chang 3
1 , CompuTherm LLC, Madison, Wisconsin, United States, 2 , GE Global Research Center, Niskayuna, New York, United States, 3 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractTitanium aluminide based alloys are candidate materials for high temperature structural applications. They are typically alloyed with Nb and Cr for enhancing the low-temperature ductility. As a first step to understand the relationship between microstructure and chemical composition/processing condition, phase equilibria in the Ti-Al-Cr-Nb system were systematically investigated using the CALPHAD approach. A thermodynamic database of the Ti-Al-Cr-Nb system was developed first based on the critically assessed binary and ternary thermodynamic models. Solidification path of selected alloys in Scheil-Gulliver and lever-rule conditions were then calculated using the currently developed database. With the guidance of thermodynamic calculation and solidification simulation, the effect of Cr and Nb on the stability of β, β0, α, α2 and γ, and the partitioning of elements in different phases during the solidification were discussed. This work provides an essential thermodynamic background on which more sophisticated kinetic simulation could be carried out.
10:45 AM - N3.5
In-situ Characterization of Phase Transformations and Thermo-mechanical Processing in Titanium Aluminium Alloys.
Klaus-Dieter Liss 1 , Saurabh Kabra 1 , Kun Yan 1 2 , Thomas Schmoelzer 3 , Mark Reid 2 , Rian Dippenaar 2 , Helmut Clemens 3
1 The Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales, Australia, 2 Faculty of Engineering, University of Wollongong, Wollongong, New South Wales, Australia, 3 Department of Physical Metallurgy and Materials Testing, Montanuniversität, Leoben Austria
Show AbstractModern neutron and synchrotron high energy X-ray methods can be used to obtain real-time information in-situ and from the bulk of the material, complemented by high-temperature laser confocal microscopy. Two-dimensional radiation detectors deliver information in multiple dimensions, bridging the scales from the atomic, nanometer, micrometer and macroscopic arrangements. Various titanium aluminium based intermetallic systems have been studied to investigate phase transformations and phase diagrams through the conventional Rietveld method. In particular, the complementarity in diffraction contrast between neutron and X-rays can be employed to emphasize the atomic order-to-disorder transformations in the α/α2 and β/β0 phases in conventional α-γ and β-stabilized alloys. Concentration changes between coexistent phases are reflected in the anomalies of the lattice parameters. Two-dimensional diffraction patterns permit to distinguish reflections from individual grains, revealing orientation correlations upon phase transformations or grain growth. Quenched α/α2 material was slowly heated and transforms into an ultra fine lamellar α-γ structure, which can be followed coherently in reciprocal space. All these aspects play a role in thermo-mechanical deformation processes, which is presented on a β-stabilized material, revealing an α+β phase field at high temperatures. The motion of the reflection spots on the Debye-Scherrer rings gives additional information about grain and subgrain evolution, grain rotation, the formation of texture, dynamic recovery and dynamic recrystallization, which is found to occur very differently in the two coexisting phases.
11:00 AM - N3 : TiAl 1
BREAK
11:30 AM - N3.6
Microstructural Optimization of a Cast and Hot-isostatically Pressed Multi-phase Titanium Aluminide Alloy by Heat Treatment.
E. Schwaighofer 1 , M. Schloffer 1 , T. Schmoelzer 1 , S. Mayer 1 , H. Clemens 1 , J. Lindemann 2 , V. Guether 3
1 Department of Physical Metallurgy and Materials Testing, Montanuniversitaet Leoben, Leoben Austria, 2 Lehrstuhl für Metallkunde und Werkstofftechnik, Brandenburgische Technische Universitaet Cottbus, Cottbus Germany, 3 , GfE Metalle und Materialien GmbH, Nuremberg Germany
Show AbstractIntermetallic γ-TiAl-based alloys are used in aircraft engines and automotive applications because of their low density and excellent specific high-temperature strength compared to steels and Ni-base superalloys. Advanced γ-TiAl-based alloys are complex structured, multi-phase materials. TiAl alloys which solidify via the β-phase, like the TNM™ alloy (Ti-43.5Al-4Nb-1Mo-0.1B, in at%) investigated in the present study, consist predominantly of γ-TiAl, α2-Ti3Al and small amounts of βo-phase, all of which are ordered at room temperature. This type of alloys shows great potential for the production of parts by a casting process since it exhibits an almost segregation-free solidification structure with small average grain size. After casting and hot-isostatic pressing, however, a small fraction of the grains exhibit large sizes and adverse shapes, thus representing internal notches which may initiate cracks under tensile loading and thereby significantly decrease the materials ductility at room temperature. In these investigations, the potential of refining coarse grains, i.e. to mitigate potential crack-initiation sites, and thus to homogenize the microstructure by means of special heat treatments is assessed. An ensuing heat-treatment step targets to adjust balanced mechanical properties, i.e. high creep strength as well as sufficient ductility at room temperature. Microstructures resulting from the various heat treatments were characterized by means of light optical and scanning electron microscopy. Hardness measurements and tensile tests were performed to relate the microstructural features to mechanical properties. The results of these experiments are presented and the impact on the establishment of a novel, cost-effective production route for a third generation γ-TiAl-based alloy is discussed.
11:45 AM - N3.7
Influence of Quenching Rates on the Transformation of Ternary Phases in Nb-rich γ-TiAl Alloys.
Andreas Stark 1 , Michael Oehring 1 , Florian Pyczak 1
1 Institute of Materials Research, GKSS Research Centre, Geesthacht Germany
Show AbstractIntermetallic γ-TiAl based alloys with additional amounts of the ternary bcc β phase attracted increasing attention in recent years due to their improved workability at elevated temperatures. At lower temperatures the ductile high-temperature β phase can transform to several ordered phases. However, actually available phase diagrams of these multiphase alloys are quite uncertain.In the present study various transformations of the third phase are observed in situ by means of high-energy x-ray diffraction (HEXRD) using synchrotron radiation. Samples of Ti-45Al-10Nb and Ti-45Al-7.5Nb-1Mo-0.1B (at.%) are subject to a temperature ramp of repeated heating cycles (700 °C - 1100 °C) with subsequent quenching at different rates.Depending on the quenching rate reversible transformations of the B2-ordered βo phase to different ω related phases are observed in Ti-45Al-10Nb. At low quenching rates the hexagonal B82-ordered ωo phase is formed while at high quenching rates the metastable intermediate trigonal ω’’ phase can be preserved. The results indicate that the complete transformation from βo to hexagonal B82-ordered ωo consists of two steps which are both diffusion controlled. No transformation of β or βo was observed in Ti-45Al-7.5Nb-1Mo-0.1B. Probably, even a small amount of Mo stabilises β or βo against transformation.
12:00 PM - N3.8
Phase Equilibria and Phase Transformations in Molybdenum Containing TiAl Alloys.
Svea Mayer 1 , Christian Sailer 1 , Hirotoyo Nakashima 2 , Thomas Schmoelzer 1 , Peter Staron 3 , Klaus-Dieter Liss 4 , Helmut Clemens 1 , Masao Takeyama 2
1 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria, 2 Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo Japan, 3 , GKSS Research Center Geesthacht, Geesthacht Germany, 4 The Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales, Australia
Show AbstractDuring the last two decades, research and development efforts have been made to generate γ-TiAl based alloys for high-temperature applications in advanced automotive combustion and aero engines. Due to the fact that γ-TiAl based alloys are intended for use in the temperature range of 600 to 900°C, there is a growing demand for these alloys, which show good hot-workability and reasonable ductility at ambient temperature. One recent approach is the design of alloys with microstructures in which homogeneously distributed β- and γ-phases are the main constituents. These alloys are termed β/γ-alloys. Since Mo is a strong β stabilizer in pure Ti, the ternary system Ti-Al-Mo is well-suited for studying this type of alloys. Despite the importance of Mo, the knowledge of its alloying effect on the Ti-Al phase diagram and on the occurring ordering and disordering reactions is rather limited. In many advanced multi-phase TiAl alloys three phases of γ (L10), α2 (D019) and βo (B2) are the major microstructural constituents, all of which are ordered at ambient temperature. At elevated temperatures the ordered hexagonal α2-phase disorders to α (A3) and the ordered cubic βo-phase disorders to the body-centered cubic β-phase (A2). In this study, representative sections of the ternary Ti-Al-Mo phase diagram, based on thermodynamic calculations and experimental results, are presented. For the prediction of the constituent phases and related transition temperatures, thermodynamic calculations based upon the CALPHAD method were conducted. The software package ThermoCalc® was also applied using a commercial TiAl database. In order to critically assess the calculated as well as the experimental phase diagram, two model alloys with a nominal composition of Ti-45Al-3Mo and Ti-45Al-7Mo (in at%) were investigated. Isothermal short- and long-term heat treatments were employed in β- and (β+α)-phase field region, followed by water quenching, leading to either martensitic or massive transformation, depending on alloy composition. All heat-treated samples were analyzed by means of scanning electron microscopy, electron back-scatter and conventional X-ray diffraction. For determining the phase evolution and phase transition temperatures, in-situ high-energy X-ray diffraction experiments were conducted during continuous heating. Complementary differential scanning calorimetry investigations were performed to verify the obtained phase transition temperatures. In addition, the order and disorder reactions (α2↔α and β0↔β) were studied by neutron diffraction. Finally, the experimental calculated results are compared to the quasi-binary phase diagrams and the differences are discussed.
12:15 PM - N3.9
TEM Studies of the Ternary Ti36Al62Nb2 Alloys.
Venkata Sai Kiran Chakravadhanula 1 , Viola Duppel 2 , Klemens Kelm 3 , Lorenz Kienle 1 , Daniel Sturm 4 , Martin Heilmaier 5 , Georg Schmitz 6 , Anne Drevermann 6 , Andriy Lotnyk 1 , Frank Stein 7 , Martin Palm 7
1 Synthesis and Real Structure, Technical Faculty of the CAU Kiel, Kiel, Schleswig Holstein, Germany, 2 Chemistry, Max Planck Institute for Solid State Research, Stuttgart, Baden-Württemberg, Germany, 3 Institute of Materials Research, German Aerospace Center, Cologne, North Rhine-Westphalia, Germany, 4 Institute for Materials Joining and Technology, Otto-von-Guericke University Magdeburg, Magdeburg, Sachsen Anhalt, Germany, 5 Physical Metallurgy, Technical University Darmstadt, Darmstadt, Hessen, Germany, 6 , ACCESS e.V., Aachen, North Rhine-Westphalia, Germany, 7 Materials Technology, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, North Rhine-Westphalia, Germany
Show AbstractAl–rich Ti–Al alloys attracted some attention during the past years due to the possibility of their application as light weight, high performance materials at elevated temperatures. While it is possible to produce microstructures in Al–rich Ti–Al alloys comparable to Ti–rich Ti–Al alloys which are nowadays entering technical applications, their mechanical properties remain unsatisfactory. The most important properties for these applications are high oxidation resistance, low creep deformation, long fatigue life and good thermal conductivity. However, refractory elements like Nb improve creep resistance as well as tensile ductility of the Ti–Al alloys. The addition of Nb enhances the room–temperature ductility, fracture toughness, elevated temperature strength and oxidation resistance. Based on our previous work1 in Al–rich Ti–Al alloys, the effect of the addition of Nb to Al–rich Ti–Al alloys has been studied in Ti36Al62Nb2 by a combined approach of transmission electron microscopy with the techniques of selected area and precession electron diffraction, high resolution imaging and EDX as well as EELS analysis for unraveling the features of the real structure in this ternary alloy. For this purpose, transmission electron microscopy analysis was first performed on as–cast ternary alloys. In these studies, h–TiAl2–, Ti3Al5– and γ–TiAl–type phases have been observed. After heat treatment of these as cast alloys, phase transformations were identified, e.g., the metastable h–TiAl2–type is replaced by the more stable r–TiAl2–type.Besides the phase transformations, changes of the microstructural features are apparent, particularly the formation of interfaces with different orientation relationships. The orientation and interfacial relationships of the phases involved are discussed and compared to those of binary Ti–Al alloys rich in Al.[1] Klemens Kelm, Julio Aguilar, Anne Drevermann, Georg J. Schmitz, Martin Palm, Frank Stein, Nico Engberding and Stephan Irsen, Mater. Res. Soc. Symp. Proc. 1128 (2009).
12:30 PM - N3.10
Two Novel Phases in the Ti and Al-rich Part of the TiNiAl Phase Diagram: Phase Equilibria and Crystal Structure.
Atta Khan 1 , Xinlin Yan 1 , Andri Grytsiv 1 , Peter Rogl 1 , Andriana Saccone 2 , Gerald Giester 3
1 Physical Chemistry, University of Vienna, Vienna, Wien, Austria, 2 Dipartimento di Chimica e Chimica Industriale, , Universita di Genova, Sezione di Chimica Inorganica e Metallurgia, Genova Italy, 3 , Institute of Mineralogy and Crystallography, Vienna, Wien, Austria
Show AbstractAlthough several research teams have investigated the Ti-Ni-Al system, phase equilibria are still far from beeing complete (for recent review see [1,2]). We have elucidated the phase relations in the Ti and Al-rich regions around novel hitherto unknown compounds Tau6 and Tau5. The crystal structure of Tau6 and Tau5 has been solved from X-ray single crystal CCD-data. Tau6 is one of the few isotypic variants of the V2(Co,Si)3 type [3,4] as a topologically dense packed structure while Tau5 is a new tetragonal structure type. Neutron powder diffraction on an almost single phase material served to unambiguously locate the Ti-atoms in the unit cell. Formation of the Tau6 phase and phase reactions were studied as a function of temperature on a series of alloys, which were annealed at carefully set temperatures and quenched. Due to sluggish reaction kinetics, the transition temperatures were defined by annealing and quenching techniques as no DTA signals could be received. The phase constitution (XRD, EMPA) is presented in form of a Schulz-Scheil diagram describing the full solidification path in the region involving the Tau6 phase. Several partial isothermal sections for the region around the Tau6 phase document the phase equilibria changing with temperature. [1]J.C. Schuster (2006). Intermetallics, (10-11), 14, 1304-1311. [2]V. Raghavan (2009). Phase Diagram Evaluations: Section II, 30, 77-78.[3] P. I. Krypyakevich, Ya. P. Yarmolyuk (1970). Dopov. Akad. Nauk. Ukr. RSR, Ser. A32, 948.[4]V. Hlukhyy, R. Pöttgen, (2004). Solid State Sciences, 6, 1175-80.
N4: Titanium Aluminides II - Structure, Properties, and Coatings
Session Chairs
Helmut Clemens
Haruyuki Inui
Tuesday PM, November 30, 2010
Room 203 (Hynes)
2:30 PM - **N4.1
Robustness Versus Performance Assessment for Different Gamma-TiAl Processing Routes.
Marc Thomas 1
1 DMSM, Onera, Châtillon France
Show AbstractOne of the main driving force for the development of advanced structural materials is weight saving especially in the transportation industry in order to reduce CO2 emission. The utilization of gamma aluminides, as good candidates for aerospace applications, is strongly related to the development of a cost-effective and robust processing route, as far as possible. It is well established that the processing route, i.e. cast, wrought or PM, has a dramatic effect on the microstructure and texture of gamma-TiAl alloys. Therefore, significant microstructural variations through post-heat treatments coupled with compositional modifications can only guarantee a proper balance of desired properties. However, a number of metallurgical factors during the processing steps can contribute to some scattering in properties. This talk will highlight several critical process variables in terms of the resulting gamma-TiAl microstructures. Of primary importance is the as-cast texture which is difficult to control and may contribute to prefer some alternative processing routes to ensure a better repeatability in mechanical results. Some innovative processing techniques for controlling the structure will then be presented. The main point which will be discussed in the presentation is whether an approach leading to a robust process would not be at the expense of the high performance of the structural material.
3:00 PM - N4.2
Effect of Microstructure on Impact Toughness of Wrought Gamma TiAl Alloys.
Masao Takeyama 1 , Shun Oinuma 1 , Tosikazu Kikugawa 1 , Hirotoyo Nakashima 1
1 Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo Japan
Show AbstractEffect of microstructure on impact properties and fracture behavior of wrought γ-TiAl based alloys have been examined by means of an instrumented Charpy impact test and an in-situ observation of three-point bend test in SEM chamber. The alloys studied were Ti-42Al-(5-8)M (at.%) having a unique transformation pathway of β-Ti → α-Ti → β+γ, based on our phase diagram study, where M is transition metals of Cr and V. These alloys can be hot free-forged by 70% reduction in height by one forging path at temperature range of α+β two-phase region. Subsequently, the transformation makes it possible to produce a fully lamellar microstructure consisting of α/γ phases by controlled cooling, and also produce the lamellar microstructure with fine β particles precipitated at the interfaces by further aging. The load-displacement curves obtained form the instrumented Charpy impact test clearly detect the difference in microstructure, and analysis of the curves revealed that the crack initiation and propagation energies remain almost unchanged with the volume fraction of the lamellar grains (VL) up to 50 vol.%, but they are sharply increases with increasing the VL of more than 50 vol.%. Further improvement of the fracture toughness was achieved by dispersion fine β particles within lamellae. In-site observation of the specimens with VL of nearly 90 vol.% clearly revealed that crack basically initiates and propagates through lamellar interfaces, although depending on the lamellar orientation of each colony with respect to the loading direction. Thus, the β particles act as an obstacle for crack propagation, which is responsible for the high facture toughness. The detailed in-site observation of the fracture behavior and lamellar orientation effect will be presented.
3:15 PM - N4.3
Comparison of Compressive and Bend Creep Behaviour of Binary Al-rich TiAl Alloys.
Martin Heilmaier 1 , Daniel Sturm 2 , Klemens Kelm 3 , Sergii Kozhar 2 , Konstantin Naumenko 4
1 Materials Science, TU Darmstadt, Darmstadt Germany, 2 , Otto-von-Guericke University Magdeburg, Magdeburg Germany, 3 , German Aerospace Centre, Cologne Germany, 4 , Martin-Luther-Universitity Halle-Wittenberg, Halle Germany
Show AbstractCompared to Ti-rich γ-TiAl-based alloys Al-rich TiAl alloys offer an additional reduction of 20% in density and a better oxidation resistance which are both due to the increased Al content. Polycrystalline material of two binary alloys, namely Al60Ti40 and Al62Ti38 was manufactured by centrifugal casting by ACCESS in Aachen. After annealing at 950 °C for 200 h the alloys possessed (thermodynamically stable) lamellar γ-TiAl + r-Al2Ti microstructures as characterized employing light-optical, scanning and transmission electron microscopy, and XRD analyses. Three point bend tests were utilized to determine the brittle-to-ductile transition temperature BDTT which was above 800°C in all cases. Due to this ambient temperature brittleness manufacturing of tensile samples was a challenging task. Hence, the high temperature creep behaviour was assessed with compression tests at constant true stress in the temperature range from 900 to 1050 °C in air. In order to assess the effect of tensile loads on creep deformation additional tests in three point bend mode were experimentally carried out and numerically modelled applying finite element methods. The numerical results coincide nicely with those of the compression creep tests under uniaxial loading and an approximate analytical solution could be developed for describing of the steady-state stresses and strains. In order to investigate the influence of lamellae orientation on creep selected samples were grown into single crystals. The comparison yields a significant influence of both microstructural features: (i) removal of grain boundaries (i.e. production of single crystals) increases the creep resistance and (ii) lamellae oriented parallel to the loading direction give rise to a substantially higher critical resolved shear stress. Finally, compared with Al60Ti40, both, the as-cast and the annealed Al62Ti38 alloy exhibit better creep resistance at 1050 °C which can be rationalized by the reduced lamella spacing.
3:30 PM - N4.4
Plastic Deformation of PST TiAl under Prescribed Strain Conditions.
Kyosuke Kishida 1 , Kengo Goto 1 , Haruyuki Inui 1
1 Department of Materials Science and Engineering, Kyoto University, Kyoto Japan
Show AbstractThe lamellar microstructure in TiAl/Ti3Al two-phase alloy is of special interest since the fracture toughness and creep resistance of the alloys with the fully lamellar microstructure are superior to those of any other microstructure types. We have been conducting systematic studies of deformation mechanisms of the fully lamellar microstructure using our polysynthetically twinned (PST) crystals through conventional uniaxial loading tests. In conventional uniaxial tension or compression, PST crystals except those with the lamellar boundaries perpendicular to the loading axis were found to deform in a highly anisotropic manner. When the compression axis is parallel to the lamellar boundaries, the macroscopic deformation occurs so that the specimen dimension measured in the direction parallel to the lamellar boundaries increase, leaving the dimension measured in the direction perpendicular to the lamellar boundaries almost unchanged. Such anisotropic macroscopic deformation was in good agreement with the anisotropy predicted on the basis of the operative deformation modes determined by TEM analysis of the deformation structures in six orientation variants in the TiAl lamellae. In the case of polycrystalline TiAl based alloys, the anisotropy of macroscopic deformation behavior of PST crystals are considered to be restricted by surrounding crystals. It is thus important to understand the deformation mechanism of PST crystals under constraint conditions. In the present study, PST crystals with the loading axis parallel to the lamellar boundaries were deformed under plane strain condition, in which the inherent anisotropic macroscopic deformation of PST crystals described above is restricted with a channel die, so as to clarify the deformation behavior of TiAl/Ti3Al lamellar under constraint conditions. The Taylor analysis was carried out to correlate quantitatively the plastic deformation behavior under the constraint condition to those under the (unconstraint) uniaxial compression. TEM analysis of deformation modes together with the Taylor analysis reveals that all TiAl orientation variants deform to yield the relaxed-constraint-type plastic strain, where three shear strain components are not zero for each TiAl variant but are macroscopically compensated to zero by the existence of twin-related TiAl lamellae at the early stage of deformation. The Taylor analysis assuming the relaxed constraint conditions is found to be adaptable for predicting the operative deformation modes in TiAl at the early stage of deformation and also for correlating quantitatively the stress-strain behavior of PST crystals under prescribed strain conditions with those under the unconstraint condition.
3:45 PM - N4.5
Microstructure and Mechanical Properties of TiAl Alloys Containing Refractory Elements Sintered by Spark Plasma Sintering.
Houria Jabbar 1 , Jean-Philippe Monchoux 1 , Marc Thomas 2 , Frank-Peter Schimansky 3 , Florian Pyczak 3 , Alain Couret 1
1 , CEMES-CNRS, Toulouse France, 2 DMSN, ONERA, Chatillon France, 3 Institute for Materials Research, GKSS, Gesthacht France
Show AbstractSpark Plasma Sintering (SPS) is used to develop intermetallic TiAl alloys, which are subjected to extensive research efforts for applications in gas turbine engines. SPS is a powder metallurgy process consisting to compact a powder by the application of direct courant pulses of high intensity under a uniaxial pressure. Its main advantage is the short processing time, allowing the minimization of physical processes as grain growth. In previous works, this technique has been used to successfully produce TiAl alloys which exhibit high mechanical strength and comfortable ductility at room temperature but a moderate creep resistance.In the present study, the potentiality of TiAl alloys containing refractory elements is investigated since these elements are expected to reduce the creep rate by limiting the mobility of dislocations moving by climb. In particular G4 (Ti-47Al-1Re-1W-0.2Si) and TNB (Ti-46Al-9Nb) prealloyed powders are used.Depending on the sintering temperature and on the chemical composition, double phased and duplex microstructures were obtained. The effect of the duration of the temperature plateau was also studied. The mechanical properties were measured by tensile tests at room temperature and creep experiments at 700°C.The TNB-SPS alloy possesses a higher creep resistance than the G4-SPS alloy, whereas the latter exhibits a better ductility. The behavior and the role of the refractory elements will be compared and discussed with respect to microstructure and elementary deformation mechanisms.
4:00 PM - N4 : TiAl 2
BREAK
4:15 PM - N4.6
Thermomechanical Characterization of β-stabilized TiAl Alloy-A Study Using Processing Map.
Bin Liu 1 2 , Akihiko Chiba 1 , Yong Liu 2 , Yuping Li 1
1 , Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan, 2 , State Key Lab of Powder Metallurgy, Central South University, Changsha, Hunan, China
Show AbstractAn alloy design strategy recently put forward for improving the hot-workability of TiAl alloys is to exploit a combination of thermo-mechanical processing and additional alloying elements to induce the disordered β-phase at elevated temperatures as ductile phase. Based on this strategy, a new β-stabilized TiAl alloy with composition of Ti–45Al–7Nb–0.4W–0.15B(at.%) was designed by Huang et al[1]. Thermomechanical treatment of TiAl alloys with normal α2/γ lamellar microstructure or with an equiaxed microstructure has been widely studied in recent decades; however, the deformation mechanisms of β-stabilized TiAl alloy, especially with regard to how the residual β phases affect the deformation, are still unclear. In this study, the correlation between hot deformation parameters and the workability of β-stabilized alloy was studied. A processing map was constructed to characterize the flow behavior accurately and to obtain the appropriate deformation parameters since the processing map based on Dynamic Materials Model (DMM) is an effective tool in characterizing flow behavior and evaluating material workability and has been widely used in researches of hot deformation of the Ti, Al, Ni alloys and etc. Also, deformation mechanisms were characterized by detailed analyses of the deformation behavior and microstructural observations. The results indicate that the flow softening of the alloy is a continuous dynamic recrystallization (DRX) process, which is different from the discontinuous DRX seen in conventional single γ phase TiAl alloys. Appropriate deformation processing parameters were suggested based on the processing map, and were successfully applied in the quasi-isothermal canned forging of industrial-scale billets. Deformation and recrystallization was found to preferentially occur in the grain boundary β phases because the good high temperature deformability enhances grain boundary gliding and migration, thus improving workability. In addition, decomposition of the β phase to α2 and γ phases partly accommodates the stress concentration and is thus beneficial in hot deformation.
4:30 PM - N4.7
Piling-up Behavior During Axisymmetric Indentation and Its Relation to the Activated Deformation Mechanisms in γ-TiAl.
Claudio Zambaldi 1 , Franz Roters 1 , Dierk Raabe 1
1 Microstructure Physics and Metal Forming, Max-Planck-Institut für Eisenforschung, Düsseldorf Germany
Show AbstractAn improved EBSD orientation mapping technique was developed to identify γ-TiAl ordering domains. The technique is based on the sensitive detection of the slight tetragonal distortion of the backscatter electron diffraction patterns. The availability of this technique enabled a study of the orientation dependent piling-up around nanoindentations in γ-TiAl as present in a Ti-46Al-8Nb two-phase alloy. The topography of the free surface around indents made with an axisymmetric indenter was measured with an atomic force microscope (AFM). The pile-up profiles of known orientations were found to be highly characteristic for the easy activation of ordinary dislocation glide in the near-stoichiometric γ-TiAl phase. The indentation process was simulated by a 3D finite element model. For the constitutive behavior, a crystal plasticity finite element (CPFE) formulation was chosen that incorporates the deformation mechanisms of the γ-TiAl phase, namely glide of ordinary dislocations and superdislocations and twinning. The simulation approach could explain the formation of the orientation dependent pile-up patterns through the spatial distribution of crystallographic shear on the competing deformation mechanisms. A quantification of the critical shear stresses of the different deformation mechanisms was possible through optimizing the used material parameters to match the simulated pile-up with the experimental surface topographies. The new method complements the analysis of dislocation-mediated plasticity in the TEM by providing information about the operation of different types of dislocations in a quantifiable way with good statistics.The orientation dependent material pile-up during axisymmetric indentation was identified as a material property that is highly characteristic for the activated deformation mechanisms in plastically anisotropic crystals. It can be efficiently displayed in the newly developed inverse pole figure of pile-up patterns. References[1] C. Zambaldi, S. Zaefferer, S.I. Wright (2009) Characterization of order domains in γ TiAl by orientation microscopy based on electron backscatter diffraction, J Appl Crystallography, 42, 1092-1101, http://edoc.mpg.de/439331[2] C. Zambaldi, D. Raabe (2010) Plastic anisotropy of γ-TiAl revealed by axisymmetric indentation, Acta Mater, 58 (9), 3516-3530, doi: 10.1016/j.actamat.2010.02.025[3] F. Roters, P. Eisenlohr, L. Hantcherli, D.D. Tjahjanto, T.R. Bieler, D. Raabe (2010) Overview of constitutive laws, kinematics, homogenization, and multiscale methods in crystal plasticity finite element modeling: theory, experiments, applications, Acta Mater, 58(4), 1152-1211
4:45 PM - N4.8
Microscale Fracture Testing of TiAl-based Alloys.
Kazuki Takashima 1 , Mitsuhiro Matsuda 1 , Masaaki Otsu 1 , Masao Takeyama 2
1 Dept. Materials Science & Engineering, Kumamoto University, Kumamoto Japan, 2 Dept. Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo Japan
Show AbstractThe mechanical properties of TiAl-based alloys with a fully lamellar structure are mainly dependant on the mechanical properties of lamellae, particularly the lamellar interface fracture strength. It is thus important to evaluate the fracture properties of the lamellae in order to improve the fracture toughness of such TiAl-based alloys. The interlamellar spacing is, however, usually less than 1 μm, and it is rather difficult to measure the fracture properties of such a small region. In this investigation, a microscale fracture testing technique, which has been developed in our laboratory, is applied to examine the fracture properties of the lamellar structure of polysynthetically twinned (PST) TiAl crystals. Micro-sized cantilever specimens (size ≈ 10 × 10 × 50 μm3) were prepared from Ti–48Al two-phase PST crystal lamellae by focused ion beam (FIB) machining. Notches with a width of 0.5 μm and a depth of 5 μm were also introduced into the specimens by FIB. Two types of notch directions (interlamellar and translamellar) were selected when introducing the notches. Fracture toughness tests were carried out for micro-sized specimens using micro-fracture testing equipment that we have developed. After the fracture tests were conducted, fracture surfaces were observed by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The fracture toughness (KQ) values of the interlamellar type specimens were obtained in the range of 1.2–4.2 MPam1/2. SEM and TEM observations revealed that the fracture surfaces with lower KQ values consist of a flat interface, while those with higher KQ values consist of several lamellar steps. This indicates that lower KQ values correspond to the intrinsic fracture toughness of the interlamellar fracture. The KQ values of the translamellar specimens were 4.7–8.4 MPam1/2. These values are lower than the values previously reported for bulk TiAl PST crystals. Extrinsic toughening mechanisms, including shear ligament bridging in the crack wake, are not operated in microscale specimens; this indicates that the obtained KQ values are related to the intrinsic fracture toughness in the translamellar direction.
5:00 PM - N4.9
Mechanical Spectroscopy in Advanced TiAl-Nb-Mo Alloys at High Temperature.
Pablo Simas 1 , Thomas Schmoelzer 2 , Maria No 3 , Helmut Clemens 2 , Jose San Juan 1
1 Fisica Materia Condensada, Universidad del Pais Vasco, Bilbao Spain, 2 Physical Metallurgy and Materials Testing, University of Leoben, Leoben Austria, 3 Fisica Aplicada II, Universidad del Pais Vasco, Bilbao Spain
Show AbstractNew advanced multi-phase γ-TiAl based alloys (TiAl-Nb-Mo), so called TNM alloys, have been developed to promote hot workability and to allow easier processing by conventional forging. However, creep behavior and room temperature ductility are very sensitive to the microstructure and in particular to the amount of β phase, which undergoes an ordering transition to a B2-structure called β0. Therefore, to control and stabilize the final microstructure, specific processing and further thermal treatments are required. In the present work we used mechanical spectroscopy techniques, measuring the internal friction and the dynamic modulus, to obtain a better understanding of the microstructural mechanisms taking place during two-step heat treatments. Internal friction spectra and dynamic modulus evolution have been measured in an inverted torsion pendulum from room temperature up to 1500 K, on samples which previously underwent different thermal treatments. A stable relaxation peak was observed in all cases at about 1050 K for 1 Hz. Spectra acquired at several frequencies between 0.01 Hz and 3 Hz allow us to measure the activation parameters of this peak. In addition, a high temperature background (HTB) has been experimentally determined for different frequencies. This HTB has been found to be dependent on thermal treatments which we have analyzed by a previously developed method to obtain the apparent activation enthalpy, which seems to be correlated to the creep behavior.Finally, we discuss the relaxation peak and the HTB in terms of the microstructural evolution during thermal treatments.
5:15 PM - N4.10
Deformation Mechanisms in Micron-sized PST-TiAl Compression Samples: Experiment and Model.
Franz Fischer 1 , Martin Rester 2 , Christoph Kirchlechner 3 , Thomas Schmoelzer 4 , Helmut Clemens 4 , Gerhard Dehm 1 3
1 Institute of Mechanics, Montanuniversität Leoben, Leoben Austria, 2 Department of Materials Physics, Montanuniversität Leoben, Leoben Austria, 3 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben Austria, 4 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria
Show AbstractTitanium aluminides are the most promising intermetallics for applications in aerospace and automobile industry. Consequently it is of fundamental interest to explore the deformation mechanisms occurring in this class of materials. One model material which is used extensively for such kind of studies is polysynthetically twinned (PST) TiAl, which consists predominantly of parallel γ-TiAl and fewer α2-Ti3Al lamellae. In the present study PST crystals with a nominal composition of Ti-50at.% Al were focussed ion beam (FIB) machined into miniaturized compression samples with a square cross-section of 8 µm x 8 µm. A compression test on the miniaturized samples was performed in situ inside a scanning electron microscope (SEM) using a microindenter equipped with a tungsten flat punch. After deformation, thin foils were cut out of the micro-compression samples and thinned to electron transparency using FIB in order to study the deformation structure by transmission electron microscopy (TEM). The TEM studies reveal mechanical twinning as the main deformation mechanism at small strains (~4%). At higher strain, however, dislocation glide becomes increasingly important. The experimentally observed twins scale in size with the width of the γ-TiAl lamella. A kinematical and thermodynamical model is developed to describe the twin-related length change of the micro-compression sample at low strains as well as the relationship of an increase of twin width with increasing γ-TiAl lamellae thickness. The developed twin model predicts a nanometer-sized width of the twins which is in agreement with the experimental findings.
5:30 PM - N4.11
Comparison of Different Fluorine-treatments for Improved High Temperature Oxidation Resistance of TiAl-alloys.
Alexander Donchev 1 , Michael Schuetze 1 , Rossen Yankov 2 , Andreas Kolitsch 2
1 Karl-Winnacker-Institut, Dechema, Frankfurt Germany, 2 , FZD, Dresden Germany
Show AbstractIntermetallic TiAl-alloys can replace the heavier Ni-based superalloys in several high temperature applications with regards to their mechanical properties but they can not be used at temperatures above 800°C in oxidizing environments for longer times because of an insufficient oxidation resistance. Despite an Al-content of about 45 at.% in technical alloys no protective alumina layer is formed because the thermodynamic stabilities of titanium oxide and aluminum oxide are in the same order of magnitude. Therefore a mixed TiO2/Al2O3-scale is formed which is fast growing so that the metal consumption rate is quite high. On the other hand the formation of a slowly growing alumina layer is promoted by a fluorine treatment. This so called fluorine effect leads to the preferential intermediate formation of gaseous aluminum fluorides at elevated temperatures if the fluorine content at the surface stays with a defined concentration range. These fluorides are converted into solid Al2O3 due to the high oxygen partial pressure of the high temperature service environment forming a protective pure Al2O3 surface scale. In this paper results of high temperature oxidations tests of several technical TiAl-alloys will be presented. Different F-treatments e.g. dipping or spaying which are easy to apply or more sophisticated ion beam techniques have been used and their results will be compared. The mass change data of the F-treated specimens are always lower than those of the untreated ones. Post experimental investigations like light microscopy, scanning electron microscopy and energy dispersive X-ray analysis reveal the formation of a thin alumina layer on the F-treated samples after optimization of the process while a thick mixed scale is found on the untreated samples. The results will be discussed in view of an optimized procedure and the future use of TiAl-components in high temperature environments.
5:45 PM - N4.12
Shark-skin Inspired Surface Engineering on Intermetallic Titanium Aluminides for High Temperature Applications Using the Fluorine Effect.
Raluca Pflumm 1 , Michael Schuetze 1
1 Karl-Winnacker-Institut, Dechema e.V., Frankfurt am Main Germany
Show AbstractIncreasing demands on technical components for high-temperature applications (e.g. turbine blades) promotes new developments not only in the field of alloy design, but also in surface engineering. This paper shows that it is possible to structure the surface topography of titanium aluminides by locally controlled oxidation of the material due to selective doping with fluorine in order to improve the surface aerodynamics. Intermetallic titanium aluminides have a remarkable potential for applications as high-temperature components due to their low specific weight and good high-temperature strength. It has been proven that their major drawback, i.e. the poor resistance against oxidation at temperatures higher than 800°C, is improved by the so called “halogen effect”. This effect not only generates a major change in the oxidation behaviour, but it also allows the extension of the service temperature up to 1050°C which qualifies these advanced light weight alloys as a replacement of the conventional Ni-based alloys used in the aero engine or automotive industry. The selective doping of the subsurface zone of titanium aluminides with fluorine for example, leads to the development of a thick mixed oxide layer on the fluorine-free domains and a thin protective alumina layer on the fluorine-rich areas. The key parameters for controlled oxidation of these alloys are the available amount of halogens and the temperature. In this paper fluorine is applied according to a defined surface pattern in a low temperature process by ion implantation or liquid phase application leading to an in-situ change in the surface topography during high temperature exposure. The aim is to reproduce a shark-skin pattern (parallel riblets with valleys in between) on titanium aluminide surfaces. The riblets result from different local oxidation rates on fluorine-free and fluorine-rich areas. Applied to an airplane turbine blade, this will lead to drag reduction effects under working conditions due to the in-situ formation of a riblet microstructure during the unavoidable high temperature oxidation of these alloys. This aerodynamic improvement will add to the major benefit of weight savings for the case of titanium aluminide engine components. The nucleation process, the aspect ratio and the stability of the generated microstructures are discussed depending on the substrate composition, the applied selective fluorination method and the oxidation conditions.
N5: Poster Session
Session Chairs
Bernard Bewlay
Sharvan Kumar
Martin Palm
Kyosuke Yoshimi
Wednesday AM, December 01, 2010
Exhibition Hall D (Hynes)
9:00 PM - N5.10
Perpendicular Magnetic Anisotropy in FePt3B Films.
Jae Young Ahn 1 , Nyung Jong Lee 1 , Taehee Kim 1 , Anny Michel 2
1 Physics, Ewha Womans University, Seoul Korea (the Republic of), 2 , PHYMAT, Poitiers France
Show AbstractHighly anisotropic FePt is a leading candidate material for ultra high density magnetic recording media of ~ 1Tbits/in2. A lot of investigations have been made to process FePt films by various thin film preparation techniques. However, to improve readback noise decrease magnetic domain size, FePtB media were subsequently needed. Here, we investigated magnetic and structural properties of FePt3 and (FePt3)1-xBx, (x=5~10) films using co-evaporation technique of UHV-MBE system. The epitaxial thin films fabricated on various buffer layer with 5nm-thick MgO underlayer onto Si substrate less than 300'C substrate temperature. The structural analysis was systematically carried out by x-ray diffractometer and transmission electron microscope and VSM has beed used for measuring magnetic properties. We observed chemically ordered of FePt3 L10 phase beyond fcc Pt buffer layer. However, FePt3B thin films shown as mixing amorphous structure. After subsequent annealing, microstructural analysis showed new fcc FePt phase with homogenous grain size, reduced RMS roughness in FePt3B thin films. PMA was observed in both films. Our results could be useful to develop high density recording media technology for mass production.
9:00 PM - N5.11
Microstructure and Deformation of the Eutectic Alloy Fe30Ni20Mn35Al15.
Yifeng Liao 2 , Ian Baker 1
2 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States
Show AbstractThe two-phase eutectic alloy Fe30Ni20Mn35Al15 has been found to have good strength and ductility. The alloy consists of alternating B2 (ordered b.c.c.) and f.c.c. lamellae. Two orientation relationships were identified between the phases in arc-melted Fe30Ni20Mn35Al15, i.e. f.c.c.(112)//B2(011), f.c.c.[111]//B2[111] and f.c.c.(011)//B2(001), f.c.c.[011]//B2[110], while for drop-cast Fe30Ni20Mn35Al15, the Kurjumov-Sachs orientation relationship, i.e. f.c.c.[111]//B2[011], f.c.c.[011]//B2[111], was found to be prevalent. Both in situ straining and post-mortem TEM tests revealed that the deformation occurred in the f.c.c. lamellae by <111> slip, while the B2 lamellae were not plastically deformed at room temperature. Nanoindentation tests on the individual phases showed that the B2 phase was much harder than the f.c.c. phase. The Vickers hardness increased with decreasing lamellar spacing, obeying the Hall-Petch relationship. Tensile tests performed at different strain rates in ambient atmosphere and dry oxygen showed that this alloy is subject to hydrogen embrittlement. Hence the ductility of drop-cast specimens increased with increasing strain rate. The embrittlement could be suppressed by testing either at high strain rate or in an oxygen environment.Research supported by NSF grant DMR-0505774
9:00 PM - N5.12
Cutting Tool Properties of Recycling-type Fe3Al Composites Using Nonferrous Metals as a Work Material.
Talkaomi Itoi 1 , Tomoalo Sudo 1 , Kyosuke Yoshimi 2 , Hisamichi Kimura 3 , Mitsuji Hirohashi 1
1 , Chiba University, Chiba Japan, 2 , Tohoku University, Sendai Japan, 3 , IMR for Tohoku University, Sendai Japan
Show AbstractRecycling-typed Fe3Al (hereinafter designated as Re-Fe3Al) composites reinforced by several carbides of TiC, ZrC, or V4C3 were prepared by the high frequency induction melting method with high-carbon Cr steel sludge, Al can scraps and the transition metals of Ti, Zr, or V, respectively. These carbides were synthesized by in-situ reactions between transition metals and carbon in molten iron aluminide alloys. The Vickers hardness of the Re-Fe3Al composites is 309HV0.5 for Re-Fe3Al/TiC , 390HV0.5 for Re-Fe3Al/V4C3 and 473HV0.5 for Re-Fe3Al/ZrC, which are much higher than that of Fe3Al prepared from pure iron and alminum. The feasibility of the Re-Fe3Al composites with TiC, ZrC, or V4C3 for practical use as cutting tools was investigated by cutting tests using pure-Cu (C1020) and a Al alloy (A5056) under a dry condition. The cutting performance of the Re-Fe3Al compoites was compared with a high-speed tool steel (HSS) at three cutting speeds of 120, 240, or 360 m/min. All of the tools examined show no visible cracks around cutting edges after the cutting tests. From the SEM images of the tools after cutting tests with Cu (C1020), the relatively sharp surfaces with parallel plough ridges and grooves were observed. This result indicates that the dominant wear mechanism is abrasive wear. Tool life is one of the most important properties of a cutting tool. In cutting tests with Cu (C1020) that the depth of cut was 0.05mm (finish machining), the tool life of Re-Fe3Al (TiC) was about three times as long as that of HSS at the cutting speed of 360/min. Moreover, the surface roughness of work material (C1020) after the cutting test by Re-Fe3Al (TiC) was smaller than that of HSS. Therefore, it is concluded that Re-Fe3Al (TiC) has excellent cutting tool properties.
9:00 PM - N5.13
Characterization of the Deformation Behavior of γ-TiAl Based Alloys by In-situ X-ray Diffraction.
Thomas Schmoelzer 1 , Klaus-Dieter Liss 2 , Martin Rester 3 , Kun Yan 2 4 , Matthew Peel 5 , Helmut Clemens 1
1 Dept. Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria, 2 , Australian Nuclear Science and Technology Organisation, Menai, New South Wales, Australia, 3 Department Material Physics, Montanuniversität Leoben, Leoben Austria, 4 Faculty of Engineering, University of Wollongong, Wollongong, New South Wales, Australia, 5 , European Synchrotron Radiation Facility, Grenoble France
Show AbstractIntermetallic γ-TiAl alloys are a class of novel high-temperature materials for use in advanced jet engines. Since their density is half that of the Ni-based alloys currently used, the potential for reductions in fuel consumption and CO2 emissions is substantial. However, uptake has been limited so far since TiAl alloys are difficult to process due to their high deformation resistance and the multitude of phase transformations occurring at high temperature. Recovery and recrystallization phenomena decidedly influence the microstructural evolution during hot-working and thereby critically influence the hot-workability as well as the properties of the finished parts. In order to promote the understanding of these phenomena and their influence on processing, a novel in-situ diffraction method has been employed. A cylindrical specimen is mounted in a load-frame and resistively heated while being illuminated by a collimated high-energy X-ray beam. During compression, a fast two-dimensional detector records the occurring diffraction patterns. The evolution of the patterns yields information on the processes occurring in different phases during deformation. By careful analysis, it is possible to determine the primary modes of deformation and to evaluate whether recovery or recrystallization occurs during compression. Complementary methods such as scanning and transmission electron microscopy as well as electron backscatter diffraction are used to verify the obtained in-situ results and confirm the great potential of this new approach to study deformation processes in real-time.
9:00 PM - N5.14
Effects of Geometric Constraint on Phase Selection and Segregation in Cast TiAl.
Sailei Zhang 1 , David Johnson 1 , Matthew Krane 1
1 , Purdue University, West Lafayette, Indiana, United States
Show AbstractSimulations of dendritic growth of TiAl alloys through porous ceramic preforms consisting of narrow straight channels with spacings on the order of the dendritic tip radius were performed with a modified cellular automaton, which includes a finite volume calculation of solute diffusion. As the width of the channels decreased, growth of the primary phase slowed due to an increased constraint on lateral solute diffusion, resulting in increased undercooling at the solid/liquid interface and an increase in solid composition. For large undercoolings, nucleation of the peritectic phase became possible. The degree of microsegregation also varied with channel width. For narrow channels, the amount of the secondary interdendritic phase was less than that for free dendritic growth. However for intermediate sized channels, an increased interdendritic phase fraction was found due to solute trapping caused by the high degree of undercooling that developed within the preform. To simulate growth of both bcc-(Ti) with four-fold symmetry and hcp-(Ti) with six-fold symmetry on the same Cartesian grid, a height function method for calculating interface curvature was applied to minimize the influence of grid dependency. Dendritic growth of hcp-(Ti) with six-fold symmetry during equiaxed, isothermal growth and during directional solidification was successfully performed on the Cartesian grid. The development and assessment of the solidification model and the growth of Ti-47at%Al and Ti-49at%Al alloys through straight narrow channels is discussed in terms of microstructure and the degree of compositional segregation.
9:00 PM - N5.15
Texture Formation of α2-Ti3Al during Hot Forming of γ-TiAl Based Alloys.
Andreas Stark 1 , Daniel Gosslar 2 , Nikolai Pashkov 2
1 Institute of Materials Research, GKSS Research Centre, Geesthacht Germany, 2 Institute of Materials Science and Technology, Hamburg University of Technology, Hamburg Germany
Show AbstractConventional titanium aluminide alloys are multiphase materials consisting of tetragonal γ-TiAl (L10 structure) and smaller amounts of hexagonal α2-Ti3Al (D019 structure). Although texture formation of γ-TiAl during hot forming was intensively studied over the past years, only little information is available concerning the texture formation of α2-Ti3Al.In the present study the α2 and the γ texture in a Ti-45Al (at.%) alloy were analyzed by means of x-ray diffraction after hot deformation. The initial Ti-45Al powder compact exhibits a random texture and shows a relatively high amount of α2 phase (about 30 vol.%). Various hot compression tests were performed at temperatures ranging from 700 °C to 1100 °C with strain rates of 5×10–4 s–1 and 5×10–2 s–1 up to a true deformation of ε = –1.Depending on the deformation temperature the γ-TiAl deformation texture consists of pure deformation components (700 °C) or components completely related to dynamic recrystallization (1100 °C). In contrast to the γ phase the α2 phase shows no remarkable changing of the deformation texture with increasing temperature. The α2 deformation texture basically consists of a similar component as it is known from hexagonal α-Ti, namely a tilted basal fibre. However, a significant influence of the deformation rate on the α2 texture formation is observed at temperatures above 800 °C. With increasing deformation temperature the α2 texture strengthens by applying a high deformation rate, whereas it weakens for a low deformation rate. This contrary behaviour is attributed to the interaction of the α2 and γ phases during texture formation.
9:00 PM - N5.16
Carbon Strengthening Mechanisms in Ti–48.5Al–0.4C and Ti–45Al–5Nb–0.5C Studied by Small-angle Neutron Scattering.
Peter Staron 1 , Frank Schimansky 1 , Christina Scheu 2 , Helmut Clemens 3
1 Institute of Materials Research, GKSS Forschungszentrum, Geesthacht Germany, 2 Department of Chemistry, Ludwig-Maximilians-University of Munich, Munich Germany, 3 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria
Show AbstractTwo-phase titanium aluminide alloys are being considered as lightweight materials to replace nickel-based superalloys in some high-temperature aero-engine applications. However, at an intended service temperature of about 700 °C the strength and creep resistance of titanium aluminides are generally inferior to those shown by superalloys. Therefore, precipitation hardening treatments using carbon additions were tested for an improvement of the high-temperature strength. The studied materials had a near-γ microstructure. Ageing of a Ti–48.5Al–0.4C (in at%) alloy for 24 h at 750 °C after a solution treatment for 4 h at 1250 °C, followed by quenching, leads to the formation of perovskite-type precipitates with sizes in the nanometre range, improving the high-temperature strength by 55% (275 MPa increase of yield stress σ1.25%). These precipitates had been observed by transmission electron microscopy (TEM) and their size distribution had been characterized by small-angle neutron scattering (SANS). Recently, Nb was added to titanium aluminides for improving the high-temperature properties, such as creep strength. It was tried to apply the same hardening by C additions to these Nb-containing alloys. Sheets made of a Ti–45Al–5Nb–0.5C alloy were investigated in the as-rolled state and after a primary annealing treatment (2 h/1050 °C). For comparison, sheets of the same alloy without C addition were produced. SANS, TEM, and atom probe tomography (3DAP) were used to examine the expected hardening precipitates in these sheets. However, amounts of precipitates with sizes in the nanometre range significant for strengthening were not detected in the Nb-containing alloy with SANS and TEM. By 3DAP it was observed that in most areas of the γ-phase the C atoms were homogeneously distributed. Only a few C-enriched features were detected that were presumably Cottrell atmospheres surrounding dislocation cores. From the SANS as well as 3DAP results it is concluded that the size of these features is above 10 nm. 3DAP yielded a C concentration within the γ-phase of ≈0.25 at%, which is a factor of 10 higher than the solubility limit reported for other TiAl alloys. This large amount of C in solid solution can explain an observed increase in yield strength of 45% (270 MPa increase of yield stress σ1.25%) compared to C-free material containing the same Ti, Al, and Nb concentrations. The remaining C seems to be concentrated in the Cottrell-like features observed by SANS and 3DAP, which are relatively large and do thus not contribute significantly to strengthening. Thus, it was shown that the dominating strengthening mechanism for C in the investigated Nb-containing titanium aluminide is solid solution hardening while in the alloy without Nb it is precipitation hardening.
9:00 PM - N5.18
Compression of Micropillars of TiAl Coexisting with Ti3Al.
Kazuki Fujimura 1 , Kyousuke Kishida 1 , Katsushi Tanaka 1 , Haruyuki Inui 1
1 Materials Science & Engineering, Kyoto University, Kyoto, Sakyo-ku, Japan
Show AbstractTiAl/Ti3Al two-phase alloys are one of the most promising materials for high temperature structural applications due to their excellent mechanical properties such as fracture toughness and creep resistance. We have systematically studied the intrinsic properties of the two-phase alloys using our polysynthetically twinned (PST) crystals composed of the lamellar structure with a single lamellar orientation over the entire crystal and also those of component TiAl and Ti3Al phases. In TiAl phase with the tetragonal L10 structure, three different deformation modes; namely {111}1/2<1-10> ordinary slip, {111}<-101> superlattice slip and {111}<11-2> twinning, have been known to be operative. Previous studies on mechanical properties of Al-rich TiAl using single crystals have revealed the temperature and composition dependences of critical resolved shear stresses (CRSS) for the three deformation modes. However, the values of CRSS for these deformation modes in nearly stoichiometric TiAl phase coexisting with Ti3Al phase have not determined yet mainly because large-scale single crystals have not been obtained so far. For further understanding of the deformation mechanisms of TiAl/Ti3Al two phase alloys, it is essential to know the detailed characteristics including the values of CRSS and their size dependence for the three deformation modes in TiAl phase with nearly stoichiometric composition. In the present study, micropillar of nearly stoichiometric TiAl single crystals with various loading axis orientations and various sizes were prepared from our TiAl PST crystals by focused ion beam (FIB) technique. These TiAl micropillars were deformed in compression using a micro hardness testing machine equipped with a flat diamond tip in order to investigate the values of CRSS for the three deformation modes as a function of specimen size. The selective operation of {111}<-101> superlattice slip and {111}1/2<1-10> ordinary slip was confirmed to be achieved by compression tests of [-110]- and [-211]- oriented micropillars, respectively. For specimens with an approximate cross section of 7x7 square micrometers, the value of CRSS for the {111}<-101> superlattice slip were estimated to be about twice as high as that for the {111}1/2<1-10> ordinary slip, which is in marked contrast to the case of Al-rich TiAl.
9:00 PM - N5.19
Experimental Reconsideration on Phase Formation in Mo-rich Portion of Mo-Si-B Phase Diagram.
Seong-Ho Ha 1 , Kyosuke Yoshimi 1 , Kouichi Maruyama 1
1 , Tohoku University, Sendai Japan
Show AbstractMo-Si-B system has attracted interest attentions for ultra-high temperature applications, and several efforts have been dedicated to understanding the phase formation, stability and solidification pathways of Mo-Si-B ternary alloys. The sequences of phase formation from liquidus phase to the final alloy product are indicated by a liquidus projection, and phase equilibrium in solids is done by phase a diagram. Unfortunately, there are some discrepancies between the liquidus projections and phase diagrams reported for the Mo-Si-B system. Meanwhile, it was observed in our recent studies that drastic microstructure development from as-cast microstructures occurred through heat treatment at 1800 °C for 24 h. Therefore, in this work, for the Mo-rich portion of the Mo-Si-B system, the reaction points in the liquidus projections are reconsidered experimentally and the phase equilibria and solubility limits of the constituent phases are investigated at 1800 °C. Careful calibration for quantitative analysis on EPMA was conducted using a Mo-9.5at%Si-14.2at%B alloy as a standard material, which had a fine and homogeneous microstructure consisting of Mo solid solution, Mo3Si and Mo5SiB2, in order to correct the low reliability of a B concentration in the Mo-Si-B system. The primary-phase regions of Mo solid solution, Mo3Si, Mo5SiB2 and Mo2B in the liquidus projection obtained in this study were in good agreement with the liquidus projection reported by Yang and Chang. On the other hand, the quantitative analyses for the as-cast microstructures by calibrated EPMA indicated that the reaction points appeared to exist between the points shown by Yang and Chang and Nunes et al. After heat treatment at 1800 °C, microstructural development drastically occurred in all of the alloys examined for the Mo-rich portion. Basically, phase equilibria at 1800 °C were the same as those shown in the phase diagram at 1600 °C. In this study, it was found by corrected EPMA that the composition of the constituent phases was relatively in good agreement with that by Nowotny et al. and the solubility limits of each element in Mo, Mo3Si and T2 became larger at 1800 °C compared with Nowotny’s at 1600 °C.
9:00 PM - N5.20
Effects of Additives on the Phase Stability of Nb3Si Intermetallic Compound and Mechanical Properties of Nb-Si Alloy.
Tatsuichi Tanahashi 1 , Seiji Miura 1 , Tetsuo Mohri 1
1 Graduate School of Engineering , Hokkaido University, Sapporo, Hokkaido, Japan
Show AbstractRecently, Nb-Si alloys have attracted attention as a substitute material of Ni based superalloys because of its low density and high melting point. For attaining good room temperature toughness of Nb-Si alloy with spheroidized Nb5Si3 strengthening phase embedded in Nb matrix, proposed is a microstructure-control technique by combining eutectic reaction (L->Nb+Nb3Si) and eutectoid reaction (Nb3Si->Nb+ Nb5Si3) [1]. For the solid solution strengthening of Nb matrix phase W and Mo are very effective, but Nb3Si phase disappears by adding these elements of as small as 3 at%. In contrast, Ti and Ta stabilize Nb3Si phase. For a further alloy development, establishment of an alloy design based on the control of phase stability of Nb3Si is needed. In the previous study [2], it was revealed that the phase stability of Nb3Si can be controlled by adding Ta and Mo simultaneously. In the present study, this approach is expanded to the other combinations of stabilizing and destabilizing elements of Nb3Si, such as W and Ti, W and Ta, and Mo and Ti. Alloy ingots of Nb-18.1Si with W, Mo, Ta or Ti are obtained by Ar-arc melting. Heat-treatments for the eutectoid reaction were conducted at 1300°C or 1650°C for 100 hours. The microstructure was observed with SEM, and compositions of phases were analyzed with EPMA. Mechanical tests were performed on the heat-treat specimens. Similarly to the case of the alloys containing Mo and Ta, it is found that the stabilizing elements enlarge Nb/Nb3Si two-phase composition area where Nb3Si exists even with the destabilizing elements. It is, therefore, confirmed that the phase stability approach can be expanded to the combinations of the other stabilizing and destabilizing elements. This study is supported by Grant-in-Aid for Scientific Research, Ministry of Education Culture, Sports, Science and Technology, Japan, No.19206071.[1] S. Miura, Y. Murasato, Y. Sekito, Y. Tsutsumi, K. Ohkubo, Y. Kimura, Y. Mishima and T. Mohri, Materials Science and Engineering A,Vol.37(2009) 317-321.[2] S. Miura, T. Tanahashi, Y. Mishima and T. Mohri, Materials Science Forum Vols. 654-656(2010)444-447
9:00 PM - N5.21
Mechanical Properties of Cr5Si3 with the D8m Structure.
Yuji Ochiai 1 , Kyosuke Kishida 1 , Katsushi Tanaka 1 , Haruyuki Inui 1
1 Materials Sciense and Engineering, Kyoto University, Kyoto Japan
Show AbstractTransition-metal silicides with a general formula of TM5Si3 (TM = transition-metal) that exhibit higher melting temperatures have become the subject of extensive study. However, because of very complex crystal structures, TM5Si3 are expected to exhibit poor fracture toughness and ductility at room temperatures in the monolithic form. In fact, most studies on TM5Si3 have focused on utilizing them as a strengthening phase in composite forms. Hence, almost nothing is known about the deformation mechanisms of TM5Si3 in the monolithic form. Among various types of TM5Si3 with different crystal structure, most of TM5Si3 including Mo5Si3, HT-Nb5Si3 have a complex tetragonal structure of the W5Si3-type (the D8m structure) with the space group of I4/mcm and therefore, it is quite important to systematically understand the mechanical properties of TM5Si3 with the D8m structure. In the present study, we prepared single crystals of Cr5Si3 (melting point: 1680 C) and investigated its mechanical properties such as elastic constants, thermal expansion properties and deformation behavior. From the values of Cauchy pressures as well as the ratio of the polycrystalline bulk modulus (B) to shear moduls (G) estimated from single-crystal elastic constants (cij), the deformation behavior of Cr5Si3 is expected to be relatively brittle compared to Mo5Si3 (melting point: 2160 C) with the same crystal structure. However, it is found that Cr5Si3 can be plastically deformed with the operation of two types of deformation modes, namely {100}[001] slip and <111> slip above 900C. Details of these newly identified deformation modes in Cr5Si3 as well as elastic properties and thermal expansion will be presented.
9:00 PM - N5.22
Alloying Effect of Ta on Microstructure and Mechanical Properties of Ni3(Si,Ti) Intermetallic Alloy.
Daiki Imajo 1 , Yasuyuki Kaneno 1 , Takayuki Takasugi 1
1 Materials Science, Osaka Prefecture University, Sakai City Japan
Show AbstractThe effect of Ta addition on the microstructure and mechanical properties of L12-type Ni3(Si,Ti) intermetallic alloy was investigated as a function of Ta content. The alloys containing up to 5 at.% Ta showed an L12 single-phase microstructure, while the alloys containing beyond 5 at.% Ta exhibited a two-phase microstructure containing Ni3Ta dispersions in the L12 matrix. At room temperature, hardness of the alloys with the L12 single-phase microstructure increased with increasing Ta content due to solid solution hardening of Ta. At high temperature, the positive temperature dependence of hardness was observed in all the alloys, irrespective of Ta addition. High temperature hardness was also enhanced by the addition of Ta. Room-temperature yield stress and tensile strength of the alloys with the L12 single-phase microstructure increased with increasing Ta content keeping a high level of tensile elongation. Further, it was found that wear resistance of the Ni3(Si,Ti) alloys under dry sliding wear condition at room temperature was markedly enhanced by the addition of Ta. These results indicate that the Ta-added Ni3(Si,Ti) alloys are expected to be a new type of high wear resistant materials.
9:00 PM - N5.23
Microstructure Dependence of High Temperature Oxidation of Mo-Ni-Al Alloys.
Kevin Severs 1 , P. Ray 1 , T. Brammer 1 , M. Akinc 1 , M. Kramer 1
1 Ames Laboratory and Department of Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractDiscovery of high temperature materials that go well beyond current state-of-the-art Ni based alloys for use in advanced coal-fired plants has proven to be an extreme materials challenge. Refractory metal silicides have good high temperature oxidation yet they are not suitable for usage in moisture containing environment, and this severely limits their applicability in coal-fired combustion environment. We attempt to design an alloy that will combine the ultra-high temperature phase stability of the silicides with adequate ductility and oxidation resistance (in dry as well as wet environments) of Ni-Al. We used an extension of the semi-empirical Miedema model to rapidly scan the ternary and higher order phase space for prospective systems. This preliminary sieving process yielded Mo-Ni-Al as a promising alloy system. We present experimental work on processing and evolution of microstructure of a two phase Mo-Ni-Al alloys, having a Mo based solid solution and the B2 NiAl phase. We will also discuss our results on the interrupted isothermal oxidation behavior of these alloys as a function of temperature, and correlate the microstructure with the oxidation behavior.
9:00 PM - N5.24
The Effect of Alloying Elements on the Formation and the Stability of γ/γ' Microstructure in Co-Al-W Base Alloys.
Yuki Tsukamoto 1 , Satoru Kobayashi 2 , Takayuki Takasugi 1 2
1 , Osaka Prefecture University, Sakai Japan, 2 , Osaka Center for Industrial Materials Research, Institute for Materials Research, Tohoku University, Sakai Japan
Show AbstractThe γ/γ' coherent microstructure was reported to form at high temperatures above 900 °C in Co-Al-W ternary alloys. Our previous studies, however, have suggested that the γ’ phase is thermodynamically metastable at 900 °C. In this study the effect of alloying elements on the formation and the stability of γ/γ' microstructure was investigated in Co-Al-W base alloys. Co-Al-W base alloys with Hf or Ta, and without the fourth alloying element were arc melted and heat treated at 900 °C for the periods up to 2000h. Differential scanning calorimetry was performed to determine the temperature of γ’ phase precipitation. In Co-9.4Al-9.6W (at.%) ternary alloys within the metastable γ/γ' two-phase field, a fine γ/γ' microstructure remained even after a heat treatment at 900 °C for 2000h. In the alloys with slightly different Al and W contents from the Co-9.4Al-9.6W alloy by ±1.1% and ±0.6%, respectively, the γ’ phase decomposed and the three phases of γ/CoAl/Co3W were coarsened and in contact with each other after the heat treatment. The formation and the stability of the γ/γ' microstructure in Co-Al-W alloys containing Hf or Ta will be presented.
9:00 PM - N5.25
Effect of Refractory Element Addition on Microstructure and Mechanical Property of Dual Two-phase Intermetallic Alloys Based on Ni3Al-Ni3V Pseudo-binary Alloy System.
Taku Moronaga 1 , Seiya Ishii 1 , Yasuyuki Kaneno 1 , Takayuki Takasugi 1
1 Materials Science, Oasaka Prefecture University, Osaka Japan
Show AbstractDual two-phase intermetallic alloys composed of geometrically closed packed (GCP) Ni3Al (L12) and Ni3V (D022) have very superior mechanical properties at high temperature and therefore practical realization as high temperature structural material is desired. The addition of the refractory metals is expected to raise the phase stability at high temperature. In this study, it was investigated how Ta and Re addition to these alloys affects microstructure and mechanical property. Transmission electron microscope was used for the observation of microstructure, and Vickers hardness test was conducted for mechanical property. It was observed that the Re added alloy after solution heat treatment was composed of Ni solid solution (i.e., channel region) and Ni3Al (L12), and showed the lowest Vickers hardness in the experimental condition investigated in this study. The Ni solid solution phase of the Re added alloy after aging at 1223 K for 2 h was decomposed to Ni3Al (L12) and Ni3V (D022) resulting in increase of Vickers hardness. Therefore it was revealed that the microstructure of dual two-phase intermetallic alloys, especially in the channel region is important for the improvement of the mechanical properties. Furthermore, the alloy simultaneously added by Ta and Re after aging at 1223 K for 2 h was composed of a very fine microstructure of the channel region, and showed extremely high Vickers hardness value.
9:00 PM - N5.26
Effect of Microstructure on Cold Workability of Ni3Si base Multi-phase Intermetallic Alloys.
Yasuyuki Kaneno 1 , Yasuyuki Matsuoka 1 , Takayuki Takasugi 1
1 Department of Materials Science, Osaka Prefecture University, Sakai Japan
Show AbstractThe effect of microstructure on cold-rolling workability of Ni3Si (L12)−Ni3Ti (D024)−Ni3Nb (D0a) multi-phase intermetallic alloys was first investigated and next, tensile properties of the cold-rolled alloys were evaluated. Cast alloys with different microstructures containing D024 phase and/or D0a phase dispersions in the L12 matrix were homogenized and then cold rolled. For the alloys with microstructure consisting of coarse plate-like D024 dispersions in the L12 matrix, serious cracks initiated at the coarse D024 dispersions at the early stage of the cold rolling process and then propagated, resulting in failure of the rolled plate. On the contrary, for the alloys with microstructure consisting of fine needle-like D024 precipitates and/or granular-shaped D0a dispersions, these second phase dispersions did not spoil the cold workability, leading to successful cold rolling of 90% reduction. After 90% cold rolling, the rolled sheets were fully recrystallized at 1173K for 1h, resulting in formation of fine grained microstructure. Room temperature yield stress of the recrystallized sheet was remarkably enhanced compared with that of the unrolled alloys, possibly due to fine-grained microstructure. Also, high temperature tensile strength and elongation were improved in the present recrystallized sheets. Consequently, it was found that the cold rolling and annealing process was beneficial to improvement of the tensile properties for the present multi-phase intermetallic alloys.
9:00 PM - N5.27
Alloying Behavior of Ni3X-type GCP Compounds with D0a Structure.
Hotaruko Sugimura 1 , Yasuyuki Kaneno 1 , Takayuki Takasugi 1
1 , OSAKA PREFECTURE UNIVERSITY, Sakai Japan
Show AbstractThe site preference of ternary additions in Ni3X-type GCP compounds with D0a structure was determined from the direction of solubility lobe of the GCP phase on the experimentally reported ternary phase diagrams. In Ni3Nb, Co and Cu preferred the substitution for Ni-site, Ti, V and W the substitution for Nb-site, and Fe the substitution for both sites. In Ni3Ta, Co, Cu, Fe, Ir, Mn and Re preferred the substitution for Ni-site, Cr, Mo and Nb the substitution for Ta-site, and Al the substitution for both sites. In Ni3Mo, Pd and W preferred the substitution for Ni-site, and Al, Nb, Ta, Ti and Zr the substitution for Mo-site. The thermodynamic model, which was based on the change in total bonding energy of the host compound by a small addition of ternary solute, was applied to predict the site preference of ternary additions. The bond energy of each nearest neighbor pair used in the thermodynamic calculation was derived from the heat of compound formation by Miedema’s formula. Good agreement was obtained between the thermodynamic model and the result of the literature search. For Ni3Mo, with a small negative heat of formation, a weak binding force between the constituent elements is often enhanced by the addition of the ternary elements that substitute for Mo-site.
9:00 PM - N5.28
The Effect of Alloying Elements M (M: Ti, Nb, Cr, Co) on the Eutectoid Reaction Temperature (A1 → L12 + D022) in Ni-Al-V Based Alloys.
Eiki Hayashi 1 , Satoru Kobayashi 2 , Yasuyuki Kaneno 1 , Takayuki Takasugi 1 2
1 , Osaka Prefecture University, Sakai Japan, 2 , Osaka Center for Industrial Materials Research, Institute for Materials Research, Tohoku University, Sakai Japan
Show AbstractThe effect of alloying elements M (M=Ti, Nb, Cr, Co) on phase equilibria among Ni(A1), Ni3Al(L12) and Ni3V(D022) phases and on the eutectoid reaction temperature (A1 → L12 + D022) was investigated in Ni-Al-V based alloys. The addition of Ti or Nb into Ni-Al-V ternary alloys shifted the three-phase coexisting region of A1 + L12 + D022 to the Ni-rich side. That of Cr or Co, on the other hand, shifted the three-phase region to the Ni-poor side. Ti and Nb partitioned into the L12 phase and D022 phase rather than the A1 phase, respectively, whereas Cr or Co partitioned into the A1 phase rather than L12 phase and D022 phase. These results suggest that the addition of Ti and Nb increases the eutectoid reaction temperature, while that of Cr and Co decreases the temperature. The vertical sections of phase diagrams between Ni3 (Al, M) and Ni3 (V, M) compositions will be experimentally determined by means of differential scanning calorimetry.
9:00 PM - N5.29
Further Strengthening and Ductilization of Ni-base Dual Two-phase Intermetallic Alloys by Carbon Addition.
Yuuta Kitaura 1 , Yasuyuki Kaneno 1 , Takayuki Takasugi 1
1 Department of Materials Science, Osaka Prefecture University, Sakai Japan
Show AbstractDual two-phase intermetallic alloys composed of geometrically close packed (GCP) Ni3Al (L12) and Ni3V (D022) phases was studied, focusing on the effect of carbon addition on high-temperature tensile properties. Carbon was charged by three kinds of raw materials, TiC, NbC and free C. In low temperature range the strength and tensile elongation showed similar curves against C content, that is, the maximum strengthening and maximum ductilization at the same time took place at a low level of C content. Further addition of carbon decreased not only strength but also tensile elongation. In high temperature range, the strength was little affected by C addition while the tensile elongation made a maximum followed by gradual decrease at high C content, therefore similar to those observed in low temperature range. Also, the C addition resulted in change of the fracture mode from brittle transgranular fracture to ductile transgranular fracture in low temperature range, and from brittle intergranular fracture to ductile transgranular in high temperature range. Possible mechanisms responsible for the further strengthening and ductilization by C addition were discussed, based on the behavior of C solutes in the matrix or at grain boundaries.
9:00 PM - N5.3
Grain Size Distribution Effects on Phase Transformation Behavior of NiTi Thin Films.
Xu Huang 1 , David Wu 2 , Ainissa Ramirez 1
1 Mechanical Engineering, Yale University, New Haven, Connecticut, United States, 2 Institute of High Performance Computing, A*STAR Agency for Science, Technology and Research, Singapore Singapore
Show AbstractThis presentation demonstrates the role of grain size and its distribution on phase transformations in sputter-deposited NiTi thin films. Two-step heat treatments—where growth and nucleation are both active in the first step, and nucleation is suspended in the second—narrow the grain size distribution, as determined by transmission electron microscopy. The associated transformation temperatures and actuation properties departed significantly from the Hall-Petch-like relationship of conventionally (one-step) annealed films. A change in texture was also apparent with two-step heat treatments. This presentation demonstrates a method for tailoring microstructures and illuminates the role of grain size distributions on properties.
9:00 PM - N5.30
First-principles Calculations and Thermodynamic Modeling of the Al-Pt and Al-Ni-Pt Systems.
DongEung Kim 1 , Venkateswara Manga 1 , Zi-Kui Liu 1
1 , Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractThe Al-Ni-Pt ternary alloys are being studied as a new bond coat in the thermal barrier coating system for jet engine and gas turbine. Thermodynamic modeling of the Al-Ni-Pt ternary system together with a remodeling of the Al-Pt binary system is carried out in the present work by combining first-principles calculations with the CALPHAD method. The four-sublattice and two-sublattice compound energy formalisms are used to model the ordered L12 and B2 phases, respectively. The enthalpy of formation for the stoichiometric compounds and end-members of ordered L12 and B2 phases are calculated from first-principles study and compared with the available experimental data. In order to obtain the mixing enthalpies of solid solution phases, the first-principles calculations of special quasirandom structures (SQS) are performed employing different relaxation schemes; full relaxation, volume plus shape relaxation and volume-only relaxation. The radial distribution function is used to check the symmetry of the relaxed SQS. The calculated model parameters, phase equilibria and thermodynamic properties are presented and discussed.
9:00 PM - N5.31
The Combined Effects of Characteristic Dissolution of Vanadium and Atomic Order on Antiphase Boundary Energies in Equiatomic FeCo Alloys.
Muratahan Aykol 1 , Amdulla Mekhrabov 1 , M. Vedat Akdeniz 1
1 Metallurgical and Materials Engineering, Middle East Technical University, Ankara Turkey
Show AbstractVanadium has been utilized as a primary alloying addition for improving the ductility of brittle B2-FeCo intermetallics for a long time. Although such a ductilizing influence can be immediately attributed to the disordering effect of V and subsequent transition from planar to wavy glide, the exact atomistic mechanism behind is still unclear and may be controlled by more complex interrelations between altered long range order (LRO), dissolution characteristics of V in the lattice, how these alter the nature of antiphase boundaries (APBs) and evolution of all of these with temperature. In this study, we investigated the possible relation between ductility, order and dissolution mode of V in (FeCo)100-xVx (x = 0-5) alloys via lattice Monte Carlo simulations combined with pseudopotential theory. It is found that there are three main factors affecting the magnitude and temperature dependence of APB energies; namely, temperature induced drop in LRO, V induced drop in LRO and V clustering with increasing temperature. We observed that V atoms prominently line up on {110} planes and in <111> directions, which coincides with the most active slip system in FeCo alloys. These V clusters grow as temperature or V content increases without deteriorating their preferential orientation. As a consequence of these factors, an optimum V content around x = 2 provides the largest drop in <111>{110} APB energies up to order-disorder transition. However, due to characteristic dissolution of V into <111>{110} system, all other slip systems require destruction of V clusters and drastic increases are observed as in {321} and {112} APB energies, which may recombine corresponding super-lattice screw dislocations and let them cross-slip more easily. These findings help explain the ductility improvement in V alloyed FeCo intermetallics.
9:00 PM - N5.32
Single Crystal Growth and Physical Property of Fe-Te-Se Materials.
Genda Gu 1
1 Condensed Matter Physics & Materials Science, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractJinsheng Wen, Zhijun Xu, M. Enoki Z. W. Lin, Qiang Li, J. M. Tranquada. BNL, USAA number of Fe-base new superconducting materials with critical superconducting temperature up to 56K have been discovered in 2008. Fe-Te-Se is “11” system superconductor which has the simplest crystal structure. We have grown a number of the FeTe1-xSex single crystals (x = 0~0.7) by using a Bridgman growth technique. The effects of the growth condition and the composition of a feed rod on the single crystal growth of Fe1+yTe1-xSex have been studied. The single crystals of the PbO-type tetragonal structure Fe1+yTe1-xSex with high extra Fe (ie y>0) are not superconducting. When Se substitutes for Te in FeTe1-xSex single crystals, the superconducting transition temperature increases with increasing Se content. The maximum Tc of FeTe0.5Se0.5 is 15K. The various physical properties are also studied by using transport measurement, Neutron, ARPES, magnetic, STM and optical methods. The work is supported by DOE under contract No. DE-AC0298CH10886
9:00 PM - N5.33
Binary (Sb2Te3)1-x-(Bi2Te3)x Thermoelectric Materials Fabricated by Mechanical Alloying and Spark Plasma Sintering.
Chia-Hung Kuo 1 2 , Yu-Jeng Lin 1 , Ya-Wen Chou 1 , Ming-Shan Jeng 1 , Chii-Shyang Hwang 2 , Masahiro Yoshimura 2
1 Energy & Environment Laboratories, Industrial Technology Research Institute, Tainan County Taiwan, 2 Department of Materials Science and Engineering, National Cheng Kung University, Tainan Taiwan
Show AbstractP-type BiSbTe bulk materials were prepared by spark plasma sintering of nanocrystalline powders attrition-milled from binary compounds of Sb2Te3 and Bi2Te3. The variation of crystal phase and related thermoelectric transport properties have been investigated by XRD, power factor and Hall measurements. Thermoelectric transport properties of BiSbTe sintered bulk samples were measured at temperatures from 300 K to 500 K. Experimental results reveal that single phase BiSbTe alloy can be obtained by prolonging milling time and increasing sintering temperature. This work shows the thermoelectric properties of BiSbTe bulk samples is according to different milling times and sintering temperature.
9:00 PM - N5.34
Microstructure Evolution in Anisotropic SmCo5 Flakes from the As-milled to the Hot Pressed State.
Wanfeng Li 1 , Liyun Zheng 1 , Baozhi Cui 1 , George Hadjipanayis 1
1 Department of Physics and Astronomy, University of Delaware, Newark, Delaware, United States
Show AbstractRecently, anisotropic SmCo5 flakes have been successfully fabricated by a surfactant assisted ball milling process. Strong out of plane [001] texture was revealed in these flakes with values of coercivity up to 18 kOe. The coercivity was found to decreasesubstantially after hot pressing. The hot pressed samples prepared from flakes ball milled for 5 hours with different surfactants, Oleic acid (OA), Oleylamine (OY), tri-octylamine (TOA), show a different magnetic behaviour. The coercivities of samples hot pressed at 650°C from ball milled flakes with OA, OY, and TOA were 6 kOe, 8.3 kOe, 9.3 kOe, respectively. In this work, we studied in detail the microstructure evolution of the flakes before and after the hot pressing process. In the as-milled flakes, the contrast difference among nanograins was very weak, which should be caused by the tiny difference in orientation among the adjacent grains. The grain sizes of the as-milled flakes are less than 20 nm. The selected area diffraction pattern indicates a [001] texture in the flakes. In all of the hot pressed samples, the contrast difference among different grains was very obvious, and the well defined grain boundaries were revealed by high resolution TEM image. Energy dispersive spectrum (EDS) analysis showed that, in the hot pressed samples, at the interface between the flakes, the composition was different from that of the central part of the flakes. More important is that the interface chemistry varies with different surfactant, and this might be the origin of the coercivity difference among the samples. The mechanism of the microstructure and chemistry evolution, and the microstructure-magnetic performance relation would be discussed in detail in this work based on the experimental resultsWork supported by DOE ARPA-E (Advanced Research Project Agency – Energy)
9:00 PM - N5.35
Hard Magnetic Materials Based on Nd9(Fe,B)87Zr2Nb2 Nanograined Intermetallic Compounds.
Shoichi Kumon 1 , Kazue Nishimoto 1 , Tomokazu Fukuzaki 2 , Ryuji Tamura 1 2
1 Department of Materials Science and Technology, Tokyo University of Science, Noda Japan, 2 Polyscale Technology Research Center, Tokyo University of Science, Noda Japan
Show AbstractLow rare-earth Nd-Fe-B nanocomposites have been excellent candidates for applications as bond magnets because of a number of advantages such as their low rare earth content, improved corrosion resistance, high magnetic polarization, etc. Among them the high magnetic polarization is due to the presence of soft magnetic phases such as α-Fe and Fe-B which are magnetically coupled with the Nd2Fe14B hard phase. Recently, we have found that simultaneous addition of the Zr and Nb to the nanocomposite of 9 at.%Nd significantly increases the coercivity up to ~1200 kA m-1. Following the result, we have then controlled the Fe concentrations of Nd-Fe-B nanocomposites in order to customize the magnetic polarization without a loss of the high coercivity. In this work, nanocomposites with various Fe concentrations were prepared with various quenching rates and annealing conditions, and the influence of the Fe concentration on the microstructure and the magnetic properties has been investigated. The optimization has shown that extraordinary high maximum energy product (BH)max of 150 kJ m-3 with an excellent coercivity of 640 kA m-1 occurs at 79 at.%Fe. Detailed relationship between the microstructure and magnetic properties of the nanocomposites will be discussed at the symposium.
9:00 PM - N5.36
Band Engineering in Metal Silicide Alloys.
Alexander Slepko 1 , Alexander Demkov 1
1 Physics, The University of Texas at Austin, Austin, Texas, United States
Show AbstractIntermetallic silicide alloy contacts to the source, drain and gate of a field effect transistor play a key role in semiconductor technology. These alloys are formed by a heat treatment of a metal-semiconductor contact. Silicide-silicon junctions behave similarly to metal-semiconductor contacts, providing low line and contact resistances. Contacts formed in this manner generally show stable electrical characteristics and exhibit excellent mechanical adhesion. Most important, however, is that the use of metal silicides allows the formation of self-aligning contacts whereas in metallic conductors their precise location usually depends on the fabrication process.Using density functional theory we study platinum monosilicides which have recently attracted significant interest. PtSi has a low p-type Schottky barrier height (SBH) (0.2eV) to Si (001) and exhibits excellent thermal stability. However, it suffers of relatively low conductivity compared for example to pure Pt which can be traced to the low density of states (DOS) at the Fermi level. We investigate the possibility of manipulating the DOS at the Fermi level by alloying PtSi with Ti. Ti is almost equal in size with Pt but contributes only 4 valence electrons (versus ten contributed by Pt). For different Ti concentrations we find a general increase in the number of conducting electrons compared to bulk PtSi. We estimate the formation energies of the compounds and determine the solubility limit of Ti in bulk PtSi at room temperature. We also analyze the changes in the work function and SBH to Si (001). Furthermore, we analyze the influence of mixing in boron, carbon, gallium and aluminum on the solubility limit of Ti in PtSi. The change in conductivity of the new compounds due to ion impurity scattering is estimated using Boltzmann transport theory with the collision integral calculated from first principles.
9:00 PM - N5.37
Reflection High Energy Electron Diffraction and Atomic Force Microscopy Studies of MnxScy Alloys Grown on MgO(001) Substrates by Molecular Beam Epitaxy.
Costel Constantin 1 , Abhijit Chinchore 2 , Arthur Smith 2
1 Physics and Astronomy, James Madison University, Harrisonburg, Virginia, United States, 2 Physics and Astronomy, Ohio University, Athens, Ohio, United States
Show AbstractWhile there is a lot of interest in alloying manganese with other metals such as iron, vanadium, and aluminum for its virtue as sulfur-fixing and increase corrosion resistance in steel production; there are not too many studies regarding manganese and scandium alloys. In this study, MnxScy films (with x = 0 – 50%) were grown on MgO(001) substrates by radio frequency plasma assisted molecular beam epitaxy. All the films were grown with a thickness of ~ 50 nm each, and at a substrate temperature of Ts ~ 520 oC. Reflection high energy electron diffraction show streaky pattern for samples with x = 3, 5, 10, 25, and 35% indicating a 2-dimentional smooth growth. Sample with x = 15% show a rather diffuse/streaky pattern indicating a combination of 2 and 3-dimentional growth on the surface. Sample with x = 50% show a spotty and ring-like pattern indicating a 3-dimentional/polycrystalline growth. Interestingly, the atomic force microscopy measurements show surface nanowires for samples with x = 10, and 50%. The MnxScy alloys could be useful for high temperature applications. Work supported by NSF.
9:00 PM - N5.38
Structure Analysis of a Long Period Stacking Ordered Phase in Mg-Al-Gd Alloys.
Hideyuki Yokobayashi 1 , Kyosuke Kishida 1 , Haruyuki Inui 1 , Michiaki Yamasaki 2 , Yoshihito Kawamura 2
1 Department of Materials Science and Engineering, Kyoto University, Kyoto Japan, 2 Department of Materials Science and Engineering, Kumamoto University, Kumamoto Japan
Show AbstractTernary Mg - TM (transition metal) - RE (rare earth) intermetallic phases with long-period stacking ordered (LPSO) structures have received a considerable amount of attentions as new types of strengthening phases in high-strength Mg-alloys. Although some beneficial effects of the LPSO phases on mechanical properties of Mg-alloys have been reported so far, inherent characteristics of the LPSO phases including crystal structure, formation process and deformation mechanism have not been fully clarified yet. Many different types of Mg-TM-RE LPSO phases with different stacking sequence including 18R- and 14H-types have been reported. Extensive studies on various Mg-TM-RE LPSO phases have revealed that the LPSO structures are characterized by a periodic arrangement of stacking fault layers on the close-packed planes in parent HCP structure of Mg. One of the most important characteristics of the LPSO structure is the fact that the stacking fault layers are preferentially enriched with TM and RE. However, the in-plane arrangement of TM and RE elements in enriched stacking fault layers is still controversial. In order to fully understand the crystal structure of the Mg-TM-RE LPSO phases, it is essential to determine the in-plane atomic arrangement in the stacking fault layers of the LPSO phases. In the present study, crystal structure of LPSO phases newly found in Mg-Al-Gd ternary alloys were investigated by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The LPSO phase in Mg-Al-Gd ternary alloy was confirmed to possess 18R-type stacking sequence by diffraction analysis in TEM and HRTEM. Atomic scale high-angle annular dark-field (HAADF) STEM imaging revealed that Gd atoms are enriched in stacking fault layers as in the cases of the previously reported Mg-TM-RE LPSO phases. In addition, periodical arrangement of Gd-enriched columns along [0-110]Mg direction was clearly observed, which indicates that the Gd atoms are distributed in an ordered arrangement in the stacking fault layers. Based on the results obtained, possible atomic arrangement in the stacking fault layers in the Mg-Al-Gd LPSO phase will be proposed.
9:00 PM - N5.39
Precipitation Sequence and Phase Evolution in Mg-Nd Alloy Containing Zn and Zr.
Galit Atiya 1 , Menachem Bamberger 1 , Alexander Katsman 1
1 Materials Engineering, Technion - Israel Institute of Technology, Haifa Israel
Show AbstractMagnesium alloys containing heavy rare earth metals are very attractive candidates for the automotive industry including racing cars and aerospace applications, due to their high strength properties combined with low density. The microstructure of the Mg-3.1%Nd-0.45%Zr-0.27%Zn (%wt) alloy has been investigated after solution treatment at 540°C for 24hr followed by isothermal aging at 175°C up to 32 days. Various electron microscopy techniques, like EDX mapping and TEM with SAED, were used to characterize the phase composition and orientation relationships between different phases. After solution treatment, the bct (Mg,Zn)12Nd phase, which was presented in the as-cast state, dissolved and small tetragonal Zn2Zr3 rod-like particles precipitated at α-Mg grain interiors. Zn2Zr3 particles are elongated along [001] direction, have an orientation relationship with the Mg matrix of [-2110]Mg‖[001]Zn2Zr3 and appear to be stable in the Mg matrix. Precipitation during isothermal aging involves the formation of metastable phases β''(Mg3Nd)hcp (DO19 structure) and β'(Mg3Nd)fcc (DO3 structure). The β'' precipitates formed during the first 8 days of aging have a platelet shape and are fully coherent with the Mg matrix, with the orientation relationship [-2110]Mg‖[-2110]β'' and [-1100]Mg‖[-1100]β''. The heterogeneous nucleation of the β'' occurs on the basal planes and on the side planes of the Zn2Zr3 rods having an orientation relationship of [-2110]β'' ‖[001]Zn2Zr3 . During 16-32 days of aging, the β'' precipitates transform to β' precipitates with an FCC structure. The β' precipitates are semi-coherent with the Mg matrix and have the following orientation relationship: [0001]Mg‖[101]β' and [-1100]Mg‖[-112]β'. After prolonged aging of 32 days, the β' precipitates transform to the stable incoherent β(Mg,Zn)12Nd phase. The growth, coarsening and phase transformations were followed by microhardness tests.
9:00 PM - N5.4
The Effect of Grain-boundary and Matrix Precipitates on High Temperature Strength in Fe3Al Based Alloys.
Ryo Makihara 1 , Satoru Kobayashi 2 , Takayuki Takasugi 1 2
1 , Osaka prefecture university, Osaka Japan, 2 , Osaca Center for Industrial Materials Research,Institute for Materials Research, Tohoku University, Osaka Japan
Show AbstractThe precipitation of carbide phases and high temperature strength were investigated in Fe3Al base alloys containing Cr, Mo and C. The κ-Fe3AlC, M2C and M6C phases precipitated in the temperature range between 600°C and 1000°C in the period range between 1min and 10 h. The κ phase tended to form in a film shape on grain boundaries by annealing at relatively high temperatures. The M2C particles precipitated with high density within the matrix grains by annealing at temperatures below 700°C. The film-like κ precipitates on grain boundaries and the finely dispersed M2C particles were found to contribute to strengthening at 600°C. Mechanisms of the grain boundary strengthening will be discussed in terms of dislocation pile-up at grain boundaries and the rule of mixture of the matrix and the grain boundary phases.
9:00 PM - N5.40
Microfracture Test of Mg12ZnY Intermetallic Compound in Mg-Zn-Y Alloys.
Hajime Yoshimura 1 , Shun Matsuyama 1 , Mitsuhiro Matsuda 1 , Masaaki Otsu 1 , Kazuki Takashima 1 , Yoshihito Kawamura 1
1 Materials Science and Engineering, Kumamoto University, Kumamoto Japan
Show AbstractMg-Zn-Y alloy including Mg12ZnY intermetallic compound has excellent mechanical properties such as strength and ductility compared with the conventional magnesium alloys. The superior mechanical properties of this alloy seem to originate in the Mg12ZnY intermetallic compound. Hagihara et al. reported Mg12ZnY intermetallic compound exhibits plastic deformation with kinking [1]. However, the fracture behavior and the fracture properties of Mg12ZnY have not been investigated yet. The size of this compound is too small to investigate the fracture properties of Mg12ZnY only by conventional fracture testing. In this study, microfracture testing method is applied to investigation of the fracture properties and the fracture behavior of Mg12ZnY which has special deformation behavior. Mg88Zn5Y7 alloy was used for the specimens. The material was extruded at 723 K with a reduction ratio of 10. Micro-sized cantilever specimens with dimensions of 10×20×50 μm3 were prepared by focused ion beam (FIB) machining. Notches with a width of 0.5 μm and a depth of 4.5-5.5 μm were also introduced into the micro-sized specimens by FIB machining. Microfracture tests were performed by the microfracture testing equipment developed by the authors. The fracture toughness values (KIC) could not be obtained as the specimen size was too small to satisfy the plane strain condition. Hence, provisional KQ values were calculated and the values of Mg12ZnY were 1.2-3.0 MPam1/2. Fracture area of the specimen having the highest KQ value was observed by TEM. The basal plane of Mg12ZnY was oriented perpendicular to crack growth direction and this result showed the orientation of the basal plane in Mg12ZnY would be effective for improving fracture toughness in Mg-Zn-Y alloy.
[1]: K. Hagihara, N. Yokotani, Y. Umakoshi : Intermetallics, 18(2010), pp.267-276.
9:00 PM - N5.41
Experimental Studies of Bonding Related Properties in Binary Intermetallics by Convergent Beam Electron Diffraction.
Xiahan Sang 1 , Jorg Wiezorek 1 , Andreas Kulovits 1
1 Mechanical Engineering and Materials Science, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractThe intrinsic properties of ordered intermetallic are related to their respective electronic structure, which differs from that of the chemically disordered solid solutions of equivalent composition. Successful support of application driven research related to intermetallics-based alloys demands detailed knowledge of fundamental aspects of their properties. Materials computation using density-functional theory is attractive approach to develop understanding of intrinsic properties. Predictions from computations require validation by suitable experimental data, which often is difficult to obtain. We use quantitative convergent beam electron diffraction (CBED) to provide experimental data, e.g. Debye-Waller and structure factors, electron and charge density distribution, suitable for validation of computational findings for binary interametallics. CBED is very attractive as it uses nano-scale crystal volumes and can be applied for a broad range of phases. Here we apply a new robust experimental CBED method for the simultaneous determination of multiple structure factors and Debye-Waller (temperature) factors of the binary intermetallic phases beta-NiAl (cubic) and gamma-1-FePd (tetragonal). Debye-Waller factors are known for most stable elemental crystals. However, the interatomic bonding alters the Debye-Waller factors of the constituent atom species in binary intermetallics from those for the same atoms in the elemental crystals. We measured the Debye-Waller factors and structure factors of the intermetallics for temperatures ranging from 100K-300K. Experimental measurements of the electron density and probing of bonding in the intermetallics requires precise structure factor measurement facilitated by the present CBED method. We acknowledge use of the facilities of the Materials Micro-Characterization Laboratory of the Department of Mechanical Engineering and Materials Science, University of Pittsburgh, and support by a grant from the Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S. Department of Energy under contract DE-FG02-08ER46545.
9:00 PM - N5.42
Indium Alloy as Cadmium Brush Plating Replacement.
Natasha Voevodin 1 3 , Melissa Klingenberg 2 , Paul Brezovec 2 , Elizabeth Berman 3
1 , UDRI, Dayton, Ohio, United States, 3 , AFRL, Dayton, Ohio, United States, 2 , CTC, Johnstown, Pennsylvania, United States
Show AbstractIt is well-known that cadmium and its corrosion product (cadmium oxide) are carcinogens and toxic. Consequently, efforts to eliminate cadmium from original equipment and repair processes have been on-going. One potential replacement is indium alloy. Indium is a soft post-transition metal, whose primary use is in the form of indium tin oxide, e.g. in liquid crystal displays (LCDs). The semi-conductive properties of indium oxide alloys makes it possible to use these for cadmium brush plating replacement in applications where contact resistance and impedance are critical parameters. Critical requirements of an alternative to cadmium brush plating in a corrosive industrial atmosphere are (1) be sacrificial to mild steel and (2) conduct electricity for bonding and grounding of the hardware assembly and remain semi-conductive through the hardware assembly’s service life. Cadmium oxide remains semi-conductive, while most other pure metal oxides are electrical insulators, such as aluminum oxide, nickel oxide and zinc oxide, and therefore, fail in meeting the requirement for bonding and grounding. Similar to cadmium oxide, indium oxide is semi-conductive, but indium exhibits other properties such as cold welding. This motivates indium to be used as part of metal alloy plating. Current work considers soluble anode adjustments to the plating processing for the adoption of a cadmium replacement candidate in DoD aerospace facilities. The presentation discusses indium- and other metal-alloy electroplating approaches to overcome the insulating limitation of pure metal plating and to replace cadmium brush plating. Test results on several alloys candidates for cadmium brush plating replacement are presented and conclusions on such replacement feasibility are provided.
9:00 PM - N5.43
Fabrication and Mechanical Properties of WC-xwt.%Co Hard Materials Prepared by Spark Plasma Sintering Process for FSW Tool Application.
Hyun-Kuk Park 1 , Seung-Min Lee 1 , Dong-In Kang 1 , Ik-Hyun Oh 1
1 Automotive Components Centers, KITECH, Gwangju Korea (the Republic of)
Show AbstractNew method of welding process is Friction Stir Welding method (FSW) that gets the limelight. FSW was developed in 1991 and is successfully being applied to an increasing number of joining applications worldwide. The FSW advantages of this solid-state joining process, which uses frictional heat generated by a rotating and traversing cylindrical tool with a profiled pin along a square butt weld joint, encompass better mechanical properties, low residual stress and deformation, weight-savings, and reduced occurrence of defects, compared to the conventional welding methods. Recently, research on the FSW process has mainly focused on the joining of low-melting metallic materials, such as aluminum alloys with steel. Tungsten carbide-cobalt hard materials (WC-Co) are widely used for a variety of welding tool, machining, cutting, drilling, and other applications. Morphologically, they consist of a high volume fraction of the “hard” hexagonal WC phase embedded within a soft and tough Co binder phase. An increase in the density of WC-xwt.%Co was not accomplished by conventional sintering method. New sintering method of Spark Plasma Sintering (SPS) was utilized to consolidate WC-xwt.%Co hard materials using various weight percent of Co powders. The demonstrated advantages of these processes are rapidly densification to near theoretical density in a relatively short time and without significant change in a grain size. The microstructure of WC-xwt% Co nano-composite prepared by high energy ball milling will be investigated by X-ray diffraction, Field Emission Scanning Electron Microscopy and Grain size analysis. WC powder on the sintering behavior and mechanical properties will be investigated. And also, the mechanical properties such as vickers hardness and fracture toughness will be investigated.
9:00 PM - N5.44
Synthesis and Microstructure of Porous Aluminum and Intermetallic Nanomaterials.
Andrew Purdy 1 , Joel Miller 1
1 Chemistry Division, Code 6123, Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractA series of porous aluminum-based materials are prepared by the reduction of solutions of metal chlorides with lithium powder in diethyl ether under dry argon. The reactants must be combined slowly, but either order of addition is used. The reduction of AlCl3 produces hollow Al balls composed of ~100 nm aluminum particles in nearly quantitative yield after the LiCl byproduct is washed away with dry tetrahydrofuran. Similar structures are formed when mixtures of AlCl3 and SiCl4, or AlCl3 and B(OEt)3 are reduced, except that the second component has the effect of reducing the Al nanoparticle size. Mixtures of AlCl3 with FeCl3 reduce to similar ball-like porous structures that are composed of Al, Fe, and Fe-Al intermetallic nanoparticles. When AlCl3 and ZnCl2 are co-reduced, flake-like nanoporous structures are obtained, and solutions of AlCl3 + VCl3 produce more compact nanoporous structures. Some side reactions involving ether cleavage that produces aluminum alkoxides and alkyls do occur, and the amount of side reaction is dependent on the identity of the second metal. The reduction of AlCl3 with excess (4 eq) Li powder produces LiAl nanomaterials. These materials are characterized by powder X-ray diffraction, scanning electron microscopy, 7Li and 27Al solid state NMR spectroscopy, density measurements, and other techniques.
9:00 PM - N5.5
Microstructure and Mechanical Behavior of Fe30Ni20Mn35Al15 Eutectic Alloy.
Fanling Meng 1 , Ian Baker 1 , Yifeng Liao 2 1
1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States, 2 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractThe flow and fracture behavior of a eutectic alloy with nominal composition Fe30Ni20Mn35Al15, consisting of alternating B2 and f.c.c. phases, was studied as a function of temperature by in-situ straining in both a transmission electron microscope and a scanning electron microscope. The room temperature yield strength and elongation to fracture of the alloy were measured to be ~450 MPa and ~12%, respectively. The hard B2 phase appears to show little signs of plastic deformation at low temperature. The flow and fracture will be discussed in terms of a dislocation pile-up mechanism and a fiber-loading mechanism, respectively. This work was supported by NSF Grant DMR-0905229.
9:00 PM - N5.6
The Formation of A2/L21 Microstructure in Fe-Al-Ti-Cr Alloys.
Suguru Hotta 1 , Satoru Kobayashi 2 , Takayuki Takasugi 1 2
1 material science, Osaka prefecture university, Sakai city Japan, 2 , Osaka center for industrial materials research, institute for materials research, Tohoku university, Osaka city Japan
Show AbstractFe-Al-Ti ternary alloys with A2/L21 coherent microstructures show good high-temperature strength, but remarkable brittleness at low temperatures. For the purpose of achieving alloys with A2/L21 microstructures with reasonable ductility, phase equilibria and the formation of A2/L21 microstructure were examined in the Fe-Al-Ti-Cr system using a diffusion-multiple technique. The microstructure was observed by SEM, and the chemical compositions of the phases present were analyzed by EPMA. By annealing at 800°C and 700°C, L21 particles precipitated within the A2 matrix phase in a wide composition range of the system. The Al content of both A2 and L21 phases decreased by a little amount with increasing Cr content and decreasing temperature. The formation and coarsening rates of L21 particles were found to depend on the volume fraction of the phase. Relationships between microstructures (the volume fraction and size of L21 phase) and mechanical properties (ductility and hardness) will be presented.
9:00 PM - N5.7
Assessment of The Phase Equilibria Among γ-Fe/Fe2Nb/Ni3Nb Phases in Fe-Ni-Nb Ternary System at Elevated Temperatures.
Yusaku Hasebe 1 , Kiyoshi Hashimoto 2 , Takashi Matsuo 1 3 , Masao Takeyama 1 3
1 Dept. of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo Japan, 2 , Materials Design Technology Co., Tokyo Japan, 3 , Consortium of the Japan Research and Development Center for Metals (JRCM), Tokyo Japan
Show AbstractPhase equilibria among γ-Fe (A1), TCP Fe2Nb Laves (C14) and GCP Ni3Nb (D0a) phases in Fe-Ni-Nb ternary system at 973 K have been calculated using the interaction parameters among the three elements of each phase, which were obtained by optimization of experimentally determined and thermodynamically calculated isothermal sections at 1473 K and 1373 K. The alloys with composition of Fe-(0~84)Ni-(15~40.5 )Nb (at.%) were prepared by arc melting, and they were equilibrated at given temperatures for up to 2400 hrs. The phase composition and phase identification was done by EPMA and XRD, respectively. The assessment and optimization of the experimental phase diagram was done by Pandat software (Ver. 8.2) using Regular solution model for Α1 and liquid phases, and sublattice model for the intermetallic compounds. At 1473 K, besides the three solid phases, a new ternary GCP compound with a crystal structure of hP24 was found to exist with a very limited composition range away from the single phase region of Ni3Nb, in addition to liquid phase. At 1373 K, the hP24 phase remains stable with no composition change, whereas the liquid phase disappears due to a temperature invariant reaction of L→hp24+C14+A1. Thus, the three three-phase tie triangles of hp24+A1+D0a, hp24+D0a+C14 and hp24+C14+A1 exist around the hp24 phase. Thermodynamic calculation using the existing database did not work to reproduce the experimental isothermal sections, and the introduction to the ternary compound into database successfully reproduces them. The isothermal section at 973 K calculated using the optimized interaction parameters reveals no existence of the hP24 phase, and instead, a three phase tie triangle of A1+C14+ D0a extended toward Fe-rich side with the terminal composition of A1 of close to 80 at. %Fe appears. This calculated results will be experimentally verified, and the change in phase equilibria with temperature will be discussed. Part of this study was carried under the research activities of the Consortium, which are financially supported by NEDO.
9:00 PM - N5.9
Analysis of Vacancy Agglomeration Processes by the Matusita's Method for B2 FeAl.
Kyosuke Yoshimi 1 , Masafumi Tsunekane 1 , Kouichi Maruyama 1
1 Graduate School of Environmental Studies, Tohoku University, Sendai, Miyagi, Japan
Show AbstractIn B2 FeAl, the agglomeration and annihilation of supersaturated vacancies forms vacancy-complexes such as voids, prismatic dislocations loops, surface pores and so on. The agglomeration processes are detectable by differential scanning calorimetry, showing an exothermic peak in the DSC curve. In this study, the exothermic peaks observed in water-quenched Fe-48.5at%Al single crystals and rapidly solidified Fe-44.9at%Al ribbons were analyzed by the Matusita's method in order to consider the kinetics of the agglomeration and annihilation processes of supersaturated vacancies, which was first adopted to the crystallization and crystal growth in amorphous materials. The reaction fraction, x, of supersaturated vacancies at a temperature, T, in an isochronal DSC measurement was related to the heating rate, α, the activation energy, E, and the nucleation and morphological factors, n and m. The Fe-48.5at%Al single crystals water-quenched from 1273 and 1373 K showed the m values ≈ 2, and the rapidly solidified Fe-44.9at%Al ribbons did the m value ≈ 3. Furthermore, the single crystals and ribbons also showed the n values ≈ 2 and 3, respectively. Based on the Matusita’s idea, the obtained m values suggested that dislocation loops grew 2-dimensionally in the single crystals and voids did 3-dimensionally in the ribbons by the agglomeration of supersaturated vacancies. These suggestions completely correspond to experimental results obtained by TEM observations. Besides, the obtained n value ≈ 3 for the ribbons suggested that the nuclei of voids existed in as-solidified ribbons, also corresponding to TEM results. The n value ≈ 2 for the single crystals suggested the preexistence of the nuclei of prismatic loops in as-water-quenched single crystals. Prismatic loops were not experimentally observed there, but it is implied from the n value that the nuclei size was much smaller than that of prismatic loops, for instance as small as vacancy clusters. Therefore, it is realized through this study that DSC analyses by the Matusita’s method are very useful for understanding the vacancy agglomeration processes in B2 FeAl.
Symposium Organizers
Bernard Bewlay General Electric Company
Martin Palm Max-Planck Institut fuer Eisenforschung GmbH
Sharvan Kumar Brown University
Kyosuke Yoshimi Tohoku University
N6: Laves Phase Alloys - Processing, Microstructure, and Properties
Session Chairs
Sharvan Kumar
Martin Palm
Wednesday AM, December 01, 2010
Room 203 (Hynes)
9:30 AM - **N6.1
Laves Phases in Binary and Ternary Transition-metal-based Systems: Stability, Structure and Disorder.
Frank Stein 1
1 , Max-Planck-Institut für Eisenforschung, Düsseldorf Germany
Show AbstractWithin the Inter-Institutional Research Initiative “The Nature of Laves Phases” of the Max Planck Society, Laves phases in a variety of binary and ternary transition metal-based systems have been investigated by a combination of experimental techniques and theoretical calculations in an attempt to improve the understanding of the stability and structure of these phases. Laves phases form the largest group of intermetallic phases and can crystallize in three different, closely related structure types, which are the cubic MgCu2 type (C15), the hexagonal MgZn2 type (C14), and the hexagonal MgNi2 type (C36). In order to get a reliable data basis, selected phase diagrams have been carefully re-investigated to study the co-existence of different Laves phase polytypes and the extension of their frequently very large homogeneity ranges. The composition dependence of the crystallographic parameters and the role of chemical disorder and site occupations are discussed showing the importance of intrinsic differences between the cubic and the hexagonal Laves phase polytypes. Other important topics of the project include a discussion of metastable Laves phases, the kinetics of phase transformations between the different polytypes, and aspects of the chemical bonding in Laves phases. In this talk, an overview will be given on the results obtained in the project.
10:00 AM - N6.2
Compositional Dependence of the Mechanical Properties of Laves Phases in the Fe-Nb(-Al) and Co-Nb(-Al) Systems.
Simon Voss 1 , Frank Stein 1 , Martin Palm 1 , Dierk Raabe 1
1 , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany
Show AbstractLaves phases are the most abundant intermetallic phases. With their high strengths at elevated temperatures, the high-melting transition metal Laves phases are supposed to be interesting materials for use in high-temperature applications. However, only few investigations have been carried out on their mechanical properties, due to the fact that their distinct brittle behaviour at lower temperatures prohibits an easy sample preparation.The present work is focussed on the investigation of the compositional dependence of the mechanical properties of single-phase Laves phase alloys in the binary Nb-Fe and Nb-Co systems and their ternary variants with Al substituting Fe or Co. As it is known for many intermetallic alloys, their mechanical properties significantly depend on their composition and the related defect mechanisms. In the systems investigated in this study, it is possible to determine the mechanical properties of single-phase alloys within wide homogeneity ranges and in addition all three polytypes of Laves phases, which are the hexagonal C14, the cubic C15 and the hexagonal C36 structure, occur. Earlier investigations on Laves phases show conflicting results concerning the compositional dependence of the mechanical properties. On the one hand, defect softening with a maximum strength at stoichiometric composition, and on the other hand, defect hardening as it is known for many B2 ordered intermetallics with a minimum in strength, was reported.Bulk, single-phase material is difficult to prepare in the case of Laves phases because of their distinctive brittle behaviour. Therefore, samples were produced by a modified levitation melting technique with in-situ heat-treatment. Materials characterization was done by light-optical and scanning-electron microscopy (SEM) as well as electron probe microanalysis (EPMA) and X-ray diffraction (XRD). The current work presents results obtained from compression and indentation experiments of polycrystalline, single-phase material as well as results on creep and the dynamic Young’s modulus. For dislocation analysis TEM experiments were performed.This work has been carried out within the Inter-Institutional Research Initiative “The Nature of Laves Phases” funded by the Max Planck Society.
10:15 AM - N6.3
Novel Concept of Creep Strengthening Mechanism Using Grain-boundary Fe2Nb Laves Phase in Austenitic Heat Resistant Steel.
Imanuel Tarigan 1 , Keiichi Kurata 1 , Naoki Takata 1 2 , Takashi Matsuo 1 2 , Masao Takeyama 1 2
1 Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo Japan, 2 , Consortium of the Japan Research and Development Center for Materials (JRCM), Tokyo Japan
Show AbstractAdvanced ultra-super critical (A-USC) power plants need materials with 10
5 h creep rupture strength more than 100 MPa at 973 K. A new type of austenitic heat resistant steels of Fe-20Cr-30Ni-2Nb (at.%) strengthened by intermetallic Fe
2Nb Laves (TCP) phases has been proposed and developed, which shows superior long-tem creep rupture strength to any of the conventional austenitic steels strengthened by metallic carbides. This superior creep strength is mainly attributed to the precipitation of the Laves phase at grain boundaries, and named it as ”
grain-boundary precipitation strengthening”. In this study, thus, the relationship between the creep resistance and the grain boundaries covered with Laves phase (
ρ: area fraction) was examined quantitatively. The steels were homogenized at 1523 K to adjust the grain size of 150 μm, and then aged at 1073 K, at which only the Laves phase is precipitated, in order to control the
ρ in the range of 40% to 90%. The preaged specimens were creep tested at 973 K at 140MPa. The creep strain was monitored by extensometer through linear variable transducer. The microstructures were analyzed by TEM and EBSD. The minimum creep rate (
m) decreases with increasing the area fraction of grain boundary Laves phase, and it can be rationalized with
ρ by the following equation:
/
0 = (1-
ρ), where
0 is the creep rate at
ρ = 0. In addition, creep rupture life is also extended with increasing
ρ with no ductility loss, which can yield up to 77% elongation even at
ρ=90%. Microstructure analysis revealed well-developed subgrains near grain boundary free from the Laves phase. Orientation analysis by EBSD also revealed that the subgrained region is deformed nearly twice as high as the average strain, indicating that the grain-boundary Laves phase acts as an obstacle for dislocation motion. Based on these observations, novel model for creep deformation and microstructure design for long-term creep rupture strength will be proposed.Part of this study was carried under the research activities of the Consortium, which arefinancially supported by NEDO.
11:00 AM - N6.4
Improvement of Cr2Nb Strengthening by Thermal Spraying in Cu-8Cr-4Nb for High Flux Applications.
Thierry Grosdidier 1 , Emilien Comoret 1 , Nathalie Bozzolo 1 , Lucas Dembinsky 2 , Christophe Verdy 2 , Christian Coddet 2 , Anne Denquin 3 , Daniel Cornu 4
1 , LETAM - CNRS / University of METZ, Metz France, 2 , LERMPS / UTBM, Belfort France, 3 , ONERA, Chatillon France, 4 , SAFRAN - Snecma, Vernon France
Show AbstractVarious dispersion strengthened copper alloys are under development for high heat flux applications such as regenerative cooled rocket motors, nozzle liners, combustion chambers as well as heat exchangers or spot welding devices. Among them, the so-called GRCop-84 alloy developed by NASA GRC contains approximately 14vol% of Cr2Nb particles reinforcing a matrix of pure copper [1]. The high melting point of the Cr2Nb particles - acting to pin grain boundaries - and the absence of mutual solubility between Cr2Nb and Cu, that prevents Ostwald ripening of the reinforcements, lead to high temperature stability of the composite microstructure. Attempts were made to produce billets having a fine microstructure though extrusion of precursors obtained by atomization and chill block melt spinning [1, 2] (rapid solidification) or planetary ball milling (mechanical milling) [3-4]. However, the strength levels are always limited by some coarsening taking place during the extrusion step while the mechanically milled powder can suffer from oxygen and iron contamination which hampers the conductivity. In this presentation, we will demonstrate that the combination of atomization + thermal spraying under vacuum is a good processing route for producing near net-shape materials having an extremely fine microstructure and high strength - conductivity balance. A detailed microstructure analysis will show the effect of the processing route on the size, distribution and crystallographic features of the Cr2Nb particles. Further improvement in strength is consequently obtained due to an optimum combination of Orowan and grain boundary (Hall-Petch) strengthening.[1] M.W. Decker, J.R. Groza, J.C. Gibeling, Mat. Sci. Eng. A 369 (2004) 101-111.[2] L.G. Vettraino, J.L. Heelan, C.A. Faconti, J.L. Walley, A. Garg, J.R. Groza, J.C. Gibeling, J. Mater. Sci. 43 (2008) 6546-6555.[3] K.R. Andrson, J.R. Groza, D.G. Ulmer, Scripta Mater. 37 (1997) 1779-185.[4] G.P. Khanra, A.K. Jha, S.Girikumar, K.T. Tharian, S. Kumar, Powder Technol., 197 (2010) 177-183.
11:15 AM - N6.5
Microstructure and Oxidation Resistance of Cr-Ta-Si Alloys.
Ayan Bhowmik 1 , Steffen Neumeier 1 , Howard Stone 1
1 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, Cambridgeshire, United Kingdom
Show AbstractOver the years, considerable research has been undertaken to identify and develop new high temperature structural alloys that possess superior temperature capabilities than nickel and cobalt based superalloys. The requirement for environmental resistance at elevated temperatures has resulted in the majority of this research being focussed upon alloys based upon aluminides, beryllides, silicides and chromides. Whilst encouraging properties have often been obtained, no alloy system has yet been developed that has the balance of properties required to supercede nickel and cobalt based superalloys in high integrity structural applications. One of the classes of alloys that have been investigated are those based upon the eutectics that exist between chromium and a refractory metal Laves phase. These enable materials to be created comprising of chromium-rich matrix reinforced by a refractory metal dichromide Laves phase. These materials are also amenable to directional solidification, enabling preferential alignment of the reinforcing phase for applications in which resistance to directional loading is required. In this study we present a series of Cr-Ta-Si ternary alloys with microstructures comprising primarily of a mixture of an A2 solid solution with C14/C15 Laves phases. For this class of alloys, silicon has previously been shown to improve ductility significantly. It may be anticipated that such additions may also have a beneficial effect upon oxidation resistance. However, to date, no phase diagram exists for this system. In the present study six alloys were prepared of base composition Cr-10 at%Ta with silicon additions of 1, 3, 5, 7, 10 and 15 at% substituted for chromium. In order to homogenise the alloys, they were subjected to heat treatments of 1300°C for 500h. The microstructures of the alloys were examined using scanning electron microscopy, the phases were identified using x-ray diffraction and the phase compositions were determined by electron probe microanalysis based on wavelength dispersive spectroscopy. Based on the results the chromium-rich end of the Cr-Ta-Si ternary phase diagram was plotted. The oxidation behaviour of the alloys was also determined using thermo-gravimetric analysis through isothermal tests at 1100°C for 120 hrs. The oxidation products were identified in order to determine the underlying oxidation mechanism.
11:30 AM - N6.6
Microstructural Studies of Multiphase (Zr,Ti)( V,Cr,Mn,Ni)2 Alloys for Ni/MH Negative Electrodes.
Leonid Bendersky 1 , Ke Wang 1 , William Boettinger 1 , Kwo Young 2 , Benjamin Chao 2 , Dale Newbury 3
1 Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 , Energy Conversion Devices Inc., Rochester Hills, Michigan, United States, 3 Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractThe solidification microstructures of multi-component Laves structures-based (Zr,Ti)(V,Cr,Mn,Ni)2 alloys intended for use as negative electrodes in Ni-metal hydride batteries was studied here; these alloys often have their best electrochemical properties in the cast state. The paper also examines the complex internal structure of the interdendritic grains formed by solid-state transformation, which plays an important role in the electrochemical charge/discharge characteristics. Solidification is accomplished by dendritic growth of a hexagonal C14 Laves phase followed by peritectic solidification of a cubic C15 Laves phase and formation of a cubic B2 phase in the interdendritic regions. Experimentally, the closeness of the compositions of the C14 and C15 phases required the use of state of the art compositional mapping with an energy dispersive detector capable of processing a high x-ray flux to locate regions in the microstructure for quantitative composition measurement and TEM examination. The observed sequence of phase C14/C15 upon solidification agrees with predictions using effective compositions and thermodynamic assessments of the ternary systems, Ni-Cr-Zr and Cr-Ti-Zr. By studying one alloy it is shown that the interdendritic grains solidify as a B2 (Ti,Zr)44(Ni,TM)56 phase, and then undergo transformation to Zr7Ni10-type, Zr9Ni11-type and martensitic phases. The transformations obey orientation relationships between the high-temperature B2 phase and the low-temperature Zr-Ni-type intermetallics, and consequently lead to a multivariant structure. Binary Ni-Zr and ternary Ti-Ni-Zr phase diagrams were used to rationalize the formation of the observed domain structure.
11:45 AM - N6.7
Formation of Planar Faults in Fe2Nb Laves Phase with Ni in Solution.
Naoki Takata 1 , Takashi Matsuo 1 , Masao Takeyama 1
1 Dept. Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo Japan
Show AbstractThe mechanical properties of Fe2Nb (C14) significantly changes, depending on the types of defect created by the substitution of solute atoms. The solution of Ni atoms, which occupy Fe2 sublattice site (kagome-nets), drastically decreases the hardness of Fe-rich Fe2Nb whereas that of Cr atoms occupying Fe1 sublattice site (36-nets) dose not change. TEM observation reveals high density of planer faults in the softened Fe2Nb with high amount of Ni in solution. There are several possibilities for the formation of planar faults: phase transformation, chemical fluctuation, thermal stress and so on. In this study, thus, we focus on the formation process of planar faults in the Fe2Nb Laves phase. The alloys used was Fe-15Nb-40Ni (at.%) with γ-Fe (fcc)+ Fe2Nb two phases. The composition of the Laves phase in equilibrium with γ-Fe is Fe-26Nb-33Ni. The alloy was partially melted and solidified with various cooling rates. Then, the samples were heat treated at 1473 K up to 240 hours, followed by water quench. In the rapidly solidified sample having eutectic lamellar microstructure, the regions with high planar fault density were sparsely observed in the Laves phase but limited to the edge of γ-Fe/Fe2Nb interface. After the isothermal aging at 1473 K, the eutectic Laves phase becomes coarsened, and the faulted regions in the Laves phase were extended with aging. No other phases were observed in the Laves phase. Effect of chemistry and interfacial dislocations on the formation of the planar faults will be presented.
12:00 PM - N6.8
Precipitation Behavior of Ir3Y in the Ir2Y Matrix.
Nobuaki Sekido 1 , Yoko Yamabe-Mitarai 1
1 , National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractLaves phase often exhibits a narrow range of homogeneity that hardly changes with temperature. This solubility behavior provides a limited opportunity for second phase precipitation within the Laves phase matrix. On the contrary, a binary C15 Laves compound Ir2Y has been found to develop off-stoichiometry toward the Ir-rich composition at high temperatures, and its range of homogeneity becomes narrow with decreasing temperatures. Because of this, Ir3Y precipitates within the Ir2Y matrix of an arc-melted Ir-30at%Y alloy. Anti-site substitution has been shown to occur as the defect mechanism in an Ir-rich Ir2Y phase. The Ir3Y precipitates exhibit a plate morphology with a typical Widmanstätten structure. The orientation relationship between the Ir3Y precipitates and the Ir2Y matrix has been identified as: (0001)Ir3Y // (111)Ir2Y and [2-1-10]Ir3Y // [-110]Ir2Y. The growth of the precipitates has been suggested to follow the ledge mechanism of which ledge height is usually equivalent to the length of the Ir3Y unit cell. Preexisting stacking faults in the Ir2Y matrix are heterogeneous nucleation sites against the Ir3Y precipitation. The nucleation on such staking faults is found to reduce the required ledge height in which the coherency of the two phases at the terrace of the interface can be preserved. The reduction in the height of a new ledge is believed to be advantageous in terms of the kinetics of this precipitation reaction.
12:15 PM - N6.9
Effect of Vacancy-site Occupation in Half-Heusler Compound ZrNiSn on Phase Stability and Thermoelectric Properties.
Yoshisato Kimura 1 , Toshiyasu Tanoguchi 1 , Yasuhiro Sakai 1 , Yaw-Wang Chai 1 , Yoshinao Mishima 1
1 Materials Science and Engineering, Tokyo Institute of Technology, Yokohama Japan
Show AbstractThermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. We focus on half-Heusler compounds such as TiNiSn and ZrNiSn, excellent n-type thermoelectric properties, which can be used at around 1000 K to directly convert waste heat into clean electrical energy. To fabricate thermoelectric modules, both n- and p-type materials need to be developed on half-Heusler compounds. Objective of the present work is to understand the conversion of thermoelectric properties of ZrNiSn from n-type to p-type by the addition of Co and Ir while our focus is placed on changes in the phase stability of the half-Heusler toward that of the Heusler according to the vacancy-site occupation of Co and Ir. Single-phase half-Heusler Zr(Ni,Mx)Sn alloys (M=Co,Ir) were fabricated by directional solidification using optical floating zone melting for crystallographic analyses and thermoelectric properties measurement. The n-type thermoelectric property of the ZrNiSn can be converted to p-type by the addition of Co and Ir. For a given amount, Ir is more effective than Co in enhancing p-type value of the Seebeck coefficient. We have found that Co and Ir atoms preferably occupy the vacancy-site instead of substituting the Ni sites. This implies that the phase stability of the compound gradually changes toward that of the Heusler compound Zr(Ni,M)2Sn due to the chemical interactions between the host and the solute elements. The total amount of Ni and M deviates from that in the stoichiometric composition of the half-Heusler phase and the lattice parameters of Zr(Ni,Mx)Sn increase depending on the M concentration. The occupation of vacancy sites by M atoms leads to a drastic reduction in the lattice thermal conductivity owing to the enhancement of phonon scattering by the solid solution effect. On the other hand, the carrier thermal conductivity increases with M concentration, i.e. together with carrier concentration.
12:30 PM - N6.10
Planarizaiton of Intermetallic Films by Gas Cluster Ion Beam Irradiations.
Kouki Naitou 1 , Noriaki Toyoda 1 , Isao Yamada 1
1 Graduate school of engineering, University of Hyogo, Himeji, Hyogo, Japan
Show AbstractIntermetallic alloys are indispensable materials for magnetic recording. Recently, it has become important to employ nano-fabrication process on these materials to realize patterned media for future high-density recording. Ar milling process has been widely used to etch or modify these materials. However, there is no surface smoothing effect with Ar milling process. Therefore, it is important to develop a surface planarization technique for intermetallic films to improve flying height stability of head on patterned media. In this study, we have studied the planarization of TiCr films by using gas cluster ion beam (GCIB). TiCr film is a non-magnetic material and was used for refill material between the patterns on magnetic layer. A gas cluster is an aggregate of a few to several thousands of gaseous atoms or molecules, and it can be accelerated to the desired energy after ionization. When the GCIB collides on a solid surface, it induces lateral motion of surface atoms and as a result, it induces surface smoothing. In addition, since a gas cluster is aggregate of several thousands of gaseous atoms or molecules, energy per atoms is quite low (energy as low as several electronvolts per atom). Therefore, a GCIB modify surface morphologies without causing deep damages. In this study, planarization of TiCr films on line-and-space patterns were demonstrated with GCIB. The pitch and depth of patterns are 100 nm and 20 nm, respectively. After irradiation of Ar and N2-GCIB, the line-and-space patterns on TiCr films were completely removed due to the surface smoothing effect of GCIBs. From the observation with a field emission secondary electron microscopy (FE-SEM), it was found that the etching depth required for planarization was only 2 nm. This is very thin compared to the initial peak to valley (4.75 nm). GCIB preferentially removes surface bumps and enhancement of the surface motion of atoms by GCIB irradiation causes refilling of grooves. From these results, it is confirmed that the GCIB process is suitable for the planarization of intermetallic films which are used for patterned medias. Surface modifications of various intermetallic films with GCIB will be also reported.
N7: Molybdenum and Niobium Silicide-Based Alloys
Session Chairs
Martin Heilmaier
Seiji Miura
Wednesday PM, December 01, 2010
Room 203 (Hynes)
2:30 PM - **N7.1
Oxidation Response and Coatings for Mo-Si-B Alloys.
John Perepezko 1 , Ridwan Sakidja 1
1 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractMo-Si-B alloys respond to high temperature oxidation in two distinct stages. First, there is a transient stage with an initial high recession rate that corresponds to the evaporation of volatile MoO3 due to the oxidation of the molybdenum rich phases. The steady state stage of the oxidation begins when a borosilica layer that initiated in the transient period becomes continuous and protects the alloy from further rapid oxidation. Then, the oxidation rate is limited by oxygen diffusion through the borosilicate layer. In order to improve the oxidation performance of the Mo-Si-B alloys, it is necessary to minimize the transient stage. The three phases, Mo (solid solution), Mo3Si (A15) and Mo5SiB2 (T2), composing the Mo-Si-B alloys play a different role in the transient stage. It has been established that variations in the microstructure size scale of the Mo-Si-B alloys, without any composition variation, can influence the formation of the protective borosilicate layer. The interaction of the three phases with a reduced microstructure scale can reduce considerably the transient oxidation stage. As a further approach to inhibit the transient stage, a kinetic biasing strategy has been developed to capitalize on the reactions between different phases to develop useful reaction products and alloy compositions that evolve toward a steady state of a compatible system. In order to achieve a compatible interface coating together with enhanced oxidation resistance, a pack cementation process has been adopted to apply diffusion coatings. Three areas are highlighted for successful coating applications on Mo-Si-B alloys and robust high temperature oxidation resistance: development of metal-rich silicide + borosilicide high-temperature coating, development of aluminide high temperature coating and development of simultaneous in-situ thermal-barrier + borosilicate coatings.
3:00 PM - N7.2
Oxidation Behavior of Mo-Si-B-(Al,Ce) Ultrafine Composites.
Jayanta Das 1 , Rahul Mitra 1 , Sanat Roy 1
1 Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721 302, West Bengal, India
Show AbstractThe effects of Al and/or Ce additions on microstructure formation of Mo--Si--B alloys and their isothermal oxidation behaviour at 500^o and 700^oC in dry air for 24 h have been investigated. Microstructure of arc-melted Mo--Si--B--(Al,Ce) alloys consists of bcc alpha-Mo dendrites embedded in ultrafine lamellar Mo3Si and Mo5SiB2 eutectic matrix. Isothermal oxidation kinetics of ultrafine structured Mo--Si--B alloy at 500^oC has been found to show hardly any mass change during 24 h exposure. However, addition of Al to Mo--Si--B alloy refines the microstructure, decreases the net mass loss at 700^oC by ~ 43\%, whereas Ce does not bring about any significant change. The enhanced oxidation resistance of Mo--Si--B--Al is due the formation of a protective and dense Al2(MoO4)3 outer layer, which reduce the sublimation of MoO3 at 700^oC. Various transient/ complex oxides formed on the alloys during their high temperature exposure have been examined to determine the oxidation mechanisms.
3:15 PM - N7.3
Deformation Behavior of Quaternary Mo-Nb-Si-B Alloys.
Padam Jain 1 3 , Ridwan Sakidja 2 , Ping Wang 1 4 , Pallavi Alur 1 3 , John Perepezko 2 , Sharvan Kumar 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States, 3 , Intel Corporation, Chandler, Arizona, United States, 2 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 4 , NETL, Albany, Oregon, United States
Show AbstractThe deformation behavior of quaternary Mo-Nb-Si-B alloys has been evaluated in compression in the temperature interval 1200°C-1600°C and in the strain rate regime 10-4 s-1 to 10-6 s-1. These alloys include a three-phase microlamellar eutectic microstructure composed of the Mo-Nb solid solution phase, the T1 and the T2 intermetallic phases. The off-eutectic alloys additionally include a primary bcc solid solution phase with a dendrite morphology. Results confirm significantly superior creep resistance for the eutectic alloy compared to ternary Mo-Si-B alloys with virtually no microstructural degradation. Post-deformation examination confirmed plastic deformation of the solid solution and T1 phases in all conditions examined whereas the T2 phase only deforms plastically in certain situations. High-temperature smooth bend tests showed that the bcc solid solution phase in the off-eutectic alloys can be beneficial to high-temperature ductility.
3:30 PM - N7.4
Microstructural and Mechanical Properties of Ternary Mo-Si-B Alloys Resulting from Different Processing Routes.
Manja Krueger 1 , Holger Saage 2 , Martin Heilmaier 3 , Heinrich Kestler 4 , Petr Loboda 5
1 Mechanical Engineering, Institute for Materials and Joining Technology, Otto-von-Guericke University Magdeburg, Magdeburg Germany, 2 Mechanical Engineering, University of Applied Sciences , Landshut Germany, 3 Materials Science, Technical University Darmstadt, Darmstadt Germany, 4 Technology Center, Plansee SE, Reutte in Tyrol Austria, 5 , National Technical University of Ukraine "Kiyv Polytechnic Institute", Kiew Ukraine
Show AbstractRefractory metals are currently being used as components for ultra-high temperature applications under protective atmosphere, taking advantage of their outstanding intrinsic properties, notably the high melting point and, thus, their excellent mechanical and creep strength. Provided that their oxidation resistance can be improved, this would make them the first choice for replacements of Ni-base superalloys in gas turbine engines with improved efficiency. In this presentation we will demonstrate how a powder metallurgical (PM) processing route (via mechanical alloying, cold isostatic pressing, sintering and hot isostatic pressing) and a zone melting (ZM) process starting from compacts of cold isostatically pressed elemental powders control the microstructure of selected Mo-Si-B compositions after consolidation. While PM processing leads to an ultrafine microstructure of Mo-Si-B alloys with a continuous Mo solid solution (“alpha-Mo”) matrix and embedded particles of the two intermetallic compounds Mo3Si and Mo5SiB2 the directional solidified (ZM) materials possess a coarse grained structure composed of an intermetallic matrix with dendritic islands of alpha-Mo. A comparative assessment of the mechanical behaviour of the alloys utilizing both the Vickers indentation fracture (VIF) technique and three-point bending tests emphasizes the beneficial effect of a continuous Mo matrix resulting in an increased room temperature fracture toughness and a reduction of the brittle-to-ductile-transition-temperature (BDTT). However, the advantageous effect of a directionally solidified microstructure is expected especially for higher temperatures above 1200°C.
3:45 PM - N7.5
Study of the Effect of Al, Cr and Sn Additions on the Microstructure and Properties of Nb Silicide Based Alloys.
Panos Tsakiropoulos 1
1 Engineering Materials, The University of Sheffield, Sheffield United Kingdom
Show AbstractNiobium silicide based alloys have the potential to replace Ni based superalloys in some structural applications at high temperatures. The development of these alloys must address the improvement of their environmental behaviour and mechanical properties at room, intermediate and high temperatures. Alloying and processing strategies have sought to enhance the performance of the solid solution, to identify optimum microstructures in terms of phase selection, stability and fractions and microstructural architecture and to understand how the latter affect performance at elevated temperatures. In this work, the role of Al, Cr and Sn in the microstructures of as-cast and heat-treated (1500 oC/100 h) Nb-24Ti-18Si (at %) based alloys were studied. The phases observed in as-cast and heat-treated alloys were the niobium solid solution, the niobium Nb3Si and 5-3 silicides, αNb5Si3 and βNb5Si3, Nb3Sn and Laves phase. The role of the aforementioned alloying elements on the selection and stability of the Nb3Si silicide, on the structure of the 5-3 silicide and in particular on the Nb3Si → Nbss + αNb5Si3 and the betaNb5Si3 → alphaNb5Si3 + (Nb,Ti)ss phase transformations and the formation of the C14-Cr2Nb Laves phase will be discussed.
4:00 PM - N7 - Silicides
BREAK
4:15 PM - N7.6
In-situ Observation of Nb/Nb5Si3 Two-phase Alloys During Bending at Various Temperatures.
Seiji Miura 1 , Yukiyoshi Tsutsumi 1 , Tetsuo Mohri 1
1 Mat.Sci. and Eng., Hokkaido Univ., Sapporo, Hokkaido, Japan
Show AbstractIn order to understand the deformation and fracture behavior of Nb-Si alloys, in-situ observation was conducted during bending of small specimens at room and high temperatures. Nb-Si alloy ingots containing 18.1at.%Si, 1.5at.%Zr and100ppmMg were prepared by arc melting, followed by uni-axial solidification of eutectic alloys in an optical floating zone apparatus. Ingots were heat-treated at 1650 °C for 100h to obtain Nb/Nb5Si3 two-phase microstructure. Chevron-notched specimens with a dimension of 1x2x10mm were prepared. A confocal laser scanning microscope (CLSM) with a small bending apparatus composed of a Piezo actuator and a force sensor was used for in-situ observation at a temperature range from room temperature up to 1000 °C. Crack propagation was recorded by a VCR with a video rate of 30 frames/sec attached to the CLSM, and a light microscope with a high-speed camera (2000 frames /sec) was also used for the in-situ observation at room temperature. In order to check the accuracy of the measurement of toughness, Si single crystal specimens were also prepared and the results were in good agreement with the reported values. At room temperature Nb-Si alloy shows a fracture toughness of 8 MPa √m. The crack propagation velocity seems to be not uniform, presumably due to the ductile Nb. At 1000°C the toughness of the alloy was about 20 MPa √m and plastic deformation accompanied by cracking was observed. This study is supported by Grant-in-Aid for Scientific Research, Ministry of Education Culture, Sports, Science and Technology, Japan, No.19206071.
4:30 PM - N7.7
The Effect of Ge and Ti Additions on the Microstructure and Properties of Nb-18Si Based Alloys.
Zifu Li 1 , Panos Tsakiropoulos 1
1 Enginering Materials, The University of Sheffield, Sheffield United Kingdom
Show AbstractNb silicide-based alloys could replace Ni-based superalloys in certain applications in gas turbine engines owing to their lower densities, high melting temperature and high temperature strength. The new alloys will need coatings, like the Ni based superalloys, and thus should have oxidation resistance to enable them to provide adequate protection in case of coating failure. Alloy development has focused on improving mechanical behaviour at room, intermediate and high temperatures as well as oxidation resistance. Alloying additions in Nb silicide-based alloys include transition metals and free electron metals. Alloying with Ti has been shown to significantly improve oxidation resistance and to affect mechanical properties. Germanium is known to have a significant beneficial effect on high temperature oxidation resistance of coatings on refractory metals owing to the formation of a GeO2SiO2 glass. There is limited work on the effects of Ge and Ti on their own and simultaneously on phase equilibria and oxidation in Nb-Si-X alloy systems. This work will study the effects of the synergy of Si with Ge and Ti on phase selection and stability and alloy hardness and microhardness of Nb solid solution and different silicides and oxidation in the pest regime and will compare the synergy of Ti and Ge with that of Ti with other free electron metals.
4:45 PM - N7.8
Compressive Deformation Behavior of Hypoeutectic and Hypereutectic Nb-Si-Mo Alloys.
Kausik Chattopadhyay 1 , Rahul Mitra 2 , Kalyan Ray 2
1 Metallurgical Engineering, Institute of Technology, Baneras Hindu University, Varanasi, Uttar Pradesh, India, 2 Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India
Show AbstractDeformation behaviour of arc-melted hypoeutectic and hypereutectic Nb-Si-Mo alloys containing Nb solid solution (Nbss) and (Nb,Mo)5Si3 in their microstructures, have been studied by carrying out at compression tests using strain rate of 10-4 /s at room temperature in air and in the range of 900-1200 oC in vacuum. The compressive yield strength and maximum stress of hypereutectic alloys with higher volume fraction of (Nb,Mo)5Si3 are found as greater than that of hypoeutectic alloys. The Mo concentration appears to affect strength of the hypoeutectic alloys, and not of the hypereutectic alloys, but its influence on strain-hardening rates of both types of alloys is more than that of (Nb,Mo)5Si3 volume fraction. The strain hardening rates appear to be strongly affected by solid solution strengthening of Nbss phase, which undergoes plastically deformation under constraint. Compressive strength and ductility are found to decrease and increase, respectively with increasing temperature. The ability for strength retention at elevated temperatures appears higher in the hypoeutectic alloys, because of the interactions of dislocations with 20-40 nm size (Nb,Mo)5Si3 precipitates dispersed in their primary Nbss, as confirmed by studies using transmission electron microscopy. Investigations of post-deformation microstructures by scanning electron microscopy have shown evidence for softening occurs by microcracking.
5:00 PM - N7.9
Application of Phase Diagram Calculation to Accelerated Development of Nb-Si Based Alloys.
Ying Yang 1 , Bernard Bewlay 2 , Y. Chang 3
1 , CompuTherm LLC, Madison, Wisconsin, United States, 2 , General Electric Global Research, Schenectady, New York, United States, 3 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractNb-Si based Refractory Metal Intermetallic Composites (RMICs) are candidate materials for ultra-high temperature applications (T>1200 degree C). To provide a balance of mechanical and environmental properties, Nb-Si composites are typically alloyed with Ti and Cr. Phase equilibria in the Nb-Cr-Ti-Si system, as prerequisite knowledge for advanced materials design and processing development, were rapidly established using the Calphad (Calculation of phase diagram) approach coupled with carefully designed experiments. The calculated isotherms, isopleths, and solidification paths were then used to aid the understanding of Cr and Ti effect on heat-treated and as-cast microstructures of selected Nb-Si alloys.
5:15 PM - N7.10
Quaternary and Quinternary Additions to Directionally-solidified X-X3Si Eutectics of Chromium and Vanadium.
Jestine Ang 1 , Vassili Vorontsov 1 , Chris Hayward 2 , Kevin Roberts 1 , Howard Stone 1 , Cathie MF Rae 1
1 Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom, 2 School of Geosciences, Grant Institute of Earth Science, University of Edinburgh, Edinburgh United Kingdom
Show AbstractAn alternative high temperature structural alloy system based on the X-X3Si eutectic compositions of chromium and vanadium is put forward. These low-density (~6g/cm3) eutectics have a bcc solid-solution to increase alloy fracture toughness, and a A15 X3Si as the high temperature load-bearing phase.
Chromium silicides typically exhibit excellent corrosion and oxidation resistance, but are unacceptably brittle and retain insufficient high temperature strength [1,2,3]. Vanadium has been chosen as an alloying addition due to its toughness and perfect miscibility with chromium. It also forms an A15 X3Si with superior high temperature hardness than Cr3Si [4].
To ascertain the limit of vanadium substitution for chromium prior to runaway oxidation, a series of alloys within the tie-tetrahedron of the (Cr,V)-(Cr,V)3Si system have been investigated. Preliminary observations indicate that a 25-50at% vanadium substitution for chromium can suppress the extensive oxide spallation of Cr-Cr3Si at 1200°C.
(½Cr,½V)-(½Cr,½V)3Si was used as the base alloy for quaternary element additions. Tantalum or tungsten were substituted in place of vanadium. Tantalum was chosen as it forms X3Si and has 12at% solubility in V3Si at 1400°C [5]. It may improve strength through solid solution strengthening and X3Si strength. Quinternary alloys were manufactured with a 10at% aluminium substitution of vanadium to both quaternary alloys.
Microstructure, elemental phase partitioning, high temperature mechanical properties and oxidation results will be discussed.
References1. M. R. Jackson et al., Journal of Metals: Volume 48, Number 1--January 1996.2. R.L. Fleischer et al., Elastic Moduli of Polycrystalline, High-Temperature Binary Intermetallic Compounds. Acta Materialia 37 (1989), 2801-2803.3. K Sadananda et al., Mat. Sci. Eng. A 261 (1999), 223-238.4. D. Shah and D. Berczik, Materials Science and Engineering A 155 (1992), 45-57.5. J.J. English, Bin. Tern. Phase Diagrams Columbium, Molybdenum, Tantalum, Tungsten, Ad 407 987, Vol. , 1963, p 1-127.6. N. Birks et al., Introduction to the High-Temperature Oxidation of Metals, 2nd ed., Cambridge University Press, 2006.
5:30 PM - N7.11
Direct Observation of an Ordered Arrangement of Vacancies and Large Local Thermal Vibration in Rhenium Silicide by Cs-corrected STEM.
Shunta Harada 1 , Katsushi Tanaka 1 , Kyosuke Kishida 1 , Norihiko Okamoto 1 , Haruyuki Inui 1 , Noriaki Endo 2 , Eiji Okunishi 2
1 , Kyoto University, Kyoto Japan, 2 , JEOL Ltd., Tokyo Japan
Show AbstractSemiconducting transition metal silicides have attracted great interest due to their potential applications of thermoelectric generation systems. Rhenium silicide is one of the candidate materials for thermoelectric application with high thermoelectric performance at high temperatures (~1000K). Our recent analysis revealed that the composition of this compound is ReSi1.75 and the crystal structure is monoclinic with space group Cm due to an ordered arrangement of vacancies in silicon sites in the underlying C11b lattice. We have proposed the crystal structure of this compound, however, the reliability of the crystal structure analyses is limited by a large atomic number difference between rhenium and silicon. Recent great enhancement of spatial resolution in transmission electron microscope by spherical aberration (Cs) correction technique makes possible to observe not only rhenium columns but also silicon columns, and silicon vacancies in rhenium silicide. In the present study, we investigate the crystal structure of rhenium silicide with an ordered arrangement of silicon vacancies and anomalous setting of the silicon atoms around the vacancies by Cs-corrected scanning transmission electron microscope (STEM). The silicon columns around the vacancies are moved from ideal C11b position and the contrast of the silicon columns around the vacancies is weaker than the other silicon columns. As the result of the detailed STEM image simulation, the reduction of the contrast of these silicon columns is due to the large local thermal vibration of the silicon atoms. By utilizing Cs-corrected STEM, not only ordered arrangement of silicon vacancies but also large local thermal vibration can be directly observed.
5:45 PM - N7.12
Microstructure and Mechanical Behavior of Refractory High-entropy Alloys.
Garth Wilks 1 2 , Oleg Senkov 1 3 , Daniel Miracle 1 , Andrew Chuang 4 , Peter Liaw 4
1 AFRL/RXLM, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States, 2 , General Dynamics Corp., Dayton, Ohio, United States, 3 , UES Inc., Dayton, Ohio, United States, 4 Materials Science & Engineering, University of Tennessee, Knoxville, Tennessee, United States
Show AbstractTwo refractory high entropy alloys, W25Ta25Mo25Nb25 and W20Ta20Mo20Nb20V20, were produced by vacuum arc-melting. Despite containing many constituents, both alloys were confirmed via high-energy X-ray diffraction to have a single-phase body-centered cubic (BCC) structure with lattice parameters of a = 3.2134(3) Å and a = 3.1832(2) Å for the quaternary and quinternary alloys, respectively. Quasi-static compression properties of both alloys over a temperature range from 20°C to 1600°C (in vacuum) were also measured. The room temperature yield strength of the four-component alloy was 1050 MPa and slowly decreased from 560 MPa to 405 MPa in the temperature range of 600 to 1600°C. The addition of V nominally increased the yield strength by ~200 MPa over the same temperature range. The room temperature compressive failure strain of the alloys was ~5% at 20°C and above 20% at elevated temperatures. The exceptional strength of these alloys is greater than that of the individual constituents, suggesting the operation of a solid-solution-like strengthening mechanism.
Symposium Organizers
Bernard Bewlay General Electric Company
Martin Palm Max-Planck Institut fuer Eisenforschung GmbH
Sharvan Kumar Brown University
Kyosuke Yoshimi Tohoku University
N8: Nickel Aluminides, Nickel Superalloys, Cobalt Alloys
Session Chairs
John Perepezko
Joerg Wiezorek
Thursday AM, December 02, 2010
Room 203 (Hynes)
9:30 AM - **N8.1
Temperature Dependence of Yield Stress and Dislocation Dissociation in L12-ordered Intermetallic Compounds.
Haruyuki Inui 1
1 Dept of MS&E, Kyoto University, Kyoto Japan
Show AbstractL12-ordered intermetallic compounds are usually classified into three groups depending on their yield stress versus temperature curves. The first category includes those L12 compounds such as Ni3Al, for which the positive temperature dependence of yield stress is observed from low temperatures. The second category includes those L12 compounds such as Pt3Al, for which the yield stress decreases rapidly with increasing temperature in the low temperature range without any yield stress anomaly at high temperatures. The third category includes those L12 compounds such as Co3Ti, for which the yield stress-temperature profile can be considered as a combination of those for the previous two categories. All these temperature dependences of yield stress have been explained in terms of the planarity of dislocation core structures. While the core of 1/2[110] dislocations separated by APB on the octahedral plane is believed to be planar, the cores of 1/2[110] dislocations separated by APB on the cube plane and 1/3[121] dislocations separated by SISF on the octahedral plane are believed to be non-planar. If the core is non-planer, the yield stress strongly depends on temperature in the low-temperature range, since thermal activation is needed for their motion. This is what is believed to be the case for the rapidly decreasing yield stress in the low temperature range for those L12 compounds of the second (Pt3Al)- and third (Co3Ti)-types. However, our recent study clearly indicates that the rapidly decreasing yield stress in the low temperature range can also be observed for many L12 compounds (such as Co3(Al,W) and Co3Ti), in which dislocations are dissociated into two collinear 1/2[110] partials separated by APB on the octahedral plane. The classification of L12 compounds in terms of the temperature dependence of yield stress is discussed in relation to the dislocation dissociation mode based on the results obtained.
10:00 AM - N8.2
Development of an Internal Reaction Zone During High-temperature Oxidation of Ni-based Superalloy TMS-75.
Karin Pruessner 1 2 3 , Hiroshi Harada 4 5
1 School of Physics, University of the Witwatersrand, Johannesburg South Africa, 2 Centre of Excellence in Strong Materials (CoE-SM), University of the Witwatersrand, Johannesburg South Africa, 3 Materials Physics Research Institute (MPRI), University of the Witwatersrand, Johannesburg South Africa, 4 High-Temperature Materials Center, National Institute for Materials Science (NIMS), Tsukuba Japan, 5 Rolls-Royce Centre of Excellence in Aerospace Materials, National Institute for Materials Science (NIMS), Tsukuba Japan
Show AbstractNi-based superalloys used in the hot section of a turbine need to form a protective alumina scale on the surface in order to withstand the high operating temperatures. Third generation Ni-based superalloys which are characterized by high Re- and Co-contents to boost creep resistance at high temperatures typically have low Cr-levels which makes formation of the protective oxide difficult. In an effort to understand the mechanism of oxidation for these alloys, we have oxidized Ni-based superalloy TMS-75 in (001) orientation at 900 C and 1000 C in air for times between 6 minutes and 100 hours. Internal and external oxidation products were characterized using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Raman Spectroscopy and Transmission Electron Microscopy (TEM) of cross-sectional samples. At temperatures between 900 and 1000 C the surface oxide on TMS-75 is non-protective (Ni,Co)O which allows inward diffusion of oxygen and nitrogen from the atmosphere. An extended internal reaction layer with oxide and nitride phases precipitated below the surface of the metal is the result. In the sub-surface region of the alloy, three morphologically different regions can be distinguished: a fine-grained mixture of oxide phases maintaining simple orientation relationships in the top region of the internal reaction layer, a coarse grained region consisting of alumina precipitates in an alloy matrix in the middle and a region containing aluminum nitride and alumina particles in an alloy matrix. In certain regions close to the initial alloy surface accumulations of heavy elements like Ta, W, and Re can also be found. Analyzing the development of the internal reaction zone over time it was found that selective oxidation of gamma’-particles in the sub-surface region of the alloy is the first precipitation step. As the reaction progresses, the near-surface region of the metal is completely oxidized. The initial orientation relationships between alloy phases and oxides are largely maintained. Diffusion and precipitation steps as the reaction progresses will be discussed.
10:15 AM - N8.3
The Effects of Alloying Elements on Microstructure and Hardness of Ni-base Dual Two-phase Intermetallic Alloys.
Kouji Kawahara 1 , Taku Moronaga 1 , Yasuyuki Kaneno 1 , Takayuki Takasugi 1
1 Department of Materials Science, Osaka Prefecture University, Sakai Japan
Show AbstractThe microstructure and hardness of dual two-phase intermetallic alloys that are composed of various kinds of volume fractions of geometrically closed packed (GCP) Ni3Al(L12) and Ni3V(D022) phases were investigated. Higher volume fraction of primary Ni3Al precipitates was observed in the Ti and Nb added alloys than base alloy when keeping Al content the same. Also, TEM microstructures in the eutectoid (channel) region consisting of Ni3Al+Ni3V were sensitive to alloying addition. The hardness of dual two-phase intermetallic alloys was basically composed of two terms, i.e., the hardness term deduced from mixture rule between primary Ni3Al precipitates and eutectoid region, and additional hardness term arising from interface between primary Ni3Al precipitates and eutectoid region. Nb and Ti addition raised hardness of dual two-phase intermetallic alloys by solid solution hardening in the constituent phases. This hardening was more significant in Nb addition than in Ti addition. As temperature increases, the additional hardening monotonously decreased for the base and Nb added alloys but little decreased for the Ti added alloy.
10:30 AM - N8.4
Microstructure Analysis of Channel Region in Dual Two-phase Intermetallic Alloys Based on Ni3Al-Ni3V Pseudo-binary Alloy System.
Taku Moronaga 1 , Yasuyuki Kaneno 1 , Takayuki Takasugi 1
1 Materials Science, Oasaka Prefecture University, Osaka Japan
Show AbstractDual two-phase intermetallic alloys based on Ni3Al-Ni3V pseudo-binary alloy system have been reported to display high phase and microstructure stability, and good mechanical properties at high temperature, and therefore are considered to be used as a next generation type of high temperature structural materials. These alloys are composed of primary Ni3Al and channel (eutectoid) region consisting of Ni3Al+Ni3V. In this study, microstructure of channel region was investigated by transmission electron microscope (TEM). The contrast of channel region in bright-field images showed complicated microstructure. -However, crystallographic coherency among the constituent phases or domains is very high because the electron beam diffraction consisted of a single set of pattern and their spots did not accompany streaks. It was also shown that the lattice misfit between a-axis of Ni3Al and c-axis of Ni3V is larger than that between a-axis of Ni3Al and a-axis of Ni3V. From dark-field observation, it was found that c-axis of Ni3V in the channel region became perpendicular to the longitudinal direction of channel. Therefore it is suggested that crystallographic orientation of Ni3V in channel region is aligned to lower an internal stress introduced by lattice strain.
10:45 AM - N8.5
Structural Trends in Topologically Close-packed Phases.
Bernhard Seiser 1 , Thomas Hammerschmidt 2 , Aleksey Kolmogorov 1 , Ralf Drautz 2 , David Pettifor 1
1 Materials, Oxford University, Oxford United Kingdom, 2 ICAMS, Ruhr-Universität Bochum, Bochum Germany
Show AbstractSingle crystal nickel-based superalloys are used in modern gas turbines because of their remarkable resistance to creep deformation at elevated temperatures, which is ensured by the addition of significant amounts of refractory elements. Too high concentrations of refractory elements can lead to the formation of topologically-close packed (TCP) phases during exposure to conditions of high temperature and stress which result in the degradation of the creep properties. In this talk we present a novel two-dimensional structure map (Navg, ΔV/Vavg) for TCP phases where Navg is the valence-electron count and ΔV/Vavg is a compositional dependent size factor. This map is found to separate the experimental data on the TCP phases of binary, ternary and multi-component TCP phases into well-defined regions corresponding to different structure types such as A15, σ, χ, δ, P, R, µ, and Laves. In particular, increasing size factor separates the A15, σ and χ phases from the δ, P, R, µ phases. The observed differences between the two groups of TCP structures with increasing ΔV/Vavg and the trend from A15 to σ to χ with increasing Navg in the first group are explained with the help of the analytic interatomic bond-order potentials (BOPs) which characterize the stability of TCP phases as a function of valence-electron count and lattice structure. The link between structural stability and local topology is thereby unraveled with the different factors controlling the formation of TCP phases being identified.
11:15 AM - N8.6
Morphology Change of γ' Precipitates in γ / γ' Two-phase Microstructure in Co-based Superalloys by Higher-order Alloying.
Katsushi Tanaka 1 , Masahiro Ooshima 1 , Norihiko Okamoto 1 , Kyosuke Kishida 1 , Haruyuki Inui 1
1 Materials Science and Engineering, Kyoto University, Kyoto Japan
Show AbstractRecently discovered cobalt base fcc/L12 two phase alloys have been attracted as a candidate of a new class of superalloys for high temperature structural materials. The alloys exhibit two-phase microstructures consisting of the solid-solution based on Co with a face-centered cubic structure (designated the γ phase) and the L12-ordered intermetallic compound based on Co3(Al,W) (designated the γ’ phase) as similar to the nickel base superalloys. In nickel base superalloys, the γ/γ’ two-phase microstructure has been believed to lead to the excellent high-temperature mechanical properties, and microstructure control has been made through higher-order alloying in very sophisticated and elegant ways, in order to further increase the temperature capability of materials. The same approach may be applied to cobalt base superalloys, however, the role of alloying elements on phase stability, lattice mismatch, mechanical properties of γ and γ' phases and so on is still unclear. Recent investigations by us and other groups indicate that alloying with tantalum strongly enhance the phase stability of the γ' phase. However, the precipitates of the γ' phase loose their lattice coherency upon alloying with a relatively large amount of tantalum, and the microstructure significantly changes. In the present study, we search alloying element to keep the coherent precipitation of the γ' phase with enhanced phase stability of the γ' phase by tantalum. According to our previous experiments about the volume fraction of the γ’ phase and γ’ solvus temperature for various quaternary alloys, we select molybdenum as an alloying element to reduce the lattice constant mismatch. As a result we succeed to develop an alloy forming a coherent cuboidal γ' precipitate with the γ' solvus temperature higher than 1150 °C.
11:30 AM - N8.7
Phase Stability of γ’ Phase in Ir-Al-W-X System.
Toshihiro Omori 1 , Komei Makino 1 , Ikuo Ohnuma 1 , Ryosuke Kainuma 1 , Kiyohito Ishida 1
1 Department of Materials Science, Tohoku University, Sendai Japan
Show AbstractPlatinum-group-metal based alloys are attractive for next-generation high-temperature materials due to their high melting temperatures. Ir alloys have L12 intermetallic compounds (γ’ phase) such as Ir3Ti and Ir3Nb, and the present authors has reported that the Ir3(Al,W) ternary compound with the L12 structure exists (J. Sato et al. Science 312 (2006) 90). The Ir-based alloys strengthened by the precipitation of the γ’ phase show very high strength at high temperatures, and therefore, these alloys are of interest for high-temperature materials. On the other hand, Ni3Al is a well-known γ’ phase utilized for superalloys and the present authors have discovered Co3(Al,W) γ’ phase. From the viewpoint of the phase stability, it is interesting to know the relation between the Ir3(Al,W) and Co3(Al,W) or Ni3Al. In this study, the phase stability of the γ’ phase in Ir-Al-W-X (X: Co, Ni, Fe) was investigated. Ir-Al-W-Co, Ir-Al-W-Ni and Ir-Al-W-Fe quaternary alloys were prepared by arc melting. Some specimens with high Ir content were homogenized at 1900°C, and heat treatment was conducted at 1000°C or 1300°C in all the specimens. Composition of the phase constituent was measured by electron probe microanalysis (EPMA) and the phase identification was carried out by X-ray diffractometry (XRD). The phase diagrams at 1000°C and 1300°C were determined in Ir-Al-W-Co, Ir-Al-W-Ni and Ir-Al-W-Fe systems. In Ir-Al-W-Co system, the γ’ phase exists in Ir-20at%Co-Al-W alloys at 1300°C, but 40Co alloys do not show it at 1300°C nor 1000°C. The same result was obtained in Ir-Al-W-Fe system. Other intermetallic compounds B2 and D019 were found to be formed from Ir-Al or Ir-W to Co-Al or Co-W systems, respectively. In Ir-Al-W-Ni system, the γ’ phase was observed at any Ni contents and it probably exists from Ir3(Al,W) to Ni3Al continuously, therefore, a wide range of alloy design from Ir-rich to Ni-rich is possible in Ir-Ni-Al-W alloys with the γ/γ’ two-phase structure.
11:45 AM - N8.8
Microstructure-driven Improvement of Catalytic Activity for Hydrogen Production Reactions in a Ni Solid Solution (γ) / Ni3Al (γ’) Two-phase Foil.
Hye-Youn Lee 1 2 , Masahiko Demura 1 , Ya Xu 1 , Dang-Moon Wee 2 , Toshiyuki Hirano 1
1 Fuel Cell Materials Center, National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 2 Department of Material Science and Engineering, KAIST, Daejeon Korea (the Republic of)
Show Abstract Microstructure is modified to mainly improve bulk properties. In addition to such traditional applications of microstructure modification, we here attempted to improve a surface property, i.e. catalytic activity, using a Ni3Al (γ’) / Ni solid solution (γ) two-phase structure. We have recently demonstrated that the surface of the γ/γ’ two-phase foil can be modified through a two-phase structure modification and a selective removal ofγmatrix phase [1]. After the selective phase removal, the surface is roughened and γ’-enriched, reflecting the original two-phase structure. Our recent studies revealed that the γ’ can show catalytic activities for hydrogen production reactions such as methanol decomposition [2] and methane steam reforming [3]. Taking into accounts that the surface area must increase by roughening, such rough and γ’-enriched surface is expected to show a high catalytic activity. The purpose of this paper is to examine the effect of this microstructure-driven surface modification on the catalytic activities for the hydrogen production reactions. The γ/γ’ two-phase foil (Ni-18at%Al) used had the γ’ block-shape particles with 1 μm in edge. After the γ matrix was selectively removed with an electrochemical etching technique, the surface was roughened and the surface area increased approximately 40%, according to an atomic force microscopic analysis. The catalytic activity measurements revealed that this surface modification yielded a significant increase in the catalytic activity for methane steam reforming. For example, a conversion of methane increased from 11 % to 32% at a reaction temperature of 973K. It should be noted that the extend of the catalytic activity improvement, approximately 300% increment, is considerably higher than the increase in the surface area, 40%. This finding might suggest that there is an additional effect of the surface modification with the selective phase removal on the catalytic activity. In the paper, we will examine the conditions in which the surface modification with the selective phase removal showed a clear effect on catalytic activities.[1] Lee et al. Adv. Mater. Res. (2010) vol. 89-91 pp. 331-336.; Lee et al. MRS Proceedings (2009) vol. 1128 pp. 269-274.[2] Xu et al. Intermetallics (2005) vol. 13 pp. 151-155.[3] Ma et al. Catalysis Letters (2006) vol. 112 pp. 31-36.
12:00 PM - N8.9
Mo Fiber Reinforced NiAl Composites Produced by Directional Solidification – Process, Microstructure and Mechanical Properties.
Lei Hu 1 , Samuel Bogner 2 , Weiping Hu 1 , Andreas Buehrig-Polaczeck 2 , Guenter Gottstein 1
1 , Institute for Physical Metallurgy and Metal Physics, RWTH Aachen University, Aachen Germany, 2 , Foundry Institute, RWTH Aachen University, Aachen Germany
Show AbstractThe purpose of this investigation was to develop a refractory metallic fiber reinforced NiAl composite with a composition of NiAl-9 at.% Mo by specially controlled directional solidification. In the composites, single crystalline Mo fibers precipitated from NiAl during solidification through eutectic transformation and aligned to grow along the solidification direction. The size (or section area) and spacing of Mo fibers were adjusted by changing solidification speed to obtain different distributing configurations of Mo fibers in NiAl matrix. Microstructure and local chemistry of the composites were examined by optical metallography and electron microscopy (SEM, TEM, EDX). Fine interface structure between Mo fiber and NiAl matrix was studied by HRTEM. Dislocations observed in NiAl near the interface indicated a thermal residual stress generated during cooling after solidification, in which NiAl matrix was tensioned along solidification direction owing to the mismatch of coefficients of thermal expansion (CTE), which was of a sufficient magnitude to cause local yielding of NiAl near the interface. The rough interface in atom scale implied a good interface shear strength, which is essential for load transfer from matrix to fiber. Such microstructure and interface structure strongly affected mechanical properties of the composites. Tensile tests of the composites at 700 °C and 1100 °C showed a relatively high strength and improved ductility compared to other NiAl composites, e.g. single crystalline Al2O3 fiber reinforced NiAl composites. The yield (σ0.2) and ultimate strength (UTS) of the composites along solidification direction were about 810 MPa and 930 MPa at 700 °C and about 340 MPa and 400 MPa at 1100 °C, respectively, while plastic elongation at ultimate tensile stress reached to 2~3% at both testing temperatures. A correlative comprehension of directional solidification processing, microstructure and mechanical properties was established according to current investigation results. It is expected to use such material with higher melting point, excellent oxidation resistance, as well as sufficient mechanical properties for structural application at the temperatures above 1000 °C.
12:15 PM - N8.10
Self-propagating High Temperature Synthesis of B2-RuAl Thin Films.
Karsten Woll 1 , Frank Muecklich 1
1 Materials Science and Engineering, Chair of Functional Materials, Saarland University, Saarbrücken Germany
Show AbstractDue to its favorable combination of properties the B2 intermetallic RuAl is superior to many other B2 aluminides. Its extraordinary high melting point of 2333 K and its high heat of formation promise an excellent thermodynamic stability up to high temperatures. Good high temperature strength is known and high creep resistance can be expected. Moreover, RuAl shows unusual high room temperature toughness compared to other intermetallics. This property profile makes that intermetallic compound interesting for high as well as room temperature thin film applications. Among the room temperature applications, the Ru/Al system is a very promising new system for soldering purposes which use metallic reactive multilayer foils (so called “reactive bonding”). Reactive bonding takes advantage of the released exothermic reaction heat of a self-propagating reaction in a metallic multilayer to melt solder locally and thereby to join parts. Due to the local nature of this process it has some advantages compared to conventional joining techniques, e.g. during joining of temperature sensitive microelectronic parts. For this application, the fundamental understanding of self-propagating reactions in Ru/Al multilayers which are characterized by reaction front velocities up to several tens of m/s is essential. High-speed methods are used to measure e.g. reaction velocities, temperatures and phase transformations in Ru/Al multilayers. Ignition thresholds are determined. Based on these results a reaction mechanism for the self-propagating reaction in Ru/Al multilayers to RuAl is proposed. Microstructural analysis of the final RuAl thin films is presented. Lastly, the findings for RuAl are compared to similar, already used systems to discuss the potential of RuAl in the mentioned application.
12:30 PM - N8.11
Role of Pd, Rh, and Ir addition on Stability and Oxidation of β-NiAl Alloys.
Travis Brammer 1 2 , P. Ray 1 2 , Y. Ye 2 , M. Kramer 1 2 , M. Akinc 1 2
1 Materials Engineering, Iowa State University, Ames, Iowa, United States, 2 , Ames Laboratory, US Department of Energy, Ames, Iowa, United States
Show AbstractThe drive for greater efficiencies and clean power generation requires the modern day gas turbines to operate at as high a temperature as possible, in harsh operating conditions of thermal power plants. Controlling oxidation and corrosion at these elevated temperatures is a prime concern for system lifetime. High temperature oxidation stability of ferrous alloys is improved by using commercial overlay coatings, typically MCrAlY alloys, whereas diffusion coatings are based on β-NiAl. In our work, we focus on the β-NiAl intermetallic. Choice of alloying additions for the current work was made using a multi-stage ‘sieving process’ to reduce the large number of potential alloying elements. Initial filtering of the prospective transition elements was done using an extended semi-empirical Miedema’s model. Promising results from this analysis were further evaluated within the framework of density functional theory using the commercially available VASP software to see if the minor element additions increased the stability of the compound. Of the eight elements tested, Pd, Rh and Ir were deemed to be the most suitable for experimental validation. In addition to studies on phase stability, the oxidation behavior of NiAl has been analyzed as functions of composition and temperature. The NiAl alloys were tested at isothermal temperatures ranging from 1100°C-1300°C, and thermal cycling at 1150°C and 1300°C at several levels of minor element (Pd, Rh, and Ir) additions.This work was supported by the DOE-FE (ARM program) through Ames Laboratory contract no.DE-AC02-07CH11358.
12:45 PM - N8.12
Three-dimensional Microstructural Observation in Crystalline Materials by Transmission Electron Microscopy.
Satoshi Hata 1 , Masatoshi Mitsuhara 1 , Tomoya Kawai 1 , Keisuke Ogata 1 , Ken-ichi Ikeda 1 , Hideharu Nakashima 1 , Minoru Nishida 1 , Syo Matsumura 2 , Minoru Doi 3 , Hiroya Miyazaki 4
1 Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka Japan, 2 The Laboratory for High Voltage Electron Microscopy, Kyushu University, Fukuoka Japan, 3 , Aichi University of Technology, Aichi Japan, 4 , Mel-Build, Fukuoka Japan
Show AbstractMicrostructure of intermetallic materials strongly depends on their crystal structures. In the case of a low-symmetry alloy phase, consideration of three-dimensional (3D) morphology of the phase is often necessary to understand the microstructure formation mechanism and its relationship to material properties. Previously, such 3D morphological information has been obtained by combination of observations from different crystallographic orientations. Recently, one can observe 3D microstructure directly from the same field of view using various microscopy techniques such as electron tomography, x-ray tomography, etc. This paper reports our activities on 3D imaging techniques using transmission electron microscopy (TEM) and their applications to microstructures in crystalline materials such as 3D morphology of domain boundaries, dislocations, precipitates, etc.
N9: Fundamental Aspects of Functional Intermetallics
Session Chairs
Bernard Bewlay
Yoshisato Kimura
Thursday PM, December 02, 2010
Room 203 (Hynes)
2:30 PM - **N9.1
Intermetallics-related Materials for Hydrogen Applications.
Chikashi Nishimura 1
1 Fuel Cell Materials Center, National Institute for Materials Science, Tsukuba Japan
Show AbstractHydrogen storage alloys are, undoubtedly the most well-known intermetallics for hydrogen energy applications. A number of review papers have already been published on many types of intermetallics for hydrogen storage. For the automobile applications, being light weight is the major requisite and magnesium-based alloys and intermetallics have been investigated. Here, I will present two other intermetallics-related materials for hydrogen applications: membrane materials for hydrogen separation and new catalysts for hydrogen production. Hydrogen diffuses much faster in bcc crystals than in fcc crystals because of the low activation energy of diffusion in bcc lattices. Therefore, alloys and intermetallics with bcc structures are favorable for membrane materials for hydrogen separation. Vanadium-based alloys1) and niobium-based alloys2) are most intensively investigated, since these metals absorb high amount of hydrogen, resulting in high hydrogen diffusion flux. In niobium-based alloys, TiNi with B2 structure works to mitigate hydrogen embrittlement2). Recently there are new movements to apply intermetallics for various catalysts, especially those for reforming reaction to produce hydrogen. Tsai and his group3) have found that PdZn with L10 structure, which exhibits a similar valence electron density of states with pure Cu, shows identical catalytic performance for methane reforming to that of Cu. It is strongly hoped to discover new intermetallics consisting of normal metals to replace catalysts based on precious metals such as Pt, Pd, Rh, and Ir.References:1)C. Nishimura, M. Komaki, S. Hwang and M. Amano, J. Alloys and Compounds, 330 (2002), 902-906.2)K. Hashi, K. Ishikawa and K. Aoki, J. Alloys and Compounds, 368 (2004), 215-220.3)A. P. Tsai, S. Kameoka and Y. Ishii, J. Phys. Soc. Japan, 73 (2004), 3270-3273.
3:00 PM - N9.2
Theoretical Calculations of Intermetallic Nanoscale Clusters for Hydrogen Storage.
Lucas Wagner 1 , Eric Majzoub 2 , Mark Allendorf 3 , Jeffrey Grossman 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , University of Missouri - St. Louis, St. Louis, Missouri, United States, 3 , Sandia National Lab, Livermore, California, United States
Show AbstractIn the search for a viable hydrogen storage method, metallic clusters are a promising field of research, potentially offering good hydrogen density by both weight and volume. However, it is challenging to find a metal hydride that is both light enough and has the proper binding energy to allow efficient hydrogen absorption and desorption. Alloys and nanoscale clusters give two additional degrees of freedom that expand the range of potential good-quality compounds for hydrogen storage, potentially enough to contain a viable hydrogen storage material.Computer simulations based on first-principles quantum calculations can aid the search in this expanded space of materials, but they must be accurate to better than ~5 kJ/mol H2, which is not attainable using traditional density functional theory methods. We apply the quantum Monte Carlo method to obtain much more accurate results for several alloyed nanoscale clusters, which allows us to precisely identify candidate cluster sizes and compositions for effective hydrogen storage.
3:15 PM - N9.3
Monte Carlo Simulations of Structural Phase Transitions in Ni-Pt Alloys.
Tongsik Lee 1 2 , Jimmie Doll 3 , Michael Baskes 1 , Steven Valone 1
1 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Department of Physics, Brown University, Providence, Rhode Island, United States, 3 Department of Chemistry, Brown University, Providence, Rhode Island, United States
Show AbstractThe Ni-Pt system, as well as several other Pt-based bimetallic alloys, has gained particular attention because of its outstanding electrocatalytic activity, and is considered to be one of the prime candidates for numerous energy conversion reactions, including the cathode material of polymer electrolyte membrane fuel cells. Catalytic reactivity and selectivity have been found to be highly structure- and composition-sensitive. This system undergoes configurational order-disorder transitions, which is a large class of structural phase transitions observed in metallic alloys.An accurate description of structural transitions and resulting changes of physical properties has been one of the central themes in computational materials modeling. A proper scheme to describe phase equilibrium must be built upon the basis of a detailed microscopic approach in terms of a reasonable model of energetics and a method of statistical mechanics. The Monte Carlo (MC) method can be an efficient yet powerful technique to determine the thermodynamic equilibrium of a given system at the atomic scale, when it is combined with a reliable interatomic potential model. In particular, if the time scale required in diffusion processes is much longer than the one that molecular dynamics (MD) can practically achieve, or if the actual kinetics of the equilibration processes need not be followed, the MC method is a superior alternative. In general, however, simulations of structural transitions in solids suffer from apparent obstacles in both aspects of energetics and statistical mechanics: (i) the lack of a suitable potential model that provides accurate phase transitions, while providing a consistent description of physical properties across multiple phases; and (ii) the inherent difficulty in effective phase space sampling in the standard Metropolis scheme.In the present study, we investigate the order-disorder transitions and associated structure-properties relationships in the Ni-Pt alloys over a broad range of temperatures, adopting the modified embedded atom method (MEAM) as an interatomic potential model. A general methodology of MC simulations for the isobaric or isotension ensemble is developed by utilizing a fully deformable periodic cell characterized by its metric tensor in accordance with the Parrinello-Rahman scheme. This MC method is further enhanced with the parallel tempering method in order to overcome sampling inefficiency. We conclude that phase transitions appear in this system as manifested through order parameters and various thermodynamic properties, such as the heat capacity, thermal expansion, and elasticity, all of which can be obtained as byproducts of a single parallel MC simulation by measuring the fluctuations of state variables.
3:30 PM - N9.4
Phase-field Simulation of Antiphase Boundary Migration in Intermetallic Compounds with Solute and Vacancy Segregation.
Yuichiro Koizumi 1 , Tatsuya Yokoi 2 , Masayuki Ouchi 2 , Yoritoshi Minamino 2 , Masato Yoshiya 2 , Samuel Allen 3
1 Institute for Materials Research, Tohoku University, Sendai Japan, 2 Adaptive Machine Systems, Osaka University, Suita, Osaka, Japan, 3 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show Abstract Recently, it has been revealed that the mechanical properties of some intermetallic compounds such as Ti3Al, Fe3Al and Fe3Ga are significantly affected by thermal antiphase domains (APDs). The critical resolved shear stress for {1-100}<11-20> slip of Ti3Al containing a high density of thermal APBs can be increased up to 6 times larger than that of an APB-free (i.e. single domain) counterpart [1]. Giant pseudoelasticity is manifested by the interaction between dislocations and fine-scale thermal APDs in Fe3Al and Fe3Ga [2, 3]. However, to apply these attractive mechanical properties to practical use at elevated temperatures, the stabilities of the fine thermal APD structure becomes important since the coarsening of the APDs degrades such attractive properties. A key factor for the stability of thermal APDs is segregation to antiphase domain boundaries (APBs). In the present study, the effects of solute and vacancy segregation on APB migration in Fe3Al and Ti3Al have been investigated by using phase-field simulation in which vacancy distribution is taken into account [4]. The simulation indicated that the migration of B2-APB (phase-shift vector: R=1/4<111>) in Fe3Al with D03 structure is slowed significantly by Fe-segregation (i.e. Al-depletion) although the boundary mobilities are nearly doubled by vacancy segregation at 673 K regardless of the composition. In contrast, the effect of solute segregation on D03-APB (R=1/2<100>) migration was indicated to be quite sensitive to the composition whereas the effect of vacancy segregation was negligibly small irrespective of composition. In Ti3Al with D019 structure in which three types of APB with different phase shift vectors of R=1/6<11-20> can be formed, the Al-depletion (i.e. Ti-segregation) and vacancy-segregation were as significant as that for B2-APB in Fe3Al. In addition, further significant vacancy segregation was indicated to occur at triple junctions of APBs. Strategies to improve the stabilities of thermal APB structures will be discussed based on these results. [1] Koizumi et al, Philos. Mag. 2008;88:465. [2] Yasuda et al, Acta Mater. 2003;51:5101. [3] Yasuda et al, ibid 2007;55:2407. [4] Koizumi et al. ibid 2009;57:3039.
4:15 PM - **N9.5
Thermomechanical Processing and Structure-property Relationships in L1o-FePd Based Ferromagnetic Intermetallics.
Jorg Wiezorek 1 , Andreas Kulovits 1 , Anirudha Deshpande 2
1 Mechanical Engineering and Materials Science, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 , GE Lighting Technology, Willoughby, Ohio, United States
Show AbstractThe L1o-ordered intermetallic phase FePd can be established in the vicinity of the equiatomic composition and is a member of a class of uniaxial ferromagnetic intermetallics that exhibit large magnetocrystalline anisotropy, which also includes FePt and CoPt. Alloys based on these uniaxial ferromagnetic intermetallics are currently of interest for applications in the information technology sector. Technologically relevant properties depend strongly on the morphology and scale of the microstructure produced during solid-state processing. Here the equiatomic FePd alloys are used as model systems to investigate aspects of the processing-structure-property relationships in this class of L1o-orderd ferromagnets during order annealing after cold-deformation. Depending on the details of the processing parameters employed during the combinations of cold-deformation and annealing treatments morphologically different microstructures with nano- to microcrystalline scales can be established. Combinations of property measurements (magnetic and mechanical) and microstructural studies by X-ray diffraction, scanning and transmission electron microscopy have been performed to establish structure-property relationships, elucidate mechanisms for defect genesis and measure activation energies for the transformation dominating elementary processes. Models based on the underlying physical metallurgy are proposed which rationalize the microstructural evolution and the enhanced properties. Implications for bulk-processing of FePd-based and other L1o-ordered intermetallic ferromagnets are discussed. We acknowledge use of the facilities of the Materials Micro-Characterization Laboratory of the Department of Mechanical Engineering and Materials Science, University of Pittsburgh, and support by a grant from the National Science Foundation, Division of Materials Research, Metals Program.
4:45 PM - N9.6
Precipitation Kinetics of an Fe-rich Phase in Ni50Al50-xFex Alloys Monitored by Magnetic Measurements.
Nagehan Duman 1 , Amdulla O. Mekhrabov 1 , M. Vedat Akdeniz 1
1 Metallurgical and Materials Engineering Dept., Middle East Technical University, Ankara Turkey
Show AbstractNickel-Aluminum (NiAl) is one of the most promising intermetallics for high temperature structural applications. It possesses high melting point, excellent oxidation and corrosion resistance and low density. However, room temperature ductility and high temperature strength requires to be improved for use as desirable high temperature materials. The incorporation of iron (Fe) in NiAl systems has been shown to induce specific alterations in structural properties. Most particularly, it might allow for the precipitation of a BCC α-phase that changes the slip system so that a higher shear stress is required which strengthens the alloy. The important phenomenon of α-phase responsible for precipitation strengthening of Ni50Al50-xFex alloys in particular at high temperatures requires a convenient annealing heat treatment in order to form desirable precipitates. Therefore, as well as its temperature dependence, kinetic aspects and mechanism of precipitation might attract interest.In a few studies, the magnetic monitoring approach was utilized to replace conventional thermal analysis as a tool to successfully reveal the kinetic aspects of such precipitation in amorphous matrix nano-crystalline alloys. However, to the best of our knowledge, there are no studies discussing the kinetics of such precipitation reactions occurring at the nano-scale in intermetallics via magnetic measurements.In this study, the kinetic aspects of α-phase precipitation in Ni50Al50-xFex alloys was studied by utilizing a novel characterization technique that reveals the isothermal kinetics by monitoring the magnetic properties using temperature-scan and isothermal holding experiments on a vibrating sample magnetometer. The precipitation was found to follow a first order kinetics where the Avrami time exponent (n) is about unity showing decreasing rate of transformation with no apparent incubation time. However, without being able to monitor the earliest stages of reaction that occur during the heat-up stage preceding isothermal holding, it does not seem possible to propose whether the transformation is of homogenous nature as in the case of spinodal decomposition or a classical diffusion controlled growth process with decreasing growth rate. Activation energies were determined using magnetization measurements at several different isothermal temperatures within the temperature range in which magnetization is favorable through the precipitation of the ferromagnetic phase. The activation energies were found to range between 74.6 kJ/mol to 82.6 kJ/mol with a maximal deviation of 3.5 kJ/mol. An increase in the Fe content of the β-phase, within which the precipitation of α-phase occurs due to the miscibility gap between the two, appears to slightly increase the activation energy for precipitation. The BCC α-phase precipitates were shown by transmission electron microscopy to be nano-sized and to coarsen with increasing annealing temperature.
5:00 PM - N9.7
The Effect of Surface Modification on Structural Stability and Magnetic Properties of Intermetallic Sm2(Co, Fe, Cu, Zr)17 Compound, Used for Permanent Magnetic Applications.
Muhammad Qadeer 1 , B. Azdhar 1 2 , Steven Savage 3
1 Materials Science and engineering, Kungliga Tekniska Högskolan, Stockholm Sweden, 2 Department of Fibre and Polymer Technology, Kungliga Tekniska Högskolan, Stockholm, Stockholm, Sweden, 3 Department of Sensor Systems, Signature materials , FOI, Swedish Defence Research, Stockholm, Stockholm, Sweden
Show AbstractThe current quest for permanent magnetic materials for applications such as more electric air craft (MEA) and more electric vehicles (MEV) has renewed the interest in Sm2Co17 alloys. We studied the effect of surface modification by glycidoxypropyltrimethoxsilane (GPTMS) on structural and magnetic properties of Sm2Co17 particles to develop useful polyamide 12 (PA12) bonded Sm2Co17 magnets. Since, during the processing of polymer bonded magnets the temperatures can exceed 400oC. It is necessary to protect the particles from oxidation. Sm2Co17 are dual phase alloys, consisting of Sm2Co17, core phase and SmCo5, boundary phase. The stability of these phases determines the magnetic properties of Sm2Co17 particles. The magnetic measurements were recorded by superconducting quantum interference device (SQUID), which showed that the uncoated particles at 400oC lost its hard magnetic character and showed soft magnetic hysteresis loop, whereas, the coated particles showed hard magnetic properties identical to that of unheated magnetic particles. The structural studies by scanning electron microscopy (SEM) and magnetic force microscopy (MFM) revealed that the poor magnetic properties attributed to uncoated particles are associated with the decomposition of boundary phase, SmCo5, which provides the hindrance to domain movement by pinning mechanism. MFM structural analysis further revealed that the dissociated phase showed soft magnetic properties. EDX analyses further revealed that the decomposed phase showed depletion of Sm and segregation of alloying elements in the decomposed phase and precipitates. Whereas, the coated particles showed better magnetic properties due to better barrier properties of coating by controlling diffusion of oxygen and hence less decomposition of SmCo5 phase. The coating of particles with GPTMS not only restricts the decomposition of SmCo5 phase but also acts as molecular bridges between the polymer matrix and magnetic filler material.Keywords: Permanent magnetic materials; Surface modification; Polymer bonded magnets; hard magnetic character; soft magnetic; Hysteresis; diffusion
5:15 PM - N9.8
The Role of Si and Sb in the Si-Sb-Te Phase-change Material.
Liangcai Wu 1 , Xilin Zhou 1 , Zhitang Song 1 , Henan Ni 1 , Feng Rao 1 , Cheng Pen 1 , Bo Liu 1 , Songlin Feng 1 , Bomy Chen 2
1 , State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai China, 2 , Silicon Storage Technology, Inc., Sunnyvale, California, United States
Show AbstractPhase-change random access memory (PCRAM) is a rapidly emerging technology which is considered as one of the most promising candidate for the next generation nonvolatile memory. For commercial application, good data retention, low power consumption, etc. are the most important issues among the critical aspects. In order to approach these goals, many efforts including improving the properties of conventional Ge-Sb-Te phase change materials, optimizing geometrical structure of the memory cell as well as inserting a heating layer between electrode and phase change materials, have been carried out. Besides Ge-Sb-Te alloy, developing new phase change material is another effective way to improve the performance of PCRAM.In this study, we proposed and fabricated Sb-rich Si-Sb-Te phase change material with different Si and Sb content, and the role of Si and Sb in the Si-Sb-Te alloy was analyzed. The resistance-temperature and retention properties of these Sb-rich Si-Sb-Te alloys were studied and compared. The devices based on these Sb-rich Si-Sb-Te alloys were fabricated by 0.18 μm CMOS technology, and device properties were studied by current-voltage (I-V) and resistance-voltage (R-V) measurements. The experimental results show that the data retention ability of the Sb-rich Si-Sb-Te alloy was obviously improved through increasing Si content, but excessive Si would degenerate the device electrical properties. And the role of Sb in the Si-Sb-Te alloy is to speed up the crystallization process, while excessive Sb would decrease the device thermal stability, thus result in bad data retention property. So, in order to obtain practicable Sb-rich Si-Sb-Te phase change material, suitable Si and Sb content is required to balance the device electrical properties, thermal stability or data retention ability.