John Lewandowski, Case Western Reserve Univ
Kyosuke Kishida, Kyoto Univ
Svea Mayer, Montanuniversitaet Leoben
Seiji Miura, Hokkaido Univ
GE Global Research, US, Hokkaido University– Faculty of Engineering, Kyoto University, Montanuniversitaet Leoben, SpringerMaterials
MB1.1: Titanium Aluminides I
Monday PM, November 28, 2016
Sheraton, 2nd Floor, Independence West
9:30 AM - *MB1.1.01
Next Generation TiAl Alloys for Low Pressure Turbine Blade Application
Bernard Bewlay 1 , Soumya Nag 1 , Akane Suzuki 1 , Michael Weimer 2
1 GE Global Research Niskayuna United States, 2 GE Aviation Cincinnati United StatesShow Abstract
In the recent years, extensive investigations of TiAl alloys have enabled their commercial implementation in the aerospace and automobile industries. Next generation alloys with a superior balance of mechanical and environmental properties has been achieved via major (group VB and VIB) and minor (group IIIA and IVA) elemental additions. It is well known that the above properties are highly sensitive to the microstructure, which in turn is dictated by the alloy chemistry and processing schemes. The ability to engineer microstructure for different alloys, and the ability to generate the required room and elevated temperature alloy property response, are key to understanding the underlying mechanisms. These themes will be discussed in the present talk. Conventional and non-conventional processing techniques like gravity casting and near net shape casting and additive manufacturing will also be highlighted.
10:00 AM - MB1.1.02
Microstructures and Fatigue Properties of TiAl Alloys Fabricated by Electron Beam Melting
Ken Cho 1 , Ryota Kobayashi 1 , Hiroyuki Yasuda 1 , Mitsuharu Todai 1 , Takayoshi Nakano 1 , Ayako Ikeda 2 , Daisuke Kondo 3 , Yuto Nagamachi 3 , Minoru Ueda 3 , Masao Takeyama 4
1 Osaka University Suit Japan, 2 National Institute for Materials Science Ibaraki Japan, 3 Metal Technology Co. Ltd Tokyo Japan, 4 Tokyo Institute of Technology Tokyo JapanShow Abstract
In this study, cylindrical rods of Ti-48Al-2Cr-2Nb alloys were fabricated by electron beam melting (EBM). Microstructures and fatigue properties at room temperature and high temperature (1023 K) of the alloys were investigated as a parameter of an angle (θ) between the building and cylinder directions. Ti-48Al-2Cr-2Nb alloys fabricated by EBM consist of duplex-like structure and coarse γ phase grains which form a chain perpendicular to the building direction. These arranged γ phase grains are referred to as the γ band. The volume fraction of the γ band is comparable in θ = 0 deg and 45 deg samples. Room temperature and high temperature fatigue properties of θ = 0 deg and 45 deg samples were evaluated by tensile-compressive (R = -1) fatigue tests. There is no significant difference in the high temperature fatigue strength between θ = 0 deg and 45 deg samples. On the other hand, the room temperature fatigue strength of θ = 45 deg sample is higher than those of θ = 0 deg sample and hot isostatic pressed sample. According to room temperature tensile test, the 0.2% proof stress of θ = 45 deg sample is same level as that of 0 deg sample. However, the elongation of θ = 45 deg sample is 4 times higher than that of θ = 0 deg sample. Therefore, it is supported that the high fatigue strength of θ = 45 deg sample is attributed to the higher fracture toughness of θ = 45 deg sample compared with that of θ = 0 deg sample. These results indicate that EBM has a great potential to be used as fabrication process for TiAl alloys.
10:15 AM - MB1.1.03
Microstructural Heterogeneity and Post Processing Effects on Mechanical Properties of Ti-48Al-2Cr-2Nb Additively Manufactured by Electron Beam Melting (EBM)
Mohsen Seifi 1 , Ayman Salem 1 , Dan Satko 2 , S. Semiatin 3 , John Lewandowski 1
1 Case Western Reserve University Cleveland United States, 2 Materials Resources LLC Dayton United States, 3 Air Force Research Laboratory Wright-Patterson Air Force Base United StatesShow Abstract
Both cast and wrought titanium aluminide alloys have been studied for more than two decades because of attractive properties that include low density, high specific strength, high specific stiffness and oxidation resistance up to about 700°C. Electron beam melting provides another processing approach to producing net shape components, although little work has been conducted to examine processing-microstructure-property relationships. This work examines as-deposited γ-TiAl (Ti-48Al-2Cr-2Nb) specimens made by Arcam AB and post-processed materials. Mechanical behavior studies on as-deposited and HIPed conditions included Vickers micro-hardness, compression, fracture toughness and fatigue crack growth testing. In addition, microstructural details were investigated over a range of scales using various microscopy tools. The presentation will summarize this evolving work on the characterization of AM γ TiAl and provide some comparison to other conventional (e.g. as-cast, wrought) γ-TiAl alloys.
10:30 AM - MB1.1.04
Mechanics and Plasticity of a TiAl Powder Processed by SPS—Application to Near-Net Shaping
Jean-Philippe Monchoux 1 , Zofia Trzaska 1 , Thomas Voisin 1 , Houria Jabbar 1 , Couret Alain 1 , Marc Thomas 2
1 CEMES-CNRS Toulouse France, 2 ONERA Châtillon FranceShow Abstract
Near-net shaping by the spark plasma sintering (SPS) technique has proven to be a promising route to obtain preforms of high added value materials. Turbine blades in TiAl have for example been shaped in one step. To master the densification of the powder in complex geometries, it is now useful to understand its plasticity at high temperature, from macro- and microscopic points of view. Therefore, the mechanical behavior in compression at 900-1100°C of a TiAl alloy (GE, Ti48Al48Cr2Nb2) has been investigated. Transmission electron microscopy (TEM) investigations of the high temperature microscopic plasticity mechanisms have been carried out, in order to account for the macroscopic mechanical behavior. Twinning, glide and climb of ordinary dislocations, have been observed. Stress relaxation by recovery and recrystallization phenomena have been shown to occur simultaneously to the deformation. Nice examples of recrystallization nuclei developing in a deformed microstructure, with curved grain boundaries having their convexity directed towards the deformed regions, have been observed. This allowed a better understanding of the densification mechanisms of the powder and of its mechanical behavior, from a fundamental point of view. Then, turbine blades near-net shaping has been attempted, leading to preforms at scale one, 100% dense and of homogeneous microstructure.
10:45 AM - MB1.1.05
In Situ Study of the Beta/Alpha Phase Transformation Kinetics in Gamma Titanium Aluminide Alloys
Michael Oehring 1 , Andreas Stark 1 , Marcus Rackel 1 , Norbert Schell 1 , Florian Pyczak 1 , Andreas Schreyer 2
1 Institute of Materials Research Helmholtz-Zentrum Geesthacht Geesthacht Germany, 2 European Spallation Source Lund SwedenShow Abstract
Due to the inherent brittleness of gamma titanium aluminide alloys part of the alloy and processing development aims at fine and homogeneous microstructures. Pronounced microstructural refinement can be achieved by the addition of B, as it is known since about two decades. Recently it has been shown that this effect can be attributed to heterogeneous nucleation of the alpha phase on borides during the beta/alpha transformation, which however is only observed for slow cooling rates. In order to understand the microstructural refinement the phase transformations kinetics was analyzed by in situ high-energy XRD on cooling the material from the high-temperature beta phase field. In the experiments an inductively heated dilatometer was used for heating and cooling specimens of the alloys Ti-43Al-5Nb and Ti-43Al-5Nb-0.2B (at.%). The specimens were heated to a temperature above 1430 °C, held for 1 min and then cooled to room temperature with several cooling rates from 1 K/s up to 30 K/s. The phase fractions were determined by HEXRD and in addition from the length signal measured by the dilatometer. The obtained transformation curves were quite similar except some characteristic differences. From the evolution of the transformed volume fraction the start temperature of the beta/alpha transformation was determined for different cooling rates. For all cooling rates the transformation starts at higher temperature in the B containing alloy. Further, the experiments indicate a change in the transformation mechanism between 5 K/s and 10 K/s the origin of which will be discussed in the contribution.
11:30 AM - *MB1.1.06
Alloy Design Rules for Advanced γ-TiAl Based Alloys
Helmut Clemens 1 , Svea Mayer 1
1 University of Leoben Leoben AustriaShow Abstract
Intermetallic TiAl alloys based on the γ-TiAl phase are already used as engineering lightweight high-temperature materials in aircraft and automotive engines. Thereby, they partly substitute the twice as heavy Ni-base superalloys. Present applications are, for example, blades in the low-pressure turbine of advanced aero-engines, turbine wheels for turbocharger systems of car diesel engines and engine parts used in racing cars. All these applications require balanced mechanical properties, i.e. certain ductility at room temperature as well as defined creep strength at elevated temperatures.
In the framework of this presentation the alloy design criteria, which have been applied for the development of a β-solidifying γ-TiAl-based alloy, the so-called “TNM alloy” with an excellent hot-deformability will be explained. The TNM alloy is already used in a particular eco-friendly and fuel-saving aero-engine, which is powering the Airbus A320neo. Besides the design criteria which have led to the selected alloying elements, the heat treatments conducted subsequent to conventional hot-forging are discussed. The microstructural parameters, which influence the elongation at fracture and creep behavior will be emphasized. Finally, the potential to improve the mechanical properties of the TNM alloy by means of micro-alloying with C and Si is addressed.
12:00 PM - MB1.1.07
Ordering and Disordering of Beta Phase in TiAl Alloys in Dependence of Alloy Composition
Victoria Kononikhina 1 , Andreas Stark 1 , Florian Pyczak 1 , Weimin Gan 1 , Andreas Schreyer 2
1 Helmholtz-Zentrum-Geesthacht Geesthacht Germany, 2 European Spallation Source ERIC Lund SwedenShow Abstract
Due to their low density (4 g per cm3), good oxidation and corrosion resistance and high specific tensile and creep strength, γ-TiAl based alloys recently have started to replace Ni-based superalloys as a material for turbine blades in aircraft engines. TiAl alloys in the range of 39 – 45 at.% Al usually contain the ordered phases γ and α2 at lower and disordered a at higher temperatures. Depending on alloy composition and temperature additionally disordered β-Ti(Al) (A2 structure) or ordered βo-TiAl (B2 structure) can occur. The ductile high temperature β phase is important for the processing of the material while the low temperature βo phase is said to embrittle the material at service temperature.
We used the good contrast of neutron diffraction for ordered and disordered β phase of TiAl-based alloys to determine the order/disorder temperatures which are not accessible by other methods like DSC measurements. Three binary TiAl alloys (Ti-xAl with x = 39, 42 and 45) and five alloys with additional alloying elements (Ti-42Al-2Y with Y = Nb, Mo, Ta, Cr and Fe) were used to investigate the influence of different Al concentrations and alloying additions on the kinetics of the occurring ordering/disordering reactions and phase transformations. In the presentation new results of neutron diffraction experiments and microstructure investigations by scanning electron microscopy will be shown.
12:15 PM - MB1.1.08
Fracture and Fatigue Crack Growth Behavior of Wrought Gamma Titanium Aluminide Ti-43Al-4Nb-1Mo in Different Microstructure Conditions
Matthew Dahar 1 , Thomas Podbesek 2 , Sesh Tamirisakandala 2 , John Lewandowski 1
1 Case Western Reserve University Cleveland United States, 2 Alcoa Titanium and Engineered Products Niles United StatesShow Abstract
Ti-43Al-4Nb-1Mo (TNM) is a third generation wrought gamma titanium aluminide being used for high performance gas turbine engine low-pressure turbine blades. This study summarizes the room temperature tension, fracture, and fatigue crack growth behavior of TNM at various stages of processing. Tension and bend bar samples were machined from the longitudinal and transverse directions of as-cast, post-hot isostatic pressing (HIP), and forge plus heat treated conditions. Fatigue crack growth tests were conducted over a range of load ratio (R) values in order to determine the dependence of fatigue threshold, Paris slope, and stress intensity at overload (Kc) on R. Optical and SEM examinations were used to determine the microstructure, fracture path, and topography and compare mechanistic behavior of TNM in various conditions.
12:30 PM - MB1.1.09
Creep Behavior of a β-Solidifying TiAl Alloy with Fully Lamellar Microstructure
Michael Burtscher 1 , Thomas Klein 1 , Helmut Clemens 1
1 Chair of Physical Metallurgy and Metallic Materials Leoben AustriaShow Abstract
The growing interest in intermetallic γ-TiAl based alloys necessitates the detailed knowledge of their mechanical properties. Since these alloys are already used in aerospace and automotive applications, where they are exposed to service temperatures up to 800°C, creep resistance is of major importance.
In this study, the effect of microstructure and composition on the creep behaviour of a so called TNM+ alloy is investigated. The nominal composition of the alloy is Ti-43Al-4Nb-1Mo-0.1B-0.3Si-0.3C (at.%), whereby the addition of C and Si significantly increases creep resistance in comparison to classic Si and C-free TNM alloys. To this end, fully lamellar microstructures with different mean interface spacings are adjusted by two-step heat-treatment and creep tests are conducted in a load controlled mode. For the prediction of the creep behaviour the one-parameter model of Y. Estrin und H. Mecking is applied . The model was adopted to the TNM+ alloying system and verified by creep tests at temperatures from 700 to 800°C using stresses of 150 to 300 MPa. For these creep parameters the mean lamellar spacing limits the free pathway of dislocations and is mainly responsible for the dislocation interaction within the individual γ-TiAl lamellae. The lamellar spacing of the different samples was measured by transmission electron microscopy. Finally, the measured creep curves are compared with model calculations and are discussed in the framework of existing literature.
 Y. Estrin, H. Mecking. “A unified phenomenological description of work hardening and creep based on one-parameter models”, Acta metall., Vol. 32, 1984, p. 57–70.
MB1.2: Silicides and Ultra-High Temperature Alloys
Monday PM, November 28, 2016
Sheraton, 2nd Floor, Independence West
2:30 PM - *MB1.2.01
The Effect of Borosilica Pack-Cementation on Creep and Oxidation Resistance of Mo-Si-B Based Alloys
Martin Heilmaier 1 , Daniel Schliephake 1 , Camelia Gombola 1 , John Perepezko 2
1 Karlsruhe Institute of Technology Karlsruhe Germany, 2 University of Wisconsin-Madison Madison United StatesShow Abstract
New high-temperature materials like Mo-Si-B alloys offer the possibility to increase the efficiency of power generation and aircraft engines by higher combustion temperatures. They show good creep and oxidation resistance at high temperatures but suffer from low ductility at ambient temperatures. By additional micro-alloying with Zr, the ductility of the Mo solid solution can be increased, thus, lowering the brittle to ductile transition temperature of Mo-Si-B alloys. More recently, besides significantly reducing density, novel Ti-rich Mo-Si-B alloys have shown an increased creep resistance compared to ternary Mo-Si-B reference alloys by the formation of Ti-silicide precipitates within the Mo solid solution during processing. However, both elements deteriorate the oxidation resistance. Hence, to provide sufficient oxidation resistance at elevated temperatures, additional coatings may be required.
This talk therefore addresses the creep and oxidation behavior of coated Mo-Si-B-X (X = Zr, Ti) alloys. Pack-cementation was done in an atmosphere of high-purity argon at 1000°C for 40h, followed by a conditioning step at 1400°C for 10h in air. The resulting layer structure depends on the substrate composition and consists of an outer borosilica layer followed by an inner MoSi2 and Ti5Si3 layer for the Ti-rich alloys. The layer structure for the Mo-Si-B-Zr alloy also consists of an outer borosilica layer but reveals an inner Mo5Si3 and Mo5SiB2 layer. Tensile creep tests under constant stress have been carried out at temperatures ranging from 1100°C to 1200°C in air. In comparison, cyclic and isothermal oxidation tests were performed in a temperature range between 800°C (the “pesting” regime) and 1200°C for up to 3000 h. Detailed microstructural analysis by SEM/EDX was employed to shed light on the interplay between substrate and coating and to identify possible mechanisms that may explain the observed superior resistance of the coated samples against thermal-mechanical loading in air.
3:00 PM - *MB1.2.02
Phase Field Simulation of Microstructure Formation in MoSi2-Based Dual Phase Alloys
Yuichiro Koizumi 1 , Toshihiro Yamazaki 1 , Akihirko Chiba 1 , Koretaka Yuge 2 , Kyosuke Kishida 2 , Haruyuki Inui 2 , Koji Hagihara 3 , Takayoshi Nakano 3
1 Tohoku University Sendai Japan, 2 Kyoto University Kyoto Japan, 3 Osaka University Suita JapanShow Abstract
MoSi2-based alloys have been proposed as candidates for next-generation gas turbine blade materials operated at extremely high temperatures beyond the maximum operation temperature of Ni-based superalloys. In C11b-MoSi2/D8m-Mo5Si3 eutectic composites, labyrinth structures (also known as script lamellar structures) with an orientation relationship of (1-10)MoSi2//(001)Mo5Si3 and MoSi2//[1-10]Mo5Si3, are formed by directional solidification. In C11b-MoSi2/C40-NbSi2 composite, lamellar structure with lamellar interfaces parallel to (1-10)MoSi2//(0001)NbSi2 planes are formed. These structures are known to enhance creep resistance. It is expected that the fracture toughness of the composite can be improved by modifying the properties of interphase boundaries. Additional elements can affect the properties of interfaces and the microstructure morphology via the changes in the lattice misfit and the interfacial energies. As a basis for controlling the interfaces and the morphology concurrently, it is important to understand the factors dominating the morphologies. Phase-field simulation is a versatile technique for examining the dominant factors of microstructure formation [1, 2]. We demonstrated that the lamellar structure in C11b-MoSi2/C40-NbSi2 is formed owing to the high anisotropy of interfacial energy, which was revealed by first principles calculation. Recently, we developed a phase-field model taking into account structural ledges, which are observed on the interfaces experimentally and believed to be responsible for the inclination of semi-coherent interfaces. By using the model, we examined the factors dominating the morphology of the C11b/D8m dual phase structure. When an isotropic interfacial energy is assumed while the ledges taken into account, a microstructure with D8m phase with interfaces inclined from the [1-10]MoSi2//Mo5Si3 direction by approximately 15° was formed, but the labyrinth structure was not formed. When a high anisotropy of interfacial energy was assumed, microstructure similar to the labyrinth structure was formed. Thus, it has been revealed that the formation of the labyrinth structure is dominated by the superposition of the anisotropy of interfacial energy and the lattice misfit with the effect of structural ledges.  T. Yamazaki et al. Intermetallics 54 (2014) 232-241,  T. Yamazaki et al. Comp. Mat. Sci. 108 (2015) 358–366.  Fujiwara et al. Intermetallics 52 (2014) 72-85.
3:30 PM - MB1.2.03
Novel V-Si-B Alloys for Ultra-High Temperature Applications
Manja Kruger 1 , Janett Schmelzer 1 , Volodymyr Bolbut 1 , Georg Hasemann 1
1 Otto-von-Guericke University Magdeburg Magdeburg GermanyShow Abstract
In this study our current understanding of the correlation between microstructures and high temperature properties of new vanadium-based materials will be presented. Vanadium alloys provide 20% to 30% density reduction compared to nickel-based alloys and steels, while the melting temperature is about ~500°C higher. Additions of silicon and boron result in the formation of a solid solution and silicide phases which affects the mechanical properties of vanadium alloys.
In a first exploratory study, some V-Si and V-Si-B alloys were processed via a powder metallurgical route. To understand the effect of mechanical alloying on the resulting microstructures different studies on binary V-Si and ternary V-Si-B are presented. For the compact V-Si-B alloys, properties like hardness, compressive strength and oxidation behavior are examined. Results are compared to CMSX-4 and other existing vanadium alloys. Compressive yield strength is comparable to the nickelbase superalloy at 1000 °C but the mass gain during isothermal oxidation at 600 °C is higher, implying the need for a protective coating for sustained high temperature exposure.
A powder metallurgical V-9Si-13B alloy with a three-phase microstructure is characterized in terms of creep behavior in the as-received and annealed state. Annealing at 1300 °C leads to grain growth and improved creep resistance. For comparison, the same alloy composition was produced via arc-melting yielding in a coarser microstructure. Compression creep tests at temperatures between 900 and 1050 °C demonstrated that these novel alloys are competitive compared to Al-Ti materials and Ni-Co superalloys.
3:45 PM - MB1.2.04
Microstructure Controlling and Oxidation Behavior of Ti-Enriched MoSiBTiC Alloy
Mi Zhao 1 , Kyosuke Yoshimi 1
1 Tohoku University Sendai JapanShow Abstract
Mo-Si-B based alloy is one of the most promising candidates for ultra-high-temperature applications. It has been clarified by our group that the addition of TiC to the Mo-Si-B system largely improved its mechanical properties . In order to further improve the strength/density ratio and high-temperature oxidation resistance, the Ti-enriched MoSiBTiC alloy was investigated in this study.
Alloy ingots with the composition of 38Mo-17Si-5B-20Ti-10TiC (at.%) were prepared by conventional Ar arc-melting. Heat treatment was carried out in Ar atmosphere at 1700 °C for 24 h. Both the as-cast and the heat-treated sample were hot-forged at 1500 °C under strain rate of 1.4×10-4 s-1 until the total strain up to 50%. Oxidation behavior was investigated at 1100 and 1300 °C in the atmosphere of p(O2)/p(Ar)=0.25. It was found that both the as-cast and heat-treated samples are composed of five phases, i.e., Mo solid solution, Mo3Si, Mo5SiB2, Ti5Si3 and TiC. Micro-cracks were often observed across Ti5Si3 phase, which were generated by thermal stress due to the strong thermal expansion anisotropy of Ti5Si3. Fortunately, these micro-cracks were largely healed by hot-forging due to microstructure development involving dynamic recovery and/or recrystallization. Grain refinement was also achieved through the hot-forging process. In addition, this alloy displayed relatively good oxidation resistance. Moreover, the hot-forged sample showed the oxidation rate 35% better than that of the unforged sample at 1100 °C. The improvement of oxidation resistance after hot-forging might be attributed to the microstructure refinement.
 S. Miyamoto et al., Metall. Mater. Trans. A, 45 (2014) 1112.
4:30 PM - *MB1.2.05
Environmental Resistant Coatings for High Temperature Mo and Nb Silicide Alloys
John Perepezko 1
1 University of Wisconsin-Madison Madison United StatesShow Abstract
The challenges of a high temperature environment impose severe material performance constraints in terms of melting point, oxidation resistance and structural functionality. In metallic systems the multiphase microstructures that can be developed in the Mo-Si-B system and Nb silicide alloys offer useful options for high temperature applications. Alloys based upon the coexistence of the high melting temperature (>2100°C) ternary intermetallic Mo5SiB2 (T2) phase with Mo and the Mo3Si phases allow for in-situ toughening and offer some oxidation resistance. For Nb alloys the toughness is satisfactory but the oxidation resistance is poor. Since the alloy compositions that exhibit the lowest oxidation rate will most likely not yield optimum mechanical properties performance, it is important to develop robust and compatible oxidation resistant coatings.. An effective strategy to achieve the necessary environmental resistance is based upon the use of an integrated Mo-Si-B based coating that is applied by pack cementation process to develop an aluminoborosilica surface and in-situ diffusion barriers with self-healing characteristics for enhanced oxidation resistance In order to control the B/Si ratio that governs the oxidation resistance, a pack cementation process was adapted for coating synthesis. During oxidation of the (Si+B)-pack alloys, the initial MoSi2 outer layer is consumed by formation of the Mo5Si3 (T1) phase and the development of the underlying T2 borosilicide and/or boride phase layer. The T1 phase that is saturated with B has excellent oxidation resistance and the loss of Si is blocked by the underlying diffusion barrier (T2). Further, any damage to the outer T1 layer can be recovered from the underlying T2 + MoB layer. In effect, the in-situ reaction that yields the T2 + MoB layer also provides a kinetic bias that allows for the continued existence of the outer T1 layer and also yields a self-healing characteristic of the coating. With coated samples the environmental resistance can be enhanced up to at least 1700°C. Moreover, the coating strategy can be adapted to apply to other refractory metal systems such as the Nb silicides where the coating is applied first by a deposition of Mo followed by a co-deposition of Si and B using the pack cementation technique. During oxidation exposure a mass change of -0.83 mg/cm2 was observed for the coated alloy after 24 hour exposure at 1300°C, demonstrating enhanced oxidation protection. . The environmental performance requires resistance not only to high temperature oxidation, but also resistance to water vapor, CMAS attack, hot corrosion and thermal cycling. Under these extended environmental conditions the Mo-Si-B based coating exhibited robust performance.
5:00 PM - MB1.2.06
Creep and Oxidation Properties of Near-Eutectic Mo-Si-B Alloy
Georg Hasemann 1 , Denys Kaplunenko 1 , Martin Palm 2 , Iurii Bogomol 3 , Manja Kruger 1
1 Otto-von-Guericke University Magdeburg Magdeburg Germany, 2 Max-Planck-Institut für Eisenforschung GmbH Düsseldorf Germany, 3 Kyiv Polytechnic Institute Kiev UkraineShow Abstract
Multiphase Mo-based alloys are potential candidates for applications in aerospace engines and the power generating industry due to their excellent creep behavior and acceptable oxidation resistance at ultrahigh temperatures. The resulting microstructures as well as the material’s properties of Mo-Si-B alloys strongly depend on the manufacturing process. Finding a eutectic composition in this alloy is expected to combine those properties with a well-defined microstructure.
The present work addresses Mo-Si-B alloys which are located in the so-called “Berczik triangle” and consist of a three-phase microstructure with a Mo solid solution phase (MoSS) and the two intermetallic phases Mo5SiB2 (T2) and Mo3Si. In our recent studies two near-ternary eutectic alloy compositions could be identified, namely Mo-17.5Si-8B and Mo-17.5Si-10B. In the present study, both alloys were prepared via conventional arc-melting (AM) and via directional solidification (DS) using a crucible-free zone-melting process. The microstructures were compared and the creep properties of the AM and DSed alloys were obtained at temperatures between 1100 °C and 1400 °C. The results were evaluated and compared with commonly used Ni-based superalloys (Rene N5 and CMSX-4) and a powder metallurgical (PM) processed alloy Mo-9Si-8B. The creep resistance of the near-eutectic DS alloys was found to be substantially improved due to their relatively coarse directionally solidified microstructure and the high volume fractions of intermetallic phases which are homogeneously distributed. Thus, near-eutectic DS Mo-Si-B alloys show great potential for applications at temperatures of around 1200-1300 °C.
In addition the cyclic oxidation behavior of the near-eutectic AM and DS alloys were investigated at 800 °C and 1100 °C in air. Due to the high volume fraction of intermetallic phases the oxidation behavior at 1100 °C is improved compared to the PM Mo-9Si-8B reference alloy. However, a strong mass loss leading to nearly complete sample degradation (pesting) could not be prevented at oxidation exposure at 800 °C longer than 20 hours.
5:15 PM - MB1.2.07
Bcc/B2 Phase Equilibria and Phase Transformation from B2 to β’ in Refractory Nb-X(Pd, Rh, Ir)-Al Alloys
Takuya Yamanouchi 1 , Seiji Miura 1 , Munekazu Ohno 1 , Ken-ichi Ikeda 1
1 Hokkaido University Sapporo-shi JapanShow Abstract
Nb-based alloys are candidates for gas turbine blade materials because they have advantages such as higher melting temperature than Ni-based superalloy and low density. However, their inadequate oxidation resistance has drawn them back from practical application. For improving oxidation resistance of Nb-based alloys it is thought to introduce B2 aluminide phase for Al reservoir layer for maintaining Al2O3 surface layer. Phase equilibrium between B2 aluminide layer and Nb matrix are required for preventing a formation of harmful intermediate layer under coating at high temperature. Previous study revealed that there is Nb-bcc / PdAl-B2 two-phase region in the Nb-Pd-Al system. However, it is known that cracks occur in PdAl alloy during casting, probably caused by phase transformation from B2 to β’. Both IrAl and RhAl are known to be B2 aluminide with higher melting temperature than PdAl, and no phase transformation from B2 to β’ have been reported[2-3]. Thus, Ir or Rh substituting for Pd is expected to solve the matter of crack formation and also to obtain much higher melting temperature than the Nb-bcc / PdAl-B2 alloys. Thus, in this study bcc / B2 two-phase equilibria and phase transformation from B2 to β’ in both Nb-Pd-Ir-Al and Nb-Pd-Rh-Al systems were investigated. Various Nb-Pd-Ir-Al and Nb-Pd-Rh-Al alloys were prepared by Ar-arc melting, followed by the heat-treatment at 1400 oC. Composition analysis of their constituent phases was conducted by using EPMA. DTA analysis was conducted to investigate phase transformation temperature including melting temperature of the alloys. Nb-bcc / (Pd,Rh)Al-B2 two-phase field was found in a wide composition ratio of Rh/(Pd+Rh) in B2 phase, while Nb-bcc / (Pd,Ir)Al-B2 two-phase field was much less because of the formation of Nb2(Ir,Al)-σ phase. With high amount of Rh no cracks were found in the alloys. It is attributed to the lack of phase transformation from B2 to β’ confirmed by DTA analysis. Improvement of melting temperature with increasing Rh amount was also found.
This work was supported by the JST-ALCA program, Ultra Heat-Resistant Materials and High Quality Recycled Steel, High temperature materials based on multi-element bcc solid solutions. A part of this work was conducted at the Laboratory of Nano-Micro Materials Analysis, Hokkaido university, supported by "Nanotechnology Platform" Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors thank Mr. N. Miyazaki at Hokkaido University for helpful discussion and technical assistance for WDS analysis.
 T. Yamanouchi and S. Miura, MRS proc., 1760 (2015), p. mrsf14-yy05-38.
 H. Okamoto, J. Phase Equilibria and Diffusion, 30.2 (2009), p. 206-207.
 V. G. Khoruzhaya et al., Powder Metallurgy and Metal Ceramics, 45.5-6 (2006), p. 251-258.
5:30 PM - MB1.2.08
Mechanical Properties and Phase Stabilities of X1-X2-B Transition Metal Monoborides (X=Ti/Fe/Mo/Nb/V) Using First Principle Calculation
Hyojung Kim 1 , Dallas Trinkle 1
1 University of Illinois at Urbana-Champaign Urbana United StatesShow Abstract
We calculate the mechanical properties and investigate the phase stabilities of different compositions of monoborides (X1x X21-x)B (X=Ti/Fe/Mo/Nb/V) to find ceramic phases and precipitates with improved strength and toughness for biomedical and aerospace applications. We use density functional theory to calculate structural information—energies, lattice and elastic constants— and estimate ductility with Pugh’s ratio and stacking fault energies. We use special quasirandom structures to generate different compositions for both the FeB and CrB crystal structures across the full phase field. We compute phase diagrams and find large solubilities in the ternary systems containing Ti compared to the ternary systems without Ti. The calculated bulk, shear, and Young's moduli for most of the mixed borides agree with the moduli values estimated from the rule of mixtures. The Pugh's ratios of the mixed borides also match well with the values from the rule of mixtures with some exceptions. TiB has the smallest Pugh's ratio of all the monoborides, which suggests that the presence of solutes or precipitates can improve the ductility of TiB. We also estimate stacking fault energies to assess the activation of possible slip systems in the ceramic phases. FeB in the FeB structure has the lowest stacking fault energy, which indicates the highest propensity for plastic slip under shear. We use our results to suggest optimal metallic boride compositions for Ti-based alloys and bulk ceramics.
5:45 PM - MB1.2.09
Influences of Minor Si and Annealing Temperatures on Fracture Mode of Nb-Si Alloys
Bin Kong 1 , Lina Jia 1 , Hu Zhang 1 , Songxin Shi 1 , Yueling Guo 1
1 School of Materials Science and Engineering, Beihang University Beijing China