Symposium Organizers
Bernard Bewlay General Electric Company
Yue-hui He Central South University
Martin Palm Max-Planck Institut fuer Esenfoschung GmbH
Masao Takeyama Tokyo Institute of Technology
Joerg Wiezorek University of Pittsburgh
U1: Intermetallics for Hydrogen Storage and Thermoelectric Applications
Session Chairs
Haruyuki Inui
Jörg Wiezorek
Monday PM, December 01, 2008
Constitution A (Sheraton)
9:45 AM - **U1.1
Crystal Structures and Hydrogenation Behaviors of the RMn (3≤n<5) (R=La, Mg; M=Ni, Co) ``Superlattice" Alloys.
Etsuo Akiba 1 , Jin Nakamura 1 , Kenji Iwase 2 , Yujun Chai 1 , Hirotoshi Enoki 1 , Kouji Sakaki 1 , Kohta Asano 1 , Yumiko Nakamura 1
1 ETRI, AIST, Tsukuba, Ibraki, Japan, 2 , Ibaraki University, Tokai, Naka-gun, Ibaraki, Japan
Show Abstract10:15 AM - U1.2
Hydrogen Storage in Zr-Ni Intermetallic Alloys.
Sesha Srinivasan 1 , Diego Escobar 1 , Yogi Goswami 1 , Elias Stefanakos 1
1 , University of South Florida, Tampa, Florida, United States
Show AbstractThe Zr-Ni intermetallic alloys namely, (i) ZrNi 70/30 and (ii) ZrNi 30/70 have been investigated for the reversible hydrogenation behavior. These composites show Zr-Ni intermetallic multi-phase formation as explored by the X-ray diffraction studies. The sorption kinetics of ZrNi 70/30 seems much faster (~3-4 times) than that of ZrNi 30/70 alloys. Again the initial desorption rate increases with increase of temperature. A well defined plateau region was obtained for the ZrNi 70/30 with an equilibrium pressure ranges from 〈1 bar (300 ○C) to 10 bars (390 ○C). For ZrNi 30/70, the sloppy plateau region extends to higher equilibrium pressures. It is estimated that the total effective hydrogen concentration for ZrNi 70/30 (~ 1.0 wt.%) is at least 2 times that of ZrNi 30/70 (~0.5 wt.%) composites. From the PCT isotherms, the enthalpy of reaction (ΔH) has been calculated to be ~39 kJ/mol H2 for the ZrNi 70/30. The surface morphologies of the hydrogenated materials exhibit the presence of cracks and particle size pulverization in comparison to the pristine alloys.
10:30 AM - U1.3
Hydrogen Permeation in Anisotropic Nb-TiNi Alloy Prepared by Forging and Rolling.
Kazuhiro Ishikawa 1 , Jun-ichi Matsuoka 1 , Sho Tokui 1 , Kiyoshi Aoki 1
1 Materials Science and Engineering, Kitami Institute of Technology, Kitami, Hokkaido, Japan
Show AbstractThe present authors have proposed Nb-TiNi alloys consisted of the primary Nb solid solution and the eutectic TiNi+Nb phases as new non-Pd hydrogen permeation alloys. Recently, it was found that an anisotropic Nb phase was formed in the Nb40Ti30Ni30 alloy after heavy forging and rolling, i.e., the Nb phase was elongated and contracted along the rolling direction (RD) and normal direction (ND), respectively. In this anisotropic alloy, hydrogen permeability strongly depends on the direction. For example, the values of hydrogen permeability along RD (ΦRD) and ND (ΦND) are about 8 times larger and 1/8 times lower than that of the original one, respectively. In the present study, hydrogen permeation in the anisotropic Nb-TiNi alloy is discussed on the basis of the experimental results and the composite law, and it is demonstrated that high hydrogen flux can be obtained in the anisotropic alloy with a good resistance to hydrogen embrittlement.The Nb40Ti30Ni30 alloy is prepared by an induction melting in an Ar atmosphere, then the alloy ingot was forged and rolled at 1173 K. The alloy samples were cut from the ingot for XRD analysis, microstructural observation and hydrogen permeation measurement. Both surfaces of the alloy disk were coated by Pd to avoid oxidation and to enhance hydrogen dissociation. Hydrogen permeation measurement was performed in 0.1-0.9 MPa H2 at 553-673 K using a mass flow meter.When the angle of the direction of hydrogen permeation against RD is defined as θ, hydrogen permeability of this alloy is decreased with increasing θ. The maximum and minimum hydrogen permeability is obtained at θ=0° and θ=90°, respectively. The value of ΦRD is independent to the thickness of the alloy membrane above 100μm. On the contrary, hydrogen flux through the alloy membrane is increased with decreasing the thickness. Large hydrogen flux of 73 ccH2cm-2min-1 is obtained at 673 K in the pressure difference of 0.8 MPa without hydrogen embrittlement.
10:45 AM - U1.4
Hydrogen Storage Properties, Metallographic Structures and Phase Transitions of Mg-based Alloys Prepared by Super Lamination Technique.
Nobuhiko Takeichi 1 , Koji Tanaka 1 , Hideaki Tanaka 1 , Nobuhiro Kuriyama 1 , Hiroshi Miyamura 2 , Shiomi Kikuchi 2
1 , AIST, Ikeda, OSAKA, Japan, 2 Department of Materials Science, University of Shiga Prefecture, Hikone, SHIGA, Japan
Show AbstractMagnesium is a one of promising materials for hydrogen storage media because it can absorb a large amount of hydrogen as MgH2 and relatively low cost. However, the hydrogen absorption/desorption kinetics is too slow for practical use and needs high temperatures such as 573K. To improve the reaction kinetics and diffusion properties, a reduction of the grain size and an addition of various catalysis have been investigated. Recently, we reported Mg-based alloys, prepared by super lamination technique that is a kind of such mechanical processes, reversibly can absorb and desorb hydrogen at 473K, which was lower than reaction temperatures of conventional Mg-based materials. In the present study, we applied the super lamination technique to form Mg-Pd and Mg-Al binary composites. The formation behavior of intermetallic compounds, metallographic structures and their hydrogen storage properties of Mg/Al and Mg/Pd laminate composites were investigated.The Mg/Al laminate composites with (Mg/Al)=17/12 and Mg/Pd laminate composites with (Mg/Pd)=6,3 and 2.5, where (Mg/Al) and (Mg/Pd) mean the ratio of the numbers of Mg to Al or Pd atoms, were prepared by repetitive-rolling method using conventional two-high rolling mill. After several stack and rolling process, the laminate composites were subjected to appropreate heat treatmens and the intermetallic compounds were uniformly formed. The growth of the intermetallic compounds and its concentration profiles were studied using a scanning electron microscope. Hydrogen storage properties (reaction kinetics and equilibrium sorption properties) of the laminate composites were evaluated by using a Sieverts’ apparatus.The phase transformations of laminate composites during hydrogen absorption and desorption was analyzed by in-situ XRD measurement.By heat treatment at 673K, intermetallic compounds of Mg17Al12 and Mg2Al3 formed between the initial Mg-Al boundaries. Also, intermetallic compounds of Mg6Pd, Mg3Pd and Mg5Pd2 were formed at the interface between Mg and Pd by interdiffusion. Those intermetallic compounds In Mg/Al composite, Mg17Al12 , was formed by heat treatment of laminate composite, can reversibly absorb and desorb hydrogen, up to 0.67 H/M, at 673K. PC-isotherms of the Mg/Pd laminate composites show single plateau at 1 MPa. On the other hand, the Mg/Pd laminate composites can reversibly absorb and desorb a large amount of hydrogen, up to 1.46~0.9 H/M, at 573K. Except Mg/Pd laminate composites with (Mg/Pd)=2.5, PC-isotherms of the Mg/Pd laminate composites show two plateaux, PL and PH. In case of Mg/Pd laminate composite with (Mg/Pd)=6, PL and PH at 573 K were 0.02 and 2 MPa, respectively. PC-isotherms for the Mg/Pd laminate composite with (Mg/Pd)=2.5 at 573K show single plateau at 2 MPa.Those intermetallic compounds can absorb and desorb hydrogen reversible through complex multistage disproportionation reaction and recombination of Mg and Al or Pd.
11:00 AM - U1.5
Thermoelectric Devices Based on Nanocomposites with Ultra High Seebeck Coefficients.
Otto Gregory 1 , Ximing Chen 1 , Gustave Fralick 2
1 Chemical Engineering, Univ. of Rhode Island, Kingston, Rhode Island, United States, 2 , NASA Glenn Research Center, Cleveland, Ohio, United States
Show AbstractThermoelectric devices based on nanocomposites comprised of wide bandgap oxide semiconductors or insulators and refractory metals, are being considered for high temperature thermoelectric devices. Very large and repeatable Seebeck coefficients have been observed using oxide semiconductor-NiCoCrAlY nanocomposites. Specifically, (n-type) ITO-NiCoCrAlY nanocomposites, (p-type) ITO-NiCoCrAlY nanocomposites, Al2O3-NiCoCrAlY nanocomposites and ZnO-NiCoCrAlY nanocomposites have been prepared by co-sputtering techniques and the thermoelectric response of these materials has been measured relative to platinum and other nanocomposites. Combinatorial chemistry techniques were used to screen the thermoelectric response of the candidate oxide semiconductor-refractory metal nanocomposites. Combinatorial nanocomposite libraries were patterned over platinum reference electrodes using microlithography techniques, and a temperature gradient was applied across the hot and cold junctions of the resulting small footprint thermocouples, generating an electromotive force. The chemical composition of the most promising combinatorial nanocomposite libraries (in terms of thermoelectric response) was analyzed by SEM/EDS. A sign change was observed in the screening experiments indicating a transition from an "n-type" to a "p-type" material with increasing NiCoCrAlY content in the ITO based nanocomposite. When a "p-type" ITO-NiCoCrAlY nanocomposite was combined with platinum to form a metal-ceramic junction, a Seebeck coefficient on the order of 1100 μV/°C was obtained (maximum electromotive force of 350 mV was achieved at a hot junction temperature of 1210 °C). When the "p-type" ITO-NiCoCrAlY nanocomposite was combined with n-type ITO, an electromotive force of 2000 mV was achieved for a hot junction temperature of 1140 °C and when combined with an n-type ITO-NiCoCrAlY nanocomposite, an electromotive force of 3400 mV was achieved for a hot junction temperature of 1160 °C. Other bi-ceramic junctions based on ITO and ZnO, also exhibited a very large thermoelectric response (1100 mV) at 1160 °C. In all cases the cold junction was maintained at room temperature, ~25°C. Seebeck coefficients of 10mV/°C were achieved for these junctions after repeated thermal cycling to 1200 °C. Therefore, ZnO-NiCoCrAlY nanocomposites have been prepared by the same co-sputtering techniques and the thermoelectric response relative to "n-type" ITO-NiCoCrAlY nanocomposites was investigated. The electrical and thermal properties of these materials are also being investigated, but the thermoelectric response of these selected metal-oxide semiconductor nanocomposites measured to date are large enough to warrant their investigation for use in energy harvesting devices.
11:45 AM - U1.7
Change in the Thermoelectric Properties with the Variation in the Defect Structure of ReSi1.75.
Shunta Harada 1 , Katsushi Tanaka 1 , Kyosuke Kishida 1 , Norihiko Okamoto 1 , Haruyuki Inui 1
1 Department of Materials Science & Engineering, Kyoto University, Kyoto Japan
Show AbstractSemiconducting rhenium silicide, ReSi1.75, is of interest owing to its potentials as a promising candidate material for thermoelectric applications. The dimensionless figure of merit (ZT) for binary rhenium silicide is as high as 0.7 at 1073K along [001]C11b. This value is higher than silicon-germanium alloy, which is practically used. The crystal structure and the stoichiometry of rhenium silicide are very controversial. The crystal structure of rhenium silicide has been reported as a tetragonal C11b (MoSi2-type, space group I4/mmm) structure and the stoichiometry was thought to be Re:Si=1:2. And other kinds of crystal structures and stoichiometry have been reported by several researchers. However, according to our recent analysis by electron diffraction by transmission electron microscopy (TEM) and high-angular annual dark field scanning transmission electron microscopy (HAADF-STEM), the crystal structure is monoclinic with the space group of Cm due to an ordered arrangement of vacancies in silicon sites in the underlying C11b structure. The reason rhenium silicide possesses such complex crystal structure with a lot of vacancies in silicon sites is that rhenium silicide belongs to the group of semiconductor with the 14 valence electron count (VEC) per metal atom. Therefore the concentration of vacancies in silicon sites is expected to change by substituting an element of rhenium and/or silicon with other elements having more or less number of valence electron. In the present study, by using transmission electron microscopy and high resolution electron microscopy (HREM), we investigate the crystal structure variations of rhenium silicide by substituting rhenium and/or silicon with ternary elements. At the same time, we investigate the effect on the thermoelectric properties by substituting rhenium and/or silicon with ternary elements. In addition we found out that the crystal structure and thermoelectric properties of binary rhenium silicide also changes by the heat treatment. Through these experiments, we figured out the significant relationship between defect structure and thermoelectric properties of rhenium silicide.
12:00 PM - U1.8
Thermoelectric Energy Recycling for Green IT Applications.
Sang Mock Lee 1 , Jong Soo Rhyee 1 , Sang Il Kim 1 , Kyu Hyoung Lee 1
1 Materials lab, Sansung Advanced Institute of Technology, YongIn Korea (the Republic of)
Show Abstract12:15 PM - U1.9
Phase Stability and Thermoelectric Properties ofHalf-Heusler (Ma0.5, Mb0.5)NiSn (Ma, Mb = Hf, Zr, Ti).
Yoshisato Kimura 1 , Hazuki Ueno 1 , Takahiro Kenjo 1 , Chihiro Asami 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 are focusing on half-Heusler compounds MNiSn (M = Hf, Zr, Ti), well-known n-type thermoelectric materials, which can be used at around 1000 K to directly convert waste heat into clean electrical power. Half-Heusler compounds have excellent electrical properties, i.e., high absolute values of Seebeck coefficient and low electrical resistivity. On the other hand, relatively high thermal conduction is a disadvantage of half-Heusler compounds. Many research groups, including ours, have succeeded to reduce the lattice thermal conductivity by substituting M site elements between the group 4 elements Ti, Zr and Hf. It is called as the solid solution effect since the differences in atomic mass and atomic size in a solid solution effectively lower the lattice thermal conduction through enhancing phonon scattering. In the present work, to evaluate actual effects of the M site substitution on thermal conduction, nearly single-phase (Ma0.5, Mb0.5)NiSn alloys were fabricated by directional solidification using the floating zone melting method, where Ma and Mb are any two of Hf, Zr and Ti. Seebeck coefficient, electrical resistivity, thermal conductivity and carrier concentration were measured in a temperature range from 300 to 1073 K. The lattice thermal conductivity can most effectively be reduced in (Ti0.5, Hf0.5)NiSn alloys most probably by the solid solution effect since the differences in atomic mass and atomic radius are maximized in the case between Ti and Hf. Moreover, we have found a tendency of the two-phase separation between Ti-rich and Ti-poor half-Heusler (Ti, Hf)NiSn phases in alloys with a nominal composition of (Ti0.5, Hf0.5)NiSn through microstructure observation and chemical concentration measurement using electron probe microanalysis. A similar tendency of the two-phase separation has also been observed in (Ti0.5, Zr0.5)NiSn alloys. We have confirmed the two-phase separation between half-Heusler phases using the powder X-ray diffractometry conducted on (Tix, Zr1-x)NiSn and (Tix, Hf1-x)NiSn alloys (x = 0 to 1). Broadening or splitting of diffraction peaks were clearly observed depending on alloy compositions. On the other hand, we previously reported that Zr and Hf are all-proportion miscible in (Zr, Hf)NiSn as a continuous solid solution phase. Related phase equilibria were also considered by the calculation of phase stability. It should be noted that the lattice thermal conduction can be reduced by microstructure factors due to the two-phase separation, such as interfaces between Ti-rich and Ti-poor (Ti, Hf)NiSn phases. Regarding the performance of thermoelectric properties, The (Zr0.5, Hf0.5)NiSn alloy has the best dimensionless figure of merit of all the alloys investigated because it has the best balance of the highest electrical power factor together with a reasonably low thermal conductivity.
12:30 PM - U1.10
Fabrication of CoTiSb, NbNiSn and FeMoSb Half-Heusler Phases for Thermoelectric Applications.
Wilfried Wunderlich 1 , Yuichiro Motoyama 1
1 Dep. Mat. Sci, Tokai University, Faculty of Engineering, Hiratsuka-shi, Kanagawa-ken, Japan
Show AbstractHalf-Heusler phases have gained recently much interest as thermoelectric materials. Screening of possible systems was performed by ab-initio simulation using VASP-software. The energy-versus-Volume (E(V)) curves were calculated and calibrated. For CoTiSb, NbNiSn, FeMoSb and others, the stability of Half-Heusler phase against concurrent crystal structures like TiNiSi, ZrCoAl, ZrBeSi, FeSiV and Full Heusler was confirmed. However, the thermo-dynamical driving force as calculated from the difference in lattice energies is less than 0.1eV/atom. Hence, the fabrication of Half Heusler phases is a challenge and requires three steps, surface activation of the raw material by ball milling, arc-melting of pressed pellets and finally long-term annealing treatment in a vacuum furnace. For the CoTiSb system, diffusion couple experiments clarified the complicated diffusion mechanism, which can lead to constitutional vacancies. The FeMoSb system lacks in slow Mo-diffusion. The specimens were characterized by SEM-EDS chemical mapping and Seebeck voltage with open and closed electric circuit were measured in a self-built device at temperature differences up to 500K. On doped CoTiSb specimens, Seebeck coefficients up to 0.1 mV/K, on NiNbSn 0.3 mV/K were measured, although the microstructure was not yet optimized.
12:45 PM - U1.11
Mechanical and Thermal Properties of Single Crystals of Some Thermoelectric Clathrate Compounds.
Norihiko Okamoto 1 , Takahiro Nakano 1 , Kyosuke Kishida 1 , Katsushi Tanaka 1 , Haruyuki Inui 1
1 Materials Science and Engineering, Kyoto University, Kyoto Japan
Show AbstractClathrate compounds have stimulated a great deal of interest as promising candidates for practical thermoelectric materials because they exhibit relatively high electrical conductivity and very low lattice thermal conductivity. These properties are considered to originate from their particular crystal structures consisting of polyhedral cages filled with a guest atom. The cages, which consist of tetrahedrally-bonded group IV/III elements, are considered to play an import role in electrical conduction while the guest atoms, which are typically alkali metals or alkaline-earth metals, are weakly bonded to the cages and vibrating inside the oversized cages to efficiently scatter the heat-carrying phonons. Among a number of clathrate compounds with various structure types, the “type-I” clathrate compounds with the cubic space group Pm-3n have been investigated the most intensively. Knowledge on the mechanical properties of these type-I clathrate compounds is indispensable when their practical application in thermoelectric devices is considered, since these clathrate compounds will inherently be subjected to thermal stresses arising mostly from their own thermal expansion and contraction within thermoelectric devices. To the best of our knowledge, however, almost nothing is known about the mechanical properties of many candidates for thermoelectric materials including these type-I clathrate compounds Ba8Ga16Ge30 and Sr8Ga16Ge30, except for low-temperature elastic constants for a few clathrate compounds. In this study, the mechanical and thermal properties of single crystals of the type-I clathrate compounds Ba8Ga16Ge30 and Sr8Ga16Ge30 have been investigated by measuring the elastic constants, coefficients of thermal expansion (CTE) and plastic deformation behavior in compression. The values of bulk moduli extrapolated to 0 K for Ba8Ga16Ge30 and Sr8Ga16Ge30 are almost identical to each other, whereas the values of Young and shear moduli for Ba8Ga16Ge30 are larger than those for Sr8Ga16Ge30. The values of CTE for Ba8Ga16Ge30 and Sr8Ga16Ge30 are virtually identical with each other. The feasibility of these two clathrate compounds as a thermoelectric material in terms of mechanical stability under possible thermal stresses is evaluated by calculating thermal stresses that are expected to develop within these compounds when used as a thermoelectric material in thermoelectric devises.
U2: Iron Aluminides - Physical Metallurgy, Processing, and Properties
Session Chairs
Ian Baker
Kyosuke Yoshimi
Monday PM, December 01, 2008
Constitution A (Sheraton)
2:30 PM - **U2.1
An Overview of the Properties of Iron Aluminides.
Ian Baker 1
1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States
Show AbstractAn overview of the properties of iron aluminides such as Fe3Al, Fe2MnAl and FeAl will be presented. These compounds exist over a wide range of composition and, thus, alloy stoichiometry is a key variable in their behavior. Several key phenomena will be discussed in detail, viz, the so-called yield anomaly, environmental embrittlement, and strain-induced ferromagnetism, in terms of dislocation behavior and the role of anti-site atoms and vacancies. Recent work on developing these materials for commercial applications will also be discussed.Research sponsored by NSF grant DMR 0552380 and DOE contract DE-FG02-07ER46392.
3:00 PM - U2.2
Forging of Steam Turbine Blades with an Fe3Al-based Alloy.
Peter Janschek 2 , Knut Bauer-Partenheimer 2 , Ronny Krein 1 , Martin Palm 1
2 , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany, 1 , Leistritz Turbinenkomponenten Remscheid GmbH, Remscheid Germany
Show AbstractForging of steam turbine blades with an Fe3Al-based alloyP. Janschek1, K. Bauer-Partenheimer1, R. Krein2, Palm21 Leistritz Turbinenkomponenten Remscheid GmbH, Lempstrasse 24, D-42859 Remscheid, Germany2 Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Strasse 1, D-40237 Düsseldorf, GermanyFe-Al-based alloys are light weight materials with outstanding corrosion behaviour. Recent advances in strengthening these alloys at high temperatures now make them suitable for applications in extreme environments such as steam turbines for power generation.In a preliminary study cylinders of 25 mm diameter and 28 mm height of two different Fe3Al-based alloys were forged between 900 and 1150 °C. The alloys behaved quite differently. Fe-25Al-20Ti-4Cr [1] (all compositions in at.%) did hardly deform even at the highest temperatures and showed numerous cracks after forging. In contrast, Fe-25Al-xTa [2,3] could easily be forged without any formation of cracks and the forged pancakes were regular shaped.From the latter alloy precursors 660 mm long each weighing about 12 kg have been produced by investment casting. The castings showed good form filling, were free of pipes and pores and had smooth surfaces. Because no measures for grain refinement were undertaken columnar grains were found in the castings which made them rather brittle.The precursors were heated up to 1100 °C and then forged on a 630 kJ counterblow hammer with 10 blows each. The dies were standard tooling used for commercially produced steam turbine blades.The forged steam turbine blades again showed very good form filling and smooth surfaces. The blades were finished by cutting off the burrs on a band saw and then grinding the edges by standard procedures. No heat treatment has been applied.Samples were cut out of certain parts of the forged turbine blades and the microstructures were examined by metallography. Basic mechanical properties such as hardness, yield stress and brittle-to-ductile transition temperatures were evaluated and are compared with those of the as-cast material.[1] R. Krein, M. Palm, Acta Mater. 56 (2008) 2400.[2] D.D. Risanti, G. Sauthoff, Intermetallics 13 (2005) 1313.[3] D.D. Risanti, G. Sauthoff, Mater. Sci. Forum 475-479 (2005) 865.
3:15 PM - U2.3
Grain Refinement for Strengthening in Fe3Al-based Alloys through Thermomechanical Process.
Satoru Kobayashi 1 , Akira Takei 2 , Takayuki Takasugi 2 1
1 , Tohoku University, Sakai Japan, 2 , Osaka Prefecture University, Sakai Japan
Show AbstractA thermomechanical process to form deformed/recovered grain structure is known to be useful in improving room temperature strength and ductility of Fe3Al-based alloys. Grain refinement, on the other hand, is also expected to improve those properties. Recently we achieved in reducing grain size to ~20μm through warm rolling and recrystallization process in Fe3Al-based alloys containing ~0.1 volume fraction of κ-Fe3AlC precipitates. Effects of grain refinement and microstructure on tensile properties were examined between room temperature and 600°C in such alloys. Three grain-size levels were prepared for a recrystallized grain structure and a deformed/recovered grain structure by changing warm rolling and/or annealing conditions. Ductility and UTS increased with decreasing grain size in each grain structure at room temperature. The properties obtained in the deformed/recovered structure were higher than those in the recrystallized structure, and the former structure with an averaged grain size of ~20μm showed more than 1200MPa tensile strength and 8% tensile ductility at room temperature in air. The strength of the structure was higher than that of conventional wrought Fe3Al alloys below 500°C and the specific tensile strength was as high as that of Ti-6Al-4V alloy at temperatures around 400°C. Correlation between microstructure, texture and tensile properties will be discussed.
3:30 PM - U2.4
Ab Initio Study on Elastic Properties of Fe3Al-based Alloys.
Martin Friak 1 , Johannes Deges 1 , Ronny Krein 1 , Frank Stein 1 , Martin Palm 1 , Georg Frommeyer 1 , Joerg Neugebauer 1
1 , Max-Planck-Institut fuer Eisenforschung, GmbH, Duesseldorf Germany
Show AbstractFe3Al-based alloys constitute a very promising class of intermetallics with great potential for substituting austenitic- and martensitic steels at elevated temperatures. A wider use of these materials is partly hampered by their moderate ductility at ambient temperatures. Theoretical ab initio based calculations are becoming increasingly useful to materials scientists interested in designing new alloys. Such calculations are nowadays able to accurately predict basic material properties by providing only the atomic composition of the material. We have therefore employed this approach to explore (i) the relation between chemical composition and elastic constants, as well as (ii) the effect transition-metal substituents (Ti, V, W, Cr, Mn) have on this relation. Using a scale-bridging approach we model the integral elastic response of Fe3Al-based polycrystals employing a combination of (i) single crystal elastic stiffness data determined by parameter-free first-principles calculations in combination with (ii) Hershey’s homogenization model. The ab initio calculations employ density-functional theory (DFT) and the generalized gradient approximation (GGA). The thus determined elastic constants have been used to calculate the ratio between the bulk and shear moduli as an indication of brittle/ductile behavior. Based on this approach we have explored chemical trends and identified optimal composition to achieve tailored mechanical properties. Using this information we have cast a selected set of Fe3Al-based ternaries, obtained for these the elastic constants by performing impulse excitation measurements at room as well as liquid nitrogen temperature and compared them with our theoretical results
3:45 PM - U2.5
Mapping the Phase Diagram Al-Fe-Ni-Ti using the Diffusion Multiples Technique.
Liliana Duarte 1 2 , Christian Leinenbach 1 , Ulrich Klotz 3 , Joerg Loeffler 2
1 Department Advanced Materials and Surfaces, Laboratory for Joining and Interface Technology , EMPA, Swiss Federal Laboratories for Materials Testing and Research, Duebendorf Switzerland, 2 Department of Materials, Laboratory of Metal Physics and Technology, ETH Zurich, Zurich Switzerland, 3 Department of Physical Metallurgy, FEM, Research Institute for Precious Metals and Metals Chemistry, Schwaebisch Gmuend Germany
Show Abstract4:30 PM - U2.6
Microstructure and Mechanical Behavior in High Strength Nanostructured Spinodal FeNiMnAl Alloys.
Xiaolan Wu 1 , Ian Baker 1 , Yifeng Liao 1 , Michael Miller 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 AbstractIn order to understand the microstructure and mechanical behavior of some recently-discovered, very high strength FeNiMnAl spinodal alloys, an ingot of Fe35Ni15Mn25Al25 was drop cast and directionally solidified under Ar using a modified Bridgman furnace. TEM studies showed that the as-cast alloy had a periodic coherent microstructure consisting of alternating B2 (ordered BCC) and BCC phases. EDS showed that the BCC phase was rich in Fe and Mn, while the B2 phase was rich in Ni and Al, features confirmed by analysis using a Local Electrode Atom Probe. Hardness measurements were performed on the alloy as a function of annealing time at 550°C. The directionally solidified alloy showed a steady increase in hardness from 437 HV with annealing time, but the as-cast alloy, which was initially harder at 523 HV, showed more complex behavior. The final hardness after 72 h anneals was very similar at 676 HV for the two initial conditions. The paper will attempt to relate the mechanical properties to the changes in microstructure.Research was supported by DOE Award #DE-FG02-07ER46392
4:45 PM - U2.7
Microstructure and Mechanical Properties of a Eutectoid FeNiMnAl Alloy.
Yifeng Liao 1 , Ian Baker 1
1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States
Show AbstractThe microstructure and mechanical properties of an as-cast eutectoid alloy with nominal composition Fe-20Ni-35Mn-15Al (at. %) have been investigated. The eutectoid transformation was shown by DTA to occur at 1502 K. The microstructure consisted of alternating 200-300 nm wide B2 (ordered body-centered cubic) and ~500 nm wide FCC lamellae, and showed good stability at temperatures up to 1033 K when heated in a TEM. The alloy exhibited a yield stress of ~750 MPa and an elongation of ~8% under tension at room temperature. Both ductile dimpled and brittle cleavage regions were observed on the resulting fracture surface. TEM in-situ straining experiments showed that deformation occurred by the glide of <110> dislocations in the FCC phase, which piled up at the interfaces with the B2 phase. Few dislocations were observed in the B2 lamellae both during TEM in-situ straining and in post-mortem TEM examinations of tensile-tested specimens. Determination of the yield stress as a function of temperature showed that it dropped only slightly to ~730 MPa at 600K but fell to ~208 MPa at 900K. The effects of different heat treatments on the mechanical properties will also be discussed.
5:00 PM - U2.8
Effect of Transition Metal Additions on B2↔A2 Order-Disorder Phase Transition Temperatures in B2 Type FeAl Intermetallic Compounds.
Mehmet Yildirim 1 , M.Vedat Akdeniz 1 , Amdulla O. Mekhrabov 1
1 Department of Material Science and Metallurgical Engineering, Middle East Technical University, Ankara Turkey
Show AbstractFe-Al intermetallic compounds are the most suitable candidates for high temperature applications due to their excellent physical, mechanical and oxidation properties in addition to their low material cost and low density. These unique properties are attributed to the long-range ordered (LRO) superlattices and strongly depend on deviation from alloy stochiometry, content and type of alloying element additions. The effect of content and type of the ternary alloying element additions on the order-disorder phase transition temperature of B2-type ordered Fe0.5(Al1-nXn)0.5 intermetallics have been determined by experimental methods. In this study B2-type ordered Fe0.5(Al1-nXn)0.5 intermetallics, where X= Cr, Ni, Nb, Hf, Mo, Ta, W, Mn and Zr are considered at n=0.01, 0.03 and 0.05 at.% concentrations, were prepared by arc melting technique and characterized by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). The order-disorder phase transition temperatures of both as-cast and heat-treated samples were determined by differential scanning calorimetry (DSC) measurements at different cooling and heating rates. Comparison of B2↔A2 order-disorder phase transition temperatures with theoretical predictions that were presented in our previous study shows good qualitative agreement with most X ternary alloying elements
5:15 PM - U2.9
Magnetism of Mechanically Deformed Powder Fe-Al Alloys.
Estibaliz Apinaniz 1 , Estibaliz Legarra 2 , Fernando Plazaola 2 , Damian Martin-Rodriguez 4 , Jose Javier Saiz Garitaonandia 3
1 Fisika Aplikatua I Saila, University of the Basque Country (UPV-EHU), Bilbao Spain, 2 Elektrizitate eta Elektronika Saila, University of the Basque Country (UPV-EHU), Bilbao Spain, 4 , ANSTO, Menai, Australian Capital Territory, Australia, 3 Fisika Aplikatua II Saila, University of the Basque Country (UPV-EHU), Bilbao Spain
Show Abstract5:30 PM - U2.10
Cracking in the Cast Fe-40Al-1C Alloys.
Jaromir Kopecek 1 , Jan Drahokoupil 1 , Karel Jurek 2 , Marketa Jarosova 2 , Petr Hausild 3 , Vladimir Sima 4 , Pavel Lejcek 1
1 Department of Metals, Institute of Physics of the AS CR, Prague Czechia, 2 Department of Structure Analysis , Institute of Physics of the AS CR, Praha Czechia, 3 Department of Materials, Faculty of Nuclear Sciences and Physical Engineering of the Czech Technical University, Prague Czechia, 4 Department of Physics of Materials, Faculty of Mathematics and Physics of the Charles University, Prague Czechia
Show Abstract5:45 PM - U2.11
Ultrasonic Vacuum Chill Casting and Hot Rolling of FeAl Based Alloys.
Vladimir Sima 1 , Premysl Malek 1 , Petr Kozelsky 2 , Ivo Schindler 2 , Petr Hana 3
1 Department of Physics of Materials, Charles University in Prague, Prague Czechia, 2 Faculty of Metallurgy and Material Engineering, VŠB - Technical University of Ostrava, Ostrava Czechia, 3 Department of Physics, Technical University of Liberec, Liberec Czechia
Show Abstract
Symposium Organizers
Bernard Bewlay General Electric Company
Yue-hui He Central South University
Martin Palm Max-Planck Institut fuer Esenfoschung GmbH
Masao Takeyama Tokyo Institute of Technology
Joerg Wiezorek University of Pittsburgh
U3: Titanium Aluminides I - Physical Metallurgy, Processing and Properties
Session Chairs
Bernard Bewlay
Masao Takeyama
Tuesday AM, December 02, 2008
Constitution A (Sheraton)
9:15 AM - **U3.1
Solidification of TiAl-based Alloys.
Ulrike Hecht 1 , Dominique Daloz 2 , Anne Drevermann 1 , Juraj Lapin 3 , Victor Witusiewicz 1 , Julien Zollinger 1
1 , ACCESS e.V., Aachen Germany, 2 LSG2M, Ecole des Mines de Nancy, Nancy France, 3 Institute of Materials and Machine Mechanics, Slovak Institute of Sciences, Bratislava Slovakia
Show AbstractNear net shape casting of TiAl-based alloys proved to be an attractive processing route for small aero-engine and automotive parts. Worldwide efforts are being made to increase not only the reliability of casting processes and the quality of cast parts, but also the size of the castings: large aero-engine blades and also industrial gas turbine blades are at the heart of today’s developments. Within the European integrated project IMPRESS*, the R&D activities related to TiAl processing equally push in this direction. The main research topics of IMPRESS will be briefly outlined. Naturally, one of the central topics relates to microstructure formation during solidification. Knowledge about the solidification behavior of TiAl-based alloys as function of alloy composition and cooling conditions is essential. Several questions emerge: they regard (i) the sequence of phase formation along the solidification path, (ii) the microsegregation that develops due to element partitioning between theses phases and diffusion inside these phases and (iii) the role of microsegregation during subsequent phase transformations in the solid state.In this presentation we give an overview of solidification characteristics of niobium containing TiAl-based alloys: in the first part of the presentation we present unidirectional solidification experiments that allowed freezing-in the sample region formed upon solidification and we show the results of microsegregation analysis for alloys with and without peritectic α(Ti) formation. The experimental results are compared to thermodynamic calculations of the solidification path, using recent thermodynamic descriptions elaborated for the ternary Ti-Al-Nb and the quaternary Ti-Al-Nb-B system, respectively.In the second part of the presentation we discuss the influence of the solidification path and of the associated microsegregation on the subsequent phase transformation, e.g. on growth of hcp α(Ti) from the parent bcc β(Ti) phase. This high temperature transformation is important for alloys with primary β(Ti) solidification, because it determines the final grain size of the material. * IMPRESS – Acronym of the European Integrated Project “Intermetallic Materials Processing in Relation to Earth and Space Solidification”
9:45 AM - U3.2
Grain Refinement of γ-TiAl Based Alloys by Inoculation.
Daniel-Hendrik Gosslar 1 , Robert Guenther 1 , Christian Hartig 1 , Ruediger Bormann 1 , Julien Zollinger 2 , Ingo Steinbach 3
1 Institute of Materials Science and Technology, Hamburg University of Technology, Hamburg Germany, 2 , Access e.V., Aachen Germany, 3 Interdisciplinary Centre for Advanced Materials Simulation, Ruhr-University Bochum, Bochum Germany
Show AbstractCasting of γ-TiAl based alloys in the composition range of 43 to 46 at.% Al would greatly benefit from the possibility of refining the primary β(Ti) phase by an appropriate inoculation of the melt. At present however, no efficient grain refinement for β(Ti) is available. In this presentation we describe first results, meant to identify and test potential grain refiners for both, the primary β(Ti) phase and the peritectic α(Ti) phase.Model calculations based on heterogeneous nucleation enable the simulation of grain size evolution. The inoculant particle size distribution, the cooling rate and the alloy constitution are the main input quantities for this model calculation.The incorporation of inoculant particles into the melt can be accomplished by a refiner master alloy. Hereby, the issue of concern is regarded to the particle stability inside the melt with respect to both, chemical stability and clustering or sedimentation processes. This may alter the particle distribution and affect strongly the model calculations. High energy ball milling has proven to be a favorable preparation method for master alloys, leading to a homogenous distribution of inoculants within a metallic matrix. Potential inoculants for the β(Ti) and / or the α(Ti) phase can be characterized based on lattice mismatch calculations. A high potency is indicated, among other physical and chemical factors, by a low lattice mismatch between substrate and nucleus. Therefore TiB2 could act as a potent substrate for heterogeneous nucleation of α(Ti) and β(Ti), if the volumetric content in the melt is sufficiently high. Adjustment of the TiB2 particle size distribution is obtained by varying milling parameters such as milling time and intensity. For model calculations the needed particle size distribution has been evaluated by SEM pictures. First results on the behavior of the master alloy dissolved in a TiAl-melt will be discussed.
10:00 AM - U3.3
Unidirectional Solidification and Single Crystal Growth of Al-rich Ti-Al Alloys.
Anne Drevermann 1 , Georg Schmitz 1 , Günter Behr 2 , Christo Guguschev 2 , Nico Engberding 3 , Martin Palm 3 , Frank Stein 3 , Martin Heilmaier 4 , Daniel Sturm 4
1 , ACCESS e.V., Aachen Germany, 2 , Leibniz-Institut für Festkörper- und Werkstoffforschung, Dresden Germany, 3 , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany, 4 , Otto-von-Guericke-Universität Magdeburg, Magdeburg Germany
Show AbstractTi-Al alloys with an aluminium content of 60 ± 2 at.% Al are considered as promising candidates for new materials and components. Compared to today’s gamma-Titaniumaluminides they offer increased oxidation resistance at high temperatures and simultaneously decreased weight.To investigate the basic mechanical and thermomechanical properties of these new materials by tensile tests, bending tests etc. sufficiently large single crystalline domains are required. To generate these sufficiently large single crystalline domains with diameters of a few millimetres and a few centimetres of length we applied the methods of unidirectional solidification of a Ti - 60 at.% Al alloy in a Bridgman Stockbarger furnace and in an optical floating zone furnace equipped with a 5 kW Xenon lamp.Several kilograms of master alloy were produced by Vacuum Arc Remelting and precursor rods for the directional solidification/ floating zone experiments were produced by re-melting the alloy in a cold-wall induction crucible and subsequent centrifugal casting into a permanent Niob mould.Experiments in the Bridgman-Stockbarger furnace were performed to optimise grain selection and minimise oxidation during directional solidification. Variable parameters were solidification velocity, temperature gradient and crucible material. Using a seed-crystal originating from one of these experiments turned out to be beneficial to generate large domains. Parallel to these experiments single crystal material was grown in an optical floating zone furnace.The focus of this article is on samples grown by unidirectional solidification with a Bridgman-Stockbarger furnace describing the process development in detail with respect to grain selection, nucleation on seed crystals, oxidation of the samples during processing, effects of different crucibles and pre-treatments. Respective results are compared to samples processed by an optical floating zone process.
10:15 AM - **U3.4
Development of TiAl Alloys by Spark Plasma Sintering.
Alan Couret 1 , Houria Jabbar 1 , Jean-Philippe Monchoux 1 , Marc Thomas 2
1 CEMES, CNRS, Toulouse France, 2 DMSN, ONERA, Chatillon France
Show Abstract10:45 AM - U3.5
Hot-die Forging of a β-stabilized γ-TiAl Based Alloy.
Wilfried Wallgram 1 , Sascha Kremmer 1 , Helmut Clemens 2 , Andreas Otto 3 , Volker Guether 3
1 , Bohler Schmiedetechnik GmbH&CoKG, Kapfenberg Austria, 2 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria, 3 , GfE Metalle und Materialien GmbH, Nuremberg Germany
Show Abstract11:00 AM - U3.6
Experimental Studies and Thermodynamic Simulations of Phase Transformations in Ti-(41-45)Al-4Nb-1Mo-0.1B Alloys.
Helmut Clemens 1 , Barbara Boeck 1 , Wilfried Wallgram 2 , Laura Droessler 1 , Gerald Zickler 3 , Harald Leitner 3 , Sascha Kremmer 2 , Andreas Otto 4
1 Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria, 2 , Bohler Schmiedetechnik GmbH&CoKG, Kapfenberg Austria, 3 , Christian Doppler Laboratory for Early Stages of Precipitation, Leoben Austria, 4 , GfE Metalle und Materialien GmbH, Nuremberg Germany
Show Abstract11:30 AM - **U3.7
The Prospects of TiAl Application for the Aerospace and the Automotive Industries.
Sadao Nishikiori 1
1 Production Technology Development Dept, IHI, Yokohama Japan
Show Abstract Gamma titanium aluminides, TiAl, have been considered as alternate material, replacing the existing Ni-base alloys. In order for TiAl alloys to be widely employed in structural materials, we have investigated the possibility of using. In 1995, ground engine test was performed with LPT #5 blades cast from the TiAl alloys, GE48-2-2 developed by GE, along with IHI alloy. We have developed a manufacturing process of both alloys which can supply sound blades on actual operation level. A lot of production technologies have shown great growth through this experience for the decade of the 90’s. With increased demands from the automotive industry, subsequently, other alloys were developed, including TiAl alloys which has a fully lamellar structure to improve creep strength up to approximately 1100K-1200K range. We have fabricated passenger vehicle turbochargers using TiAl turbine wheels since 2004. As of today, a total number of this product is well over 100,000 units. New production methods which add to the applicability of TiAl are indispensable in actual use, such as the blades of aero-engines and the impellers of automotive turbochargers and must be economically manufacturable. The present status of TiAl application for the aerospace and the automotive industries is reviewed.
12:00 PM - U3.8
Microstructure Stability and Tensile Ductility of a Ti-43Al-4Nb-1Mo-0.1B Alloy.
Laura Droessler 1 , Limei Cha 1 , Thomas Schmoelzer 1 , Wilfried Wallgram 2 , Gopal Das 3 , Helmut Clemens 1
1 Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben Austria, 2 , Bohler Schmiedetechnik GmbH&CoKG, Kapfenberg Austria, 3 , Pratt & Whitney, East Hartford, Connecticut, United States
Show Abstract12:15 PM - U3.9
Unique Plastic Deformation Behavior in TiAl-PST Crystals with Aligned Elongated Pores.
Takayoshi Nakano 1 , Takahiro Tachibana 1 , Koji Hagihara 1 , Yukichi Umakoshi 1 , Patrick Veyssiere 2
1 Course of Materials Science & Engineering, Division of Materials & Manufacturing Science, Graduate School of Engineering, Osaka University, Suita, Osaka, Japan, 2 Laboratorie d’Etude des Microstructures, CNRS-ONERA, Chatillon cedex France
Show Abstract12:30 PM - U3.10
Effect of Microstructural Parameters on Toughness of Ti-Al-Nb-Cr Alloys.
Michael Kesler 1 , Sonalika Goyel 1 , Orlando Rios 1 , Hans Seifert 2 1 , Fereshteh Ebrahimi 1
1 Materials Science and Engineering, University of Florida, Gainesville , Florida, United States, 2 , Technische Universitaet Bergakademie Freiberg, Freiberg Germany
Show Abstract Titanium Aluminides are candidate materials for aerospace turbine engine applications due to their high strength to weight ratio and strength retention at high temperatures. Currently, high Nb γ+α2 alloys are being considered, but their applications are limited to relatively low temperatures because of loss of creep resistance. It has been shown that alloys consisting of γ-TiAl and σ-Nb2Al phases render excellent high temperature creep resistance; however, these alloys have unacceptable room temperature toughness and tensile ductility. These properties are anticipated to be strongly dependent on the volume fraction and connectivity of the hard and brittle σ-phase. In this study, alloys based on Ti-Al-Nb-Cr were designed and prepared by arc melting. The compositions of these alloys were selected such that different volume fractions of the σ-phase were obtained during the aging process. These alloys solidified as a single β-phase, wherein they could be solutionized and retained upon quenching. DTA was conducted to determine phase transformation temperatures. Based on these results temperatures for solutionizing and aging heat treatments were identified. The scale and morphology of the microstructure was controlled by varying the aging temperature. The results revealed that the morphology of the γ-phase is sensitive to whether this phase precipitates before, simultaneously or after σ-phase formation. Furthermore, the heating rate during aging significantly affected the nucleation rate and hence the size of the phases. It was also found that the σ-phase does not dissolve easily upon heating to the β+γ phase region, a quality which is desirable for creep resistance. Apparent toughness of the different microstructures was evaluated by indentation technique. In this presentation, the dependence of toughness on the volume fraction and particle size of σ as well as its dependence on the γ-phase morphology will be elucidated.The supported by NSF/AFOSR under grant number DMR-0605702 is greatly appreciated.
12:45 PM - U3.11
An Overview of the ESA-ESRF-ILL Collaboration in the Framework of the IMPRESS Integrated Project.
Guillaume Reinhart 1 2 3 , Gail Iles 1 2 3 , Garry McIntyre 1 , Andrew Fitch 2 , David Jarvis 3
1 ILL diffraction group, ILL (Institut Laue-Langevin), Grenoble France, 2 Material science group, ESRF (European Synchrotron Radiation Facility), Grenoble France, 3 Physical Science Unit, Directorate of Human spaceflight, Microgravity and exploration, ESA/ESTEC, Noordwijk Netherlands
Show AbstractThree intergovernmental research organisations from the EIROforum collaboration : the European Space Agency (ESA), the European Synchrotron Radiation Facility (ESRF) and the Institut Laue-Langevin (ILL), are cooperating to perform advanced experimental characterisation in the field of materials science within the framework of the IMPRESS integrated Project. IMPRESS is an acronym for Intermetallic Materials Processing in Relation to Earth and Space Solidification. This project is coordinated by ESA within the European Commission’s 6th Framework Programme (FP6). The main scientific objective of IMPRESS is to gain a better understanding of the links between material processing routes, structures, and final properties of intermetallic alloys. The project aims to develop and test two distinct prototype-based intermetallic materials : (i) γ-TiAl turbine blades for aero-engines and stationary gas turbines, and (ii) Raney-type Ni-Al catalytic powder for use in hydrogen fuel cell electrodes and hydrogenation reactions.The opportunity to carry out investigations combining the use of both synchrotron radiation at ESRF and neutrons at ILL provides unique experimental data to complement other benchmark experiments performed on the ground and in microgravity. We present an overview of the different synchrotron X-ray and neutron characterisation techniques implemented at ESRF and ILL which are used to understand the way in which these materials form during solidification processes. Preliminary qualitative and quantitative analyses are reported. They show that precise and critical information on the studied systems can be collected, which can eventually be used as references to validate models and numerical simulations developed within the IMPRESS collaboration.
U4: Titanium Aluminides II - Structure, Properties, and Coatings
Session Chairs
Helmut Clemens
Tresa Pollock
Tuesday PM, December 02, 2008
Constitution A (Sheraton)
2:45 PM - U4.2
Precipitation Reactions in Ti40Al60 and Ti38Al62 Alloys and Symmetry Relations of the Phases Involved.
Klemens Kelm 1 2 , Stephan Irsen 2 , Anne Drevermann 3 , Julio Aguilar 3 , Georg Schmitz 3 , Martin Palm 4 , Frank Stein 4 , Nico Engberding 4 , Martin Heilmaier 5 , Daniel Sturm 5 , Holger Saage 5
1 Center for Materials Analysis, Technische Fakultät, Christian-Albrechts-Universität Kiel, 24143 Kiel Germany, 2 , Stiftung caesar, 53175 Bonn Germany, 3 , ACCESS e.V, 52072 Aachen Germany, 4 , Max-Planck-Institut für Eisenforschung GmbH, 40237 Düsseldorf Germany, 5 Institut für Werkstoff- und Fügetechnik, Otto-von-Guericke-Universität Magdeburg, 39016 Magdeburg Germany
Show AbstractIn the system Al-Ti, several phases occur at compositions between 50 at.-% Al and 66 at.-% Al. While at room temperature g-TiAl is the stable phase near 50 al.-% Al, r-Al2Ti is the stable phase near 66 at.-% Al up to 1215_C. Above this temperature, g-AlTi becomes the stable phase even at 66 at.-% Al. Usually, the metastable phases h-Al2Ti and Al5Ti3 first precipitate in the compositions. Furthermore, the metastable Phase is frequently observed in specimens with depicted compositional range. [1]Despite the fact that the crystallographic structures of all the mentioned compounds are known, the descriptions of their structural interrelationships are limited. While the transformation from h-Al2Ti to r-Al2Ti has been attributed to the elimination of non-conservative antiphase boundaries [2], the interrelations of Al5Ti3 and h-Al2Ti to g-AlTi have only been stated to be “superstructures” and the topological arrangement of building blocks of Al5Ti3 and h-Al2Ti has been analyzed [3].We studied the phase evolution of two alloys with compositions of 60 at.-% Al and 62 at.-% by in situ transmission electron microscopy in the range between room temperature and 900_C. Only the three phases g-TiAl, h-Al2Ti and Al5Ti3 have been observed besides some spurious oxidation effects. While in the sample containing 62 at.-% the h-Al2Ti phase persisted besides g-AlTi, in the 60 at.-% sample Al5Ti3 and h-Al2Ti precipitated from the g-AlTi matrix and dissolved within the matrix in dependency from sample temperature within minutes. By these experiments using transmission electron microscopy, we were able directly to follow the solution and re-precipitation steps of the involved superstructure phases for the first time. We attribute the appearance of the metastable phases to the kinetic effects of the ordering of the crystallographic structures. While h-Al2Ti and Al5Ti3 can be related to g-AlTi by a group-subgroup-relationship, the same is not possible for the r-Al2Ti phase. A relation by a group-subgroup-relationship means that the crystallographic positions of the atoms split due to symmetry reduction. Thus, diffusion only has to take place among these splitting positions, which in the case of Al5Ti3 and h-Al2Ti are confined to a single crystallographic plane. This means that the atomic motion necessary to achieve this ordering can be confined to the length of the unit cell dimensions. I contrast, the formation of the stable phase r-Al2Ti needs the coordinated three dimensional movement half of the atoms of every unit cell and as well as of half of the atoms of every crystal upon the transformation from h-Al2Ti, resulting in the long transformation times of several hours observed.[1] J. C. Schuster, M. Palm, Journal of Phase Equilibria and Diffusion 27, 255 (2006).[2] C. Loiseau, C. Vanuffel, phys. stat. sol. a, 107, 655 (1988).[3] U. D. Kulkarni, Acta mater. 46, 1193 (1998).
3:15 PM - U4.4
Observation of <2c+a> Dislocations Glide in Duplex Ti-48Al After Room Temperature Tensile Deformation.
Andreas Kulovits 1 , Jorg Wiezorek 1
1 MEMS, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractTiAl based engineering alloys offer attractive combinations of density-specific properties rendering them attractive for selection in structural components of advanced transportation systems to realize energy savings during operation. TiAl based alloys typically comprise at least two phases, namely the tetragonal ordered phase gamma-TiAl and a minority fraction of the hexagonal ordered phase alpha-2 Ti3Al. During general shape changes at room temperature both constituent phases must accommodate plastic deformation and the deformation behavior of the alpha-2 phase has important implications for the mechanical behavior of TiAl based alloys. Ti3Al exhibits a yield stress anomaly for the <2c+a> dislocation slip systems, the activation of which is required for general shape change. Furthermore, transmission electron microscopy (TEM) after deformation of single crystals of Ti3Al revealed an interesting tension-compression anomaly for the glide activation of <2c+a> dislocation; at room temperature <2c+a> dislocations glide on {11-21} type pyramidal planes under compression loading and on {2-201} type pyramidal planes under tensile loading. We previously reported the choice of pyramidal glide plane critically influences details of the transfer of plastic deformation induced shears associated with dislocation glide and mechanical twinning activity from the gamma-phase to the alpha-2 phase in lamellar grains of TiAl alloys during room temperature compression. Here we used TEM to investigate the dislocation glide activity in the alpha-2-phase constituent in polycrystalline two-phase Ti-48at.%Al with a duplex microstructure after room temperature tensile deformation to failure. The results are discussed in relation to the mechanical behavior of Ti3Al, including the choice of <2c+a> dislocation glide plane, intraphase interface shear transfer and mechanical properties of two-phase TiAl. We gratefully acknowledge support from the National Science Foundation, Division of Materials Research, and helpful discussions with Drs. H.L. Fraser and X.D. Zhang of The Ohio State University.
3:30 PM - U4.5
TEM of C-component Dislocations Associated with Pyramidal Slip Activity in Hexagonal Alpha-2-Ti3Al.
Jorg Wiezorek 1 , Andreas Kulovits 1
1 MEMS, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractThe hexagonal ordered phase Ti3Al is a major constituent in super-alpha-2 alloys and a minor constituent in TiAl-based alloys, which are currently considered for high temperature structural applications in advanced airborne and ground transportation system. The lamellar grains in morphologically fully-lamellar and duplex microstructures of polycrystalline TiAl-based alloys consist of thin slabs of gamma-TiAl and alpha-2-Ti3Al and exhibit anisotropic mechanical properties. Single crystals of Ti3Al are also mechanically strongly anisotropic. The (0001)<11-20> and {1-100}<11-20> slip systems can be activated with relative ease in Ti3Al, whereas c-component dislocation slip, {2-201}<11-26> and {11-21}<11-26>, operates only for loading close to the c-axis. The latter slip system is associated with anomalous yielding and the deformation behavior of lamellar grains in two-phase TiAl may be affected by the behavior of the minority phase to a significant degree. Thus, here we used binary Ti-48at%Al model alloys to study the fine structure of c-component dislocations activated during c-axis loading of lamellar grains in TiAl-alloys by diffraction contrast and high-resolution transmission electron microscopy. In addition to the large amounts of debris characteristically associated with pyramidal slip in Ti3Al non-planar configurations of c-component dislocations have been observed. The results are discussed in relation to the mechanical behavior of Ti3Al and two-phase TiAl. We gratefully acknowledge support from the National Science Foundation, Division of Materials Research, and helpful discussions with Drs. H.L. Fraser and M.J. Mills of The Ohio State University.
3:45 PM - U4.6
High Carbon Solubility in a Gamma-TiAl Based Ti-45Al-5Nb-0.5C Alloy and its Effect on Hardening.
Christina Scheu 2 1 , Erich Stergar 2 , Michael Schober 2 , Limei Cha 2 , Helmut Clemens 2 , Arno Bartels 3 , Frank-Peter Schimansky 4 , Alfred Cerezo 5
2 Department of Physical Metallurgy and Materials Testing , University of Leoben, Leoben Austria, 1 Department of Chemistry and Biochemistry, University of Munich, Munich Germany, 3 Materials Science and Technology , Technical University Hamburg Harburg , Hamburg Germany, 4 GKSS Research Center, Institute of Materials Research, Hamburg Germany, 5 Department of Materials, Oxford University, Oxford United Kingdom
Show AbstractThe C distribution within the gamma-TiAl-phase of a Ti-45Al-5Nb-0.5C alloy with near-gamma microstructure has been studied by atom probe measurements. Volumes of up to 60x60x750nm3 were analysed and revealed that in most areas the C atoms are homogenously distributed. Only a few C enriched features were detected which are most likely Cottrell atmospheres surrounding dislocation cores. The C concentration within the gamma-phase is about 0.25 at.%, which is a factor of ten higher than the solubility limit reported for other TiAl alloys. The reason for this unusual high C solubility is explained with the help of an existing model which relates the number of octahedral sites consisting of six Ti atoms to the solubility limit of interstitials. Nb is known to locate exclusively at Ti sites and induces Ti antisites, i.e. Ti on Al sites, which in turn increases the number of octahedral sites surrounded by Ti atoms. The large amount of C in solid-solution can explain the 30% increase in yield strength for the C-containing sheet in comparison to a C-free sheet containing the same Ti, Al and Nb concentration.
4:30 PM - U4.7
Nano-scaled γ-TiAl: New Insights.
Limei Cha 1 , Christina Scheu 2 , Helmut Clemens 1 , Gerhard Dehm 3
1 Metallkunde und Werkstoffprüfung, Montanuniversität Leoben, Leoben Austria, 2 Department Chemie und Biochemie, Ludwig-Maximilians-Universität München, München Germany, 3 , Erich Schmid Institut für Materialwissenschaft, Leoben Austria
Show AbstractIntermetallic titanium aluminides are promising candidates sustaining high mechanical and severe thermal conditions as e.g. occurring in aerospace, spacecraft and automotive applications. Especially γ-TiAl based alloys provide excellent strength up to temperatures of 800°C. In addition, they possess a high melting point, good corrosion resistance and low density. The mechanical properties are often determined by the presence of a lamellar microstructure which can be substantially varied by modifying the length scale between the internal γ-TiAl/α2-Ti3Al heterophase as well as γ-TiAl/γ-TiAl homophase boundaries. In this fundamental study, nanometer-scaled lamellar microstructures were adjusted in Ti-45Al-7.5Nb-(0-0.5)C alloys (in at.%) and analyzed mainly by means of transmission electron microscopy. Different annealing temperatures subsequent to oil quenching from the single α-phase region were used to obtain well-defined lamellar microstructures, which display ultrafine widths of a few tens of nanometers to a few nanometers.Surprisingly, the hardness of these alloys is enhanced by increasing the volume fraction of γ-TiAl laths. This behavior is different from earlier studies and is a consequence of the occurrence of a size effect: The nanometer-sized width of the γ laths prevents dislocation plasticity and deformation is carried by the wider α2-Ti3Al laths.
4:45 PM - U4.8
Ductility of Gamma-TiAl-Based Microstructures in the Light of Deformation Mode Interaction—Crystal Plasticity Modeling and Micro-Mechanical Experiments.
Claudio Zambaldi 1 , Franz Roters 1 , Stefan Zaefferer 1 , Dierk Raabe 1
1 Microstructure Physics and Metal Forming, Max-Planck-Institute for Iron Research, Düsseldorf Germany
Show AbstractThe deformation behavior of a gamma-TiAl based alloy (Ti-46Al-8Nb, at.-pct.) is incorporated into an elastic-viscoplastic crystal plasticity formulation coupled with the finite-element method. The deformation by ordinary dislocation glide, super dislocation glide as well as unidirectional deformation on four twinning systems are assigned different strength and hardening characteristics. For the hexagonal alpha2 phase prismatic, basal and pyramidal deformation modes are taken into account. The interaction of deformation mechanisms during deformation is captured by the formulation. The single phase constitutive behavior is calibrated by nano-indentation experiments in differently oriented single phase regions. Nano-indentation experiments are evaluated by means of a three-dimensional crystal-plasticity finite-element model of the deformation during nano-indentation. Load-displacement curves as well as the material pile-up around the indents are matched with the experimental data. The simultaneous activation of deformation mechanisms during nano-indentation is used to clarify their relative strengths and cross-hardening behavior. Two kinds of microstructures are investigated. The lamellar microstructure, resulting from slow cooling, is analyzed in terms of kinematic constraints imposed by the lamellar interfaces. The strong anisotropy of lamellar material is analyzed by calculating two-phase representative volume elements as well as material-point simulations for different sets of constitutive parameters. Secondly, the mechanical behavior of massively transformed and annealed microstructures is modeled. Microstructures obtained via a massive transformation step, exhibit a lower degree of kinematic constraints in terms of parallel gamma/alpha2 interfaces. On a grain-scale, this results in a less anisotropic behavior for plastic deformation and possibly improved ductility. Additionally, an attempt is described to include into the model the internal stresses that are known to reach significant values in TiAl-based alloys. The influence of the internal stress state on the activation of deformation modes is analyzed.The modeling work is complemented by mechanical characterization through nano-indentation and small-scale tensile tests. The microstructural characterization is greatly enhanced by the development of a novel orientation mapping technique. It makes possible the reliable determination of all six orientational variants of pseudo-cubic gamma-TiAl in a scanning electron microscope setup for electron backscatter diffraction.
5:00 PM - U4.9
Surface Treatment of TiAl with Fluorine for Improved Performance at Elevated Temperatures.
Alexander Donchev 1 , Pattrick Masset 1 , Michael Schuetze 1 , Hans-Eberhard Zschau 1
1 Karl-Winnacker-Institut, Dechema, Frankfurt/Main, Hessen, Germany
Show AbstractAlloys based on aluminium and titanium are potential materials for several high temperature applications. The use of TiAl would increase the efficiency of e.g. aero turbines, automotive engines and others due to their properties, among others low specific weight and good high temperature strength. The oxidation resistance is low at temperatures above approximately 800°C so that no long term use of TiAl-components is possible without improvement of the oxidation behaviour. Several ways are cited in the literature e.g. alloying with Nb. To produce TiAl-components specially designed for their later use surface treatment procedures are better because they do not interfere with the bulk properties. Small amounts of halogens in the surface zone of TiAl-samples lead to a dramatic improvement of the oxidation resistance at temperatures up to 1100°C for more than 8000 hours in air. In this paper results of the work on the halogen effect over the last years are presented. The results of thermogravimetric measurements, thermocyclic oxidation tests of small coupons and thermodynamic calculations for different atmospheres (e.g. air, H2O, SO2) are shown and the halogen effect mechanism is discussed. The postulated mechanism is in good agreement with the results of the oxidation tests. The limits of the halogen effect will be also mentioned. Predictions for the halogenation of TiAl-components can be given so that the processing can be planned in advance.
5:15 PM - U4.10
The Role of Fundamental Material Parameters for the Fluorine Effect in the Oxidation Protection of Titanium Aluminides.
Hans-Eberhard Zschau 1 , Michael Schuetze 1 , Alexander Donchev 1
1 Karl-Winnacker-Institut, DECHEMA e. V., Frankfurt am Main Germany
Show Abstract5:30 PM - U4.11
Analysis of the Liquid, Gamma, Sigma and eta Phase Equilibrium in the Ti-Al-Nb Alloy System.
Orlando Rios 1 , Damian Cupid 2 , Hans Seifert 2 , Fereshteh Ebrahimi 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Institute of Materials Science , Freiberg University of Mining and Technology , Freiberg, Saxony, Germany
Show AbstractTi-Al-Nb alloys have shown promising properties for high temperature applications. This alloy system is thermodynamically complex with multiple invariant reactions involving the liquid phase, solid state transformations and strongly temperature dependent solubility limits. An accurate knowledge of the equilibrium phase diagram is essential to the design of Ti-Al-Nb alloys and the development of effective thermo-mechanical processing and heat treatment schemes. Although the phase diagram has been well researched several controversies exist in the high temperature phase equilibria and the resulting liquidus projection. Extensive thermodynamic optimization incorporating experimental data available in the literature has indicated that a clear description of the invariant reaction involving L, γ, σ, η phases is not fully developed, yet characterizing this reaction is important to the understanding of high temperature stability in the Ti-Al-Nb system. Based on the existing assessments this invariant reaction can be either a ternary eutectic or a transition reaction.To address the above controversy, three alloy compositions were selected in the vicinity of different assessments of the invariant point, such that each of which crossed through the isothermal four phase field. DTA analysis was conducted at different rates for these alloys. The curves for all three alloys exhibited two main groups of peaks. Different heat treatments were designed to distinguish the phase transformations associated with these peaks. The microstructure and the phase components of the alloys in as-arc-melted slowly solidified and variously heat treated samples were examined using XRD, OP, SEM and TEM. Based on these analyses, the temperature of the invariant reaction was found to be close to 1527 οC. Samples were heat treated slightly below this temperature and then water quenched. EPMA analysis showed that the compositions of the three phases in each alloy were similar, which placed them in the three phase field resulting from the invariant reaction. In this presentation the phase transformation paths in the three alloys and the nature of the invariant reaction will be discussed.This work has been supported by NSF/AFOSR under grant number DMR-0605702.
5:45 PM - U4.12
The Effect of Cr Addition on Phase Stability of Ti-Al-Nb Alloys.
Sonalika Goyel 1 , Michael Kesler 1 , Orlando Rios 1 , Hans Seifert 2 1 , Fereshteh Ebrahimi 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 , Technische Universitaet Bergakademie Freiberg, Institut fuer Werkstoffwissenschaft, Freiberg Germany
Show AbstractHigh Nb TiAl-based alloys show promising high temperature strength and oxidation resistance. Previous studies have demonstrated that alloys with a γ+σ microstructure exhibit excellent creep properties at high temperatures. To impart good high temperature workability and microstructural control upon aging, it is necessary for the alloy to solidify as the single β-phase and be able to retain this phase upon quenching to room temperature. We have substituted Nb with Cr in selected Ti-Al-Nb alloys and studied the effect of Cr addition. The thermodynamics and kinetics of phase transformation in these alloys were evaluated using various techniques including DTA, high temperature and room temperature XRD, TEM and SEM. The analysis of the results has revealed that Cr addition affects thermodynamics as well as kinetics of β to γ and β to γ+σ phase transformations. In general, these transformation temperatures were reduced however the degree of influence was dependent on the alloy composition. Microstructural studies showed that the γ phase nucleates preferentially at the β grain boundaries and grows with Widmanstätten morphology even at very high temperatures. This observation suggests that the growth of this phase is mainly interfacial controlled. The addition of Cr was found to somewhat decrease this tendency. Compositional analysis of individual phases using EPMA revealed that, consistent with previously reported investigations, Cr partitions into the β-phase. This presentation discusses the possible mechanisms responsible for the kinetics and morphological effects of Cr addition on the β to γ phase transformation.This work has been supported by NSF/AFOSR under grant number DMR-0605702.
U5: Poster Session:
Iron Aluminides, Titanium Aluminides, Nickel Aluminides, and Silicides
Session Chairs
Bernard Bewlay
Yuehui He
Martin Palm
Tresa Pollock
Masao Takeyama
Jörg Wiezorek
Wednesday AM, December 03, 2008
Exhibition Hall D (Hynes)
9:00 PM - U5.1
Crystal Structure and Thermoelectric Properties of Mn-Substituted Ru2Si3 with the Chimney-Ladder Structure.
Tatsuya Koyama 1 , Norihiko Okamoto 1 , Kyosuke Kishida 1 , Katsushi Tanaka 1 , Haruyuki Inui 1
1 Kyoto University, Materials Science and Engineering, Kyoto Japan
Show AbstractChimney-ladder compounds with the general chemical formula of MnX2n-m (n, m: integers) possess tetragonal crystal structures which consist of two types of subcells; one composed of transition metal atoms (M) with the β-Sn structure and the other composed of group 13 or 14 atoms (X) with a helical arrangement along the tetragonal c-axis. Since the chimney-ladder compounds generally exhibit very low thermal conductivity, presumably due to its long periodicity along the c-axis, they have been extensively investigated as promising thermoelectric materials. The high-temperature (HT) phase of Ru2Si3 is one of the chimney-ladder compounds with n=2 and m=1. Recently we have found that the HT-Ru2Si3 phase is stabilized by substituting Ru with Re so as to exist even at low temperatures in a wide compositional range of the Re content (Re: 14 to 73%), and that the thermoelectric power factor for alloys with high Re contents increases with the Re content and the highest value was obtained for the alloy with the highest Re content (73%), which is the solubility limit of Re in the chimney-ladder phase. In order to further enhance the thermoelectric properties, another ternary element which extends the solid solubility region of the HT-Ru2Si3 phase is favorable. We have chosen Mn as the ternary element because Mn4Si7 with the chimney-ladder structure exists as a counterpart of HT-Ru2Si3 in the Ru2Si3-Mn4Si7 pseudo-binary system so that the solid solubility region of the chimney-ladder phase is anticipated to extend in a wider composition range than the Re case. Our study, in fact, shows that the Mn-substitution stabilizes the HT-Ru2Si3 phase in a wide compositional range of the Mn content; 12 to 100%. Compositional analyses indicate that the Si/M ratio gradually increases as the Mn content increases. This is considered to be due to the addition of Si atoms in the Si subcell in order to compensate the decrease in the valence electron concentrations (VEC) per M atom by the substitution of Ru (group 8) with Mn (group 7) with fewer valence electrons. The Seebeck coefficient and electrical resistivity of the Mn-substituted Ru2Si3 are explained in terms of the VEC deviation from the idealized value, 14, which is expected for intrinsic semiconductors with the chimney-ladder structure. The highest dimensionless thermoelectric figure of merit (ZT=0.76) is obtained for 90%Mn-substituted alloy. The relationships between the microstructure and thermoelectric properties will be discussed.
9:00 PM - U5.10
High Temperature Oxidation of Fe-28Al-W Alloys Containing 0.1 % Y.
Remzi Gurler 1 , Nese Korpe 2 , Ibrahim Celikyurek 1
1 , Metallurgical Institute, Eskisehir Turkey, 2 Metallurgy and materials, Engineering Faculty, Eskisehir Turkey
Show Abstract9:00 PM - U5.11
Effects of Initial Grain Size on Recrystallization and Tensile Properties in Rolled Fe3Al-based Alloys Containing κ-Fe3AlC Particles.
Akira Takei 1 , Satoru Kobayashi 2 , Takayuki Takasugi 1
1 Materials science, Graduate School of Engineering,Osaka Prefecture University, Sakai Japan, 2 , Osaka Center for Industrial Materials Research,Insititute for Materials, Tohoku University, Sakai Japan
Show Abstract9:00 PM - U5.12
Grain–boundary Precipitation Strengthening in Creep of Fe-20Cr-30Ni-2Nb Steel Strengthened by Intermetallic Phases.
Keiichi Kurata 1 , Naoya Hashizume 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 Metals(JRCM), Tokyo Japan
Show AbstractAdvanced ultra-super critical (A-USC) power plants require wrought materials with 105 h creep rupture strength more than 100 MPa at 973 K. Nickel-base alloys strengthened by GCP phases meet the condition, whereas none of the conventional austenitic heat resistant steels strengthened by transition metal carbides do not. Thus, a new class of austenitic steels strengthened by intermetallic phases of carbon free Fe-20Cr-30Ni-2Nb (at%) was proposed. In this steel only Fe2Nb Laves phase (C14) finely precipitates at grain boundaries and within grain interiors above 1073 K, whereas very fine metastable Ni3Nb-γ" phase (D022) precipitates coherently within grains, in addition to the grain boundary precipitation of Laves phase, at 973 K. In this study, the creep properties of the steel have been examined at 973 K under stresses above and below the yield strength (150 MPa). Regardless of the stress, the creep rate significantly reduced more than a few orders of magnitude during transient creep region and reaches minimum of about 10-6 h after 100 h. However, following creep behavior in accelerated stage is quite different, depending on the stress level. The creep rate increases aggressively to time to rupture of 340 h at 200 MPa. The acceleration in creep slows down with decreasing stress, and it obviously ceases for a certain period of time before reaching fracture (4594 h) at 120 MPa. The reduction in creep rate during the transient stage is caused by homogeneous precipitation of γ"phase in grain interiors. However, this precipitation strengthening is effective only for short-term creep since the γ"phase transforms soon to stable Ni3Nb-δ (D0a) phase. The microstructure observation of the ruptured specimen at 120 MPa clearly revealed the decoration of grain boundaries with stable Laves phase particles. It was also found that a pre-aged specimen at 1073 K having many Laves phase particles at grain boundaries exhibits more than 50 % creep elongation at 973 K / 140 MPa, indicating that the grain boundary Laves phase is not the factor to embrittle the sample. The projection of the creep rupture strength at 973K for 105 h is about 80 MPa, much superior to conventional 347 steels strengthened by carbides (40 MPa). Thus, the grain-boundary precipitation strengthening by stable Laves phase is responsible for the superior long-term creep strength of this steel. The detailed strengthening mechanism will be discussed in terms of the quantitative analysis of the area fraction of Laves phase at grain boundaries.
9:00 PM - U5.13
Consolidation of Mechanical Alloyed Ti-Al Intermetallic Compound by Electro Discharge Sintering(EDS).
Jang Hyoung-Soon 1 , Lee Won-Hee 1 , Kang Tae-Ju 1 , Cho Yoo-Jung 1
1 Sejong University, Department of Advanced Materials Engineering, Seoul 143-747 Korea (the Republic of)
Show Abstract9:00 PM - U5.14
Microscale Fracture Toughness Testing of TiAl PST Crystals.
Daisuke Miyaguchi 1 , Masaaki Otsu 1 , Kazuki Takashima 1 , Masao Takeyama 2
1 Dept. Materials Science & Engineering, Kumamoto University, Kumamoto Japan, 2 , Tokyo Institute of Technology, Tokyo Japan
Show AbstractTiAl based alloys with a fully lamellar structure exhibit superior fracture properties compared to those with other microstructures. This is mainly due to the activation of extrinsic toughening mechanisms including crack deflection and shear ligament bridging. These extrinsic toughening mechanisms are controlled by the mechanical properties of lamellae, in particular the lamellar interface fracture strength. It is, therefore, extremely important to evaluate the fracture properties of lamellar to improve the fracture toughness of such TiAl based alloys. In this investigation, a microscale fracture testing technique is applied to examine the fracture properties of lamellar in TiAl PST crystals. Micro-sized cantilever specimens with a size ≈ 10 × 15 × 50 μm3 were prepared from Ti-48Al two-phase single crystals (PST) lamellar 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 tests were successfully completed using a mechanical testing machine for micro-sized specimens at room temperature. The fracture toughness (KQ) values of the interlamellar type specimens were obtained in the range 1.2 − 3.6 MPam1/2, while those of the translamellar specimens were 5.0 − 8.1 MPam1/2. These fracture toughness values are lower than those having been previously reported in conventional TiAl PST samples. For macro-sized specimens, extrinsic toughening mechanisms, including shear ligament bridging, act in the crack wake, and the crack growth resistance increases rapidly with increasing length of crack wake for lamellar structured TiAl alloys. In contrast, the crack length in microsized specimens is only 2 − 3 μm. This indicates that extrinsic toughening mechanisms are not activated in micro-sized specimens. This also indicates that intrinsic fracture toughness can be evaluated using microscale fracture toughness testing.
9:00 PM - U5.15
Microstructure and Compression Behavior of Ti3Al Based Ti-Al-V Ternary Alloys.
Tohru Takahashi 1 , Ayumu Kiyohara 1 , Daisuke Masujima 1 , Jun Nagakita 1
1 Dept. Mechanical Systems Engg., Tokyo Univ. Agric. & Tech., Koganei, Tokyo, Japan
Show AbstractOrdered alloy phase of Ti3Al shows a rather wide solid solubility range in aluminum and also in vanadium. Several Ti-Al-V ternary alloys have been prepared to investigate the alloy composition effect upon the microstructure, crystallography, and mechanical characteristics. The materials containing 75, 70 or 65 mol% titanium, and 0 or 5 mol% vanadium were prepared by arc melting. Metallographic observation has revealed that the binary Ti-Al alloys contained somewhat coarse grains with about 100 μm grain diameters. In contrast to this, ternary alloys containing 5 mol% vanadium showed smaller grained microstructures with grain diameters around 15 μm. The grain size could not be normalized to a unified value in the present study. X-ray diffraction study and microanalysis showed that the alloys contained single phase α2. Not every possible peak of the D019 ordered structure has been observed in the XRD patterns. The lattice parameters, a and c, were observed to become a little smaller as the aluminum content increased and also when vanadium was added. Compression tests have been performed at various temperatures ranging from ambient temperature up to 1200K on rectangular parallelepiped specimens with 2mm×2mm×3mm dimensions. Alloys containing more aluminum showed higher strength, and vanadium addition enhanced the strengths of the alloys. Deformability and strength are both enhanced by vanadium addition in some alloys. Temperature dependence of strength showed a little variation upon chemical compositions.
9:00 PM - U5.17
Identification of Ordering Domains in Gamma-TiAl by Crystal Orientation Mapping.
Claudio Zambaldi 1 , Stefan Zaefferer 1 , Stuart Wright 2
1 Microstructure Physics and Metal Forming, Max-Planck-Institute for Iron Research, Duesseldorf Germany, 2 , EDAX/TSL, Draper, Utah, United States
Show AbstractTetragonal gamma-TiAl exhibits cubic pseudo-symmetry due to its c over a ratio being just