Symposium MB1 : Intermetallic-Based Alloys---From Fundamentals to Applications

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.

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.

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.

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.

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.

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.

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 β_o-TiAl (B2 structure) can occur. The ductile high temperature β phase is important for the processing of the material while the low temperature β_o 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.

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 (K_c) 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.

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 [1]. 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.

[1] 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.

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.

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 C11_b-MoSi₂/D8_m-Mo₅Si₃ eutectic composites, labyrinth structures (also known as script lamellar structures) with an orientation relationship of (1-10)_MoSi2//(001)_Mo5Si3 and [110]_MoSi2//[1-10]_Mo5Si3, are formed by directional solidification. In C11_b-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 C11_b-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 C11_b/D8_m dual phase structure. When an isotropic interfacial energy is assumed while the ledges taken into account, a microstructure with D8_m phase with interfaces inclined from the [1-10]_MoSi2//[001]_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. [1] T. Yamazaki et al. Intermetallics 54 (2014) 232-241, [2] T. Yamazaki et al. Comp. Mat. Sci. 108 (2015) 358–366. [3] Fujiwara et al. Intermetallics 52 (2014) 72-85.

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.

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 [1]. 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.
[1] S. Miyamoto et al., Metall. Mater. Trans. A, 45 (2014) 1112.

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.

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 (Mo_SS) 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.

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^[1]. 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.

[1] T. Yamanouchi and S. Miura, MRS proc., 1760 (2015), p. mrsf14-yy05-38.
[2] H. Okamoto, J. Phase Equilibria and Diffusion, 30.2 (2009), p. 206-207.
[3] V. G. Khoruzhaya et al., Powder Metallurgy and Metal Ceramics, 45.5-6 (2006), p. 251-258.

We calculate the mechanical properties and investigate the phase stabilities of different compositions of monoborides (X¹_x X²_1-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.

Controlling the distribution of elements in Nb solid solution (Nb_SS) and optimizing the microstructure by appropriate heat treatment are significant for the achievement of outstanding comprehensive properties of Nb-Si based alloys. In this paper, the influences of minor Si and annealing temperatures on microstructures, tensile properties and fracture behaviors of Nb_SS alloys at room temperature have been investigated. Nb-(0/0.5/1)Si (at.%) alloys were well arc-melted, and Nb-1Si alloy was annealed at different temperatures (1300°C, 1400°C and 1500°C) for 10 hours. With the increase of Si content, the ultimate tensile strength (UTS) enhances and the elongation decreases, both of which are mainly because of the increase of Si content in Nb_SS. Particularly, with the addition of Si from 0.5 to 1 at.%, the elongation reduces sharply (21.9% to ~0%) and the fracture mode transforms from ductile fracture to brittle fracture. After annealing, the elongation increases significantly from ~0% to 12.2%, 5.1% and 1.1% after annealing at 1300°C, 1400°C and 1500°C, respectively, which is mainly because that Si content in Nb_SS decreases after annealing and then increases with the annealing temperature. In particular, the fracture feature transforms from cleavage to dimples after annealing at 1300°C. The results mentioned above indicate that Si displays an adverse effect on the ductility of Nb_SS, while annealing at 1300°C would transform the fracture mode of Nb-1Si alloy from brittle fracture to ductile fracture and improve the ductility in this paper.

There are many hard materials that are considered to be candidates for structural applications under extreme conditions such as very high temperatures. This stems from the fact that many of them possess peculiar properties such as high hardness, high melting temperature, and so on. But, one of the common characteristics for these hard materials is their brittleness. They usually fail in cleavage without showing any plastic deformation at ambient temperature. So, even, fundamentals for plasticity such as operating slip systems and their CRSS values have yet to be known for many of them. However, there is a chance for these hard materials to plastically deform in the form of micropillars of the micron-meter size even at ambient temperature, from which we can obtain the information of operating slip systems and their CRSS values. We have investigated the compression deformation behavior of brittle transition-metal silicides of the M₅Si₃-type such as Mo₅Si₃, Nb₅Si₃ and Mo₅SiB₂ and those of the MSi₂-type such as MoSi₂, VSi₂, CrSi₂, NbSi₂ and TaSi₂. Although none of them listed above deform plastically at room temperature in the bulk form, plasticity is clearly observed at room temperature for all of them in the micropillar forms. Plasticity observed in the micropillar form at room temperature has made us to clearly identify their operative slip systems with their CRSS (critical resolved shear stress) values. For transition-metal silicides of the MSi₂-type, slip systems operative at high temperatures in the bulk form are observed also to operate in the micropillar form at room temperature. The room-temperature bulk CRSS values for these slip systems can be obtained by extrapolating the power-law of the CRSS-specimen size dependence to the bulk size, which can be estimated to be 30-50 mm.

Laves phases are the largest class of intermetallic phases, showing very high strength at elevated temperatures, but being very brittle at room temperature. They can be encountered e.g. as strengthening precipitates in steels.
A unique single phase material of NbFe₂ Laves phase with hexagonal C14 structure was subjected to compression test at elevated temperatures. The compressive stress-strain curves are characterized by a pronounced stress peak in the stress-strain curves at temperatures up to 1200 °C and by steady state flow above 1200 °C.
The deformed microstructure was monitored by the means of transmission electron microscopy in order to reveal the deformation mechanism. Undeformed NbFe₂ is almost free of dislocations. After deformation at 1200 °C new dislocations are introduced into the material. Widely-extended stacking faults on the basal plane (0001) dominate the microstructure. Material deformed at 1300 °C shows a high density of dislocations, which are split into pairs of partial dislocations that bound stacking faults on the basal plane. With increasing deformation temperature the width of stacking faults decreases. Moreover the dislocation networks are formed. The main deformation mechanism in the stoichiometric NbFe₂ Laves phase is basal slip.
Increased content of Nb in the material leads to softening of the material and enables activating of other slip systems – prismatic {1-100} and pyramidal {1-101}.

Zinc-coated (galvanized) steel is widely used in applications in automotive and building industries. The galvanized steel is sometimes heat-treated (galvannealed) to alloy the zinc coating with the substrate iron by diffusion, resulting in improved coating adhesion and weldability. The coating layer of galvannealed steel consists of a lamellar series of intermetallic compounds in the Fe-Zn system; Γ (Fe₃Zn₁₀), Γ₁ (Fe₅Zn₂₁), δ_1k (FeZn₇), δ_1p (Fe₁₃Zn₁₂₆) and ζ (FeZn₁₃) in decreasing order of the iron content. The crystal structures of all the intermetallic compounds consist of Fe-centered Zn₁₂ icosahedra, which share their faces or vertices with one another [1-3]. Because deformation and fracture properties of these intermetallic compounds determine the press formability response of the galvannealed steel, we have been investigating deformation behavior of each intermetallic phase through compression tests of polycrystalline/single-crystal micropillar specimens in order to elucidate the optimum microstructure of the coating layer [3,4]. Compression tests of single-crystal micropillars of each of the compounds have indicated that, if they exhibit plastic deformation, slip systems are selected so that the Fe-centered Zn₁₂ icosahedra are not destroyed [3]. This implies that the Fe-centered Zn₁₂ icosahedra might behave as if they were large-sized atoms during the plastic deformation. In the present study, we have observed the atomic arrangement of the cores of dislocations and stacking faults bounded by partial dislocations with an aberration-corrected canning transmission electron microscope, and found that the Fe-centered Zn₁₂ icosahedra indeed seem to behave as if they were large-sized atoms during the plastic deformation.

[1] R. Belin, M. Tillard, and L. Monconduit, Acta Crystallogr. C 56, 267 (2000).
[2] N. L. Okamoto, K. Tanaka, A. Yasuhara, and H. Inui, Acta Crystallogr. B 70, 275 (2014).
[3] N. L. Okamoto, M. Inomoto, H. Adachi, H. Takebayashi, and H. Inui, Acta Mater. 65, 229 (2014).
[4] N. L. Okamoto, D. Kashioka, M. Inomoto, H. Inui, H. Takebayashi, and S. Yamaguchi, Scripta Mater. 69, 307 (2013).

A novel complex intermetallic compound is known to possess unique chemical, physical, electronic, and magnetic properties, and is regarded as an excellent candidate material for future devices. Nevertheless, their extreme brittleness at room temperature has significantly limited their practical applications. However, the recent micro-/nano-pillar studies at small length scales revealed that a brittle material could become ductile as a result of size reduction. For instance, metallic glass, nanocrystalline metals, Si and GaAs, which are extremely brittle at bulk scale, can exhibit flaw insensitivity as well as the reasonable amount of ductility at the sub-micron length scale. Thus, this brittle-to-ductile transition due to the size reduction enables the investigation on the intrinsic plastic deformation behavior of brittle materials, such as intermetallic compounds, at small length scales.

Quasicrystals and their approximants are intermetallic compounds that have been widely studied due to their unique atomic arrangement and related physical properties. Especially, Y-Cd binary systems have been studied as an ideal system to understand the relationship between aperiodic crystal structure and magnetic properties. It is also of great interest to understand its mechanical properties due to their extraordinary crystal structures. In this study, we used a solution growth in the binary Y-Cd melt to synthesize a single crystal of YCd₆ quasicrystal approximant that has a body-centered cubic crystal structure with the Tsai-type icosahedral cluster consisting of Cd and Y shells. Then, we investigated its intrinsic mechanical properties using micropillar fabrication and in-situ micropillar compression in scanning electron microscope. We found the highly localized and extensive plastic deformation in the <10-1>/{101} slip system. We confirmed that the crystal structure contains {101} planes with the large interplanar spacing, which would lead the observed preferential slip. We also found the size dependence of yield strength with the power law exponent near 0.3, which would be related to the weakest link statistics. Advanced transmission electron microscopy was used to visualize the defect structure and to understand the plasticity of YCd₆ at small length scales. The fundamental mechanisms of plastic deformation will be discussed in terms of two possible viewpoints: dislocation slip vs. rigid translational slip. This study will give an insight into understanding of a new plasticity mechanism in a novel complex intermetallic compound.

Transition-metal-based Laves phases show excellent mechanical properties at high temperature, but their brittleness at low temperature is a great drawback. Our knowledge about the plasticity of transition-metal-based Laves phases and the way mechanical deformation proceeds is quite incomplete. Moreover, the existing literature indicates their mechanical properties strongly depend on the composition but the underlying mechanism is not fully understood yet.
A severe challenge for a systematic study of the mechanical properties of the Laves phases is the preparation of appropriate samples. Their distinct brittleness makes it difficult to prepare flawless bulk samples for mechanical tests. Besides, sample condition including grain size, second phases and impurities influences the mechanical behavior of bulk samples and it is hard to make sure all samples have the same condition. The uncertainty of sample condition may mask the composition and crystal structure dependence of their mechanical properties. Moreover, the homogeneity range of a transition-metal-based Laves phase is usually quite large. In order to get statistically significant conclusions, a lot of repetitive and time-consuming work has to be done. Therefore, we propose a new method for a comprehensive and precise study of the composition dependence of mechanical properties of intermetallic phases. The idea is to grow diffusion layers of target intermetallic phases with concentration gradient using the diffusion couple technique, cut a series of micro-sized, single-phase and single-crystalline samples with different compositions in the diffusion layers using the focused ion beam (FIB) technique and then study their mechanical properties with micromechanical testing.
In the present work, the composition dependence of mechanical properties of NbCo₂ Laves phases was studied using this method. With the diffusion couple technique, layers of C36, C15 and C14 NbCo₂ Laves phases with extended diffusion zones and coarse grains were obtained. Hardness was measured in the diffusion layers along the concentration gradient using nanoindentation. A series of micropillars and micro-sized cantilever beams with different compositions were fabricated by FIB. The critical resolved shear stress and fracture toughness were measured by pillar compression and single cantilever bending, respectively. The influences of sample size and dislocation density on the mechanical behavior of the micro-sized samples were also studied.

Magnesium alloys are very attractive in such applications as automotive, railway and aerospace technologies. However, their low ignition temperature and low mechanical strength have restricted their use. New high-strength magnesium-base alloys with heat resistance and flame resistance have been developed in Japan and are now the focus of wide attention in many parts of the world. These new alloys are strengthened by a unique phase having long-period stacking ordered (LPSO) structure. These alloys are therefore called LPSO-type Mg alloys. The LPSO structure, which is formed in Mg-M-RE alloys (M is Co, Ni, Cu or Zn, and RE is limited to Y, Gd, Tb, Dy, Ho, Er or Tm), features synchronization with respect to chemical and stacking modulations. The LPSO phase has better mechanical properties than α-Mg phase and undergoes kink deformation. This kink deformation drastically improves the mechanical properties of LPSO phase. This kinking can be a new concept for the strengthening of metals. The LPSO-type Mg alloys produced by extrusion of cast ingots have high yield strength (350-520 MPa for 0.2% proof strength) and reasonable elongation (5-15 %) at room temperature, and high elevated-temperature yield strength (250-350 MPa at 473 K). These mechanical properties are superior to those of ordinal magnesium alloys such as AZ91, and high strength aluminum alloys such as super duralumin and extra-super duralumin (7075-T6). Moreover, the LPSO-type Mg alloys produced by rapidly solidified powder metallurgy (RS P/M) processing exhibit higher mechanical properties and corrosion resistance than the LPSO-type Mg alloys produced by ingot metallurgy (I/M) processing. The ignition temperature of the LPSO-type Mg alloys is ranging from 1053 to 1213 K, which is much higher than that of ordinary magnesium alloys (743-823 K).

Intermetallics are stable compounds composed of metals organized in a long range order. They are favored structures for their mechanical, catalytic and other properties due to the high dispersion of one metal in another. However, the formation of intermetallic compounds is often elemental specific. For example, Cu forms alloys and intermetallic compounds with Au but it is immiscible with Ag based on the binary phase diagram. Like Cu, Pt is also immiscible with Ag. Previous studies showed the existence of an intermetallic L1₁ phase within a very narrow composition window. However, due to the limitations in traditional metallurgy process, phase pure Ag-Pt intermetallic has never been observed. Here, we demonstrated a bottom-up approach to the production of Ag-Pt compositional intermetallic phase. By thermally treating the alloy nanoparticles in an inert atmosphere, we successfully obtained an Ag-Pt intermetallic judging from the powder X-ray diffraction patterns of the thermally treated samples. Powder pattern simulations show this structure best resembles the synthetic materials constructed from unique stacking of interchangeable close-packed Ag and Pt layers. This rather unique stacking results in wavy patterns of Ag and Pt planes in the scanning transmission electron microscopy (STEM) micrographs. The intermetallic Ag-Pt obtained is highly active for the electrochemical oxidation of formic acid at low anodic potentials, 5 times higher than its alloy nanoparticles, and 29 times higher than the commercial Pt/C at 0.4 V (vs RHE) in current density. The high activity is very likely due to the high dispersity of Pt atoms in the intermetallic compound.

High-strength high-temperature alloys are often strengthened by intermetallic compounds, thus an effective search strategy for stable intermetallic compounds that can precipitate at desired temperatures can lead to new classes of high-temperature alloys. The morphology of the precipitates and their microstructure stability are also important consideration for high-temperature applications. A dual-anneal diffusion-multiple (DADM) approach will be described as an effective way to screen large composition spaces in order to discover new strengthening precipitates. An example will be used to illustrate the DADM approach in discovering the Chi-phase strengthened Fe-Cr-Mo based ferritic steels.

Tempered martensitic high Cr heat resistant ferritic steels have are conventionally strengthened with fine carbonitride and carbide particles such as M₂₃C₆, VN and NbC phase particles. Such fine particles are, however, not stable and coarsened after long-term creep exposure at a temperature above 600 °C in recent steam power plants and thereby losing their strengthening effects. Fe₂M type Laves phase may be a good candidate to replace such carbide and carbonitride phases since slower coarsening rates are expected in the Laves phase due to slower diffusivity of substitutional elements than interstitial elements. Nevertheless Fe₂Mo and Fe₂W form in coarsened globular shapes in the conventional ferritic steels and are therefore not considered effective for creep strengthening.
We have recently found periodically arrayed rows of very fine Fe₂Hf Laves phase particles formed in 9 % chromium ferritic matrix through interphase precipitation in a reaction path of delta ferrite -> gamma austenite + Fe₂Hf with a subsequent phase transformation of the austenite into alpha ferrite. In the present paper creep test samples with the fine Fe₂Hf particles dispersion in 9% Cr ferritic matrix were prepared by careful heat treatments based on time-temperature-transformation diagram that was previously investigated by our recent works. Creep tests were performed at a temperature of 700 °C under stress conditions of 60, 40 and 30 MPa. The lower the stress condition is the more effective the introduction of the fine particles becomes; the time to rupture increases from 150 to 350 h at 40 MPa and from 500 to ~1500h at 30 MPa by introducing the fine particles into the ferritic steel. The fine Laves phase particles remain in grains after the creep tests, whereas they were coarsened at grain boundaries and particle free zones were formed close to the grain boundaries. Local deformation behaviors with respect to grain boundary characters and fine particle distribution will be explained, and a new strategy to enhance the creep strength of high Cr heat resistant ferritic steels will be discussed in the paper.

Ferritic materials showing a disordered matrix and a significant fraction of intermetallic precipitates are promising candidates for applications at elevated temperatures. Among their most beneficial properties are excellent oxidation and corrosion resistance, low material and production cost, reasonable strength, good thermal conductivity, and small thermal expansion. However, one of the main drawbacks of these alloys is the decline in strength above 600-700 °C. Our approach to overcome this problem was to investigate different alloy compositions in the system Fe-Al-Ti-Ni(-Cr). In particular, the microstructures and mechanical properties of cast and heat-treated material were analyzed. To describe transformation temperatures and the microstructural evolution of the prevailing phases, differential scanning calorimetric measurements as well as special heat treatments were performed. The final microstructures were characterized employing scanning and transmission electron microscopy along with hardness measurements. Nanoindentation and compression tests were further conducted to obtain a better understanding of the mechanical properties in terms of thermally activated deformation processes. Finally, a relationship between microstructure and mechanical properties is presented.

Although the benefits of titanium aluminides for high temperature applications were well conceived and significant research and development activities were conducted in the past four decades, they remained as developmental materials due to barriers associated with melting, processing, scale-up, and affordable productionization. Demanding requirements of efficient aero-engines and extensive risk reduction demonstrations paved the path for commercial introduction of gamma titanium aluminides, the single most attractive current application is for low pressure turbine blades in aero-engines replacing conventionally cast nickel superalloys. Bringing a new material from lab to production often reveals critical missing technical elements and these issues must be addressed and resolved prior to product introduction. This talk provides a snapshot of gamma TiAl journey from R&D to productionization for critical aerospace applications.

Advanced TiAl alloys are nowadays successfully used for aero engine low pressure turbine blades in new generation of civilian aircrafts [1]. Nevertheless, TiAl alloys show only limited room temperature ductility. This fact makes further alloy and microstructure optimisation indispensable [2]. One way to improve the properties is to generate an alternative microstructure design, by utilizing the cooling-rate dependent massive transformation and subsequent annealing heat treatments. Such a microstructure is known as a “convoluted microstructure” and shows improved mechanical properties [3]. During slow cooling from the high-temperature α-Ti(Al) (hcp structure) phase field, lamellae of the tetragonal γ-TiAl phase (L1₀ structure) precipitate on α-basal planes according to the Blackburn orientation relationship: {111} γ || (0001) α and <1-10> γ || <11-20> α. Faster cooling can result in a sudden massive transformation of α-phase into γ-phase. During subsequent annealing, α₂ lamellae precipitate on all four tetrahedral planes (111) of the γ phase resulting in the aforementioned “convoluted microstructure”. In this study a quaternary Ti-45Al-4Nb-4Ta (at.%) alloy was investigated using in situ high-energy synchrotron X-ray diffraction (HEXRD). Only in situ experiments enable the continuous monitoring of phase evolution during heating and cooling with high frame rates up to 10 Hz. In particular the transformation start temperature, the critical cooling rate, and the undercooling required for the massive transformation, as well as the subsequent α₂ precipitation during annealing have been investigated directly. According to the information received from the in situ HEXRD experiments a CCT-diagram was calculated. In addition the microstructure developed after the massive transformation and after subsequent annealing was characterised by EBSD and TEM investigations.

[1] P. Janschek, Wrought TiAl Blades, Materials Today: Proceedings 2, Supplement 1 (2015), S92-S97.
[2] F. Appel, J.D.H. Paul, M. Oehring, Gamma Titanium Aluminide Alloys, Wiley-VCH (2011).
[3] Hu, D., et al., On the massive phase transformation regime in TiAl alloys: The alloying effect on massive/lamellar competition, Intermetallics 15(3) (2007), S327-332.

The study of creep behaviour of materials for high-temperature applications, usually requires a large series of long creep tests under different conditions of temperatures and stresses, involving a non-negligible amount of samples and time, which very often constitute a problem, particularly for the development of new prototype alloys.
In this work we present a new methodology to study the mechanisms responsible of creep behaviour by a non-destructive technique called Mechanical Spectroscopy, which requires only one or few small samples for the complete study. Furthermore, from the high-temperature background (HTB) of the internal friction spectra, it can be predicted the comparative creep resistance behaviour of different materials. Initially the theoretical model, which allows an analysis of the HTB and its comparison with creep, will be presented in the frame of the thermally activated motion of defects [1]. A short description of the specific equipment developed to work under high vacuum up to 1700 K, will be also presented.
Then the analysis of the HTB will be applied to study several γ-TiAl intermetallics up to 1675 K. In particular, measurements on some TNM alloys, as well as on a series of Mo bearing (1% to 7%) special TiAl alloys, will be presented and analysed. Finally, the results obtained by mechanical spectroscopy will be compared with those obtained in the literature by classical creep tests. This comparison will allow us to validate the presented methodology as an alternative method to study the microscopic mechanisms involved during creep of intermetallics.
[1] M. Castillo-Rodríguez, M.L. Nó, J.A. Jiménez, O.A. Ruano, J. San Juan. Acta Mater. 103, 46-56 (2016).

Light-weight alloys based on intermetallic titanium and aluminides (TiAl) are structural materials used for several high-temperature applications, e.g. in aero engines or automotive engines. Technical TiAl alloys consist normally of two phases, the γ- and the α₂-phase. Recent developments have led to the so-called TNM alloys (N = Nb and M = Mo) with a rather low Al-content. These alloys possess the disordered body centered cubic β-phase at elevated temperatures. This third phase ensures better processing compared to conventional two-phase alloys. However, the relatively low Al content (< 45 at.%) limits the high temperature capability due to reduced oxidation resistance. This impedes their application in the desired temperature range above 800°C. This work shows how the fluorine effect counteracts this disadvantage due to the formation of a protective alumina layer. The performance of the TNM alloy with the nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (at %) is compared with another TNM alloy containing additional elements, such as Si and C, and the so-called GE alloy (Ti-48Al-2Cr-2Nb) which is already in use for turbine blades. The results of isothermal and thermocyclic high temperature exposure tests of untreated and fluorine treated specimens will be compared and the effects of composition and microstructure of the alloys on the oxidation behavior with and without fluorine treatment will be addressed.

TiAl alloys are increasingly used as light weight material, for example in aero engines, which also leads to a request for suitable repair methods. For this purpose, diffusion brazing is a promising method for the closure of cracks (in noncritical or not highly loaded areas) as it is already used for Ni-base superalloys. Therefore, two different brazing solders based on Ti-Fe and Ti-Ni were investigated for joining the alloy Ti-45Al-5Nb-0.2B-0.2C (in at. %).
Tensile tests at room temperature show different mechanical strength depending on the brazing solder. Furthermore, the phases and their development and distribution over the brazing zone were investigated time and space resolved during the brazing process. Here, the two brazing solders show differences in development and fraction of the phases. These analyses were performed with high-energy X-ray diffraction using the HZG-run materials science beamline HEMS at the synchrotron radiation facility at DESY in Hamburg, Germany.
In addition, analysis with electron microscopy and electron backscatter diffraction show significantly different grain sizes in the brazed region for both brazing solders.
The results of phase analysis and electron backscatter diffraction can be combined to explain the different microstructure after the brazing process and thus the different mechanical strength depending on the two brazing solders.

Demands for innovation of structural materials for worldwide issues on energy, environment and security/safety are becoming extremely strong to build up sustainable and low-carbon societies. In fact, advanced ultra supercritical (A-USC) power plant projects are in progress in US, Europe and Japan, at which the steam temperature is raised up to 1073 K from current 873 K in order to improve the efficiency of power generation. High performance jet engine development to have larger thrust-to-weight ratio is also on this platform, since more than 30,000 new airplanes are to be produced by 2030. In Japan, several national projects related to these issues are currently going on, one is “Advanced Low Carbon Technology R&D Project (ALCA) and the other “Structural Materials for Innovation (SM⁴I)” in Cross-ministerial Strategic Innovation Promotion Program (SIP), where author is committed to both. In this talk, the target and outline of these projects are briefly introduced, and our recent progress on the materials innovation technologies using intermetallics for power plant and for jet engine materials are reviewed. In power plant materials, a novel design concept for the development of a new class of austenitic steels strengthened by TCP Laves (C14, hP12) and Sigma (D8_b, tP30) phases is presented, where the both phases are implicitly believed to deteriorate the mechanical properties, but this thought is not necessarily true. The model steels exhibits excellent creep properties at 1073 K, equivalent to Ni-based alloys. The mechanism of the superior creep strength of “Grain-boundary precipitation strengthening” is presented. In jet engine materials, another novel design concept for the development of wrought TiAl alloys to be used for LPT and HPC blades is presented. Introduction of bcc b-Ti phase makes it possible to have excellent hot workability in process and good creep and crack propagation resistance in service temperatures in our model alloys, through the microstructure design using a unique phase transformation pathway. In both cases of the Laves phase steels and titanium aluminides, phase diagram studies on the ternary systems of Fe-Ni-M and Ti-Al-M (M: transition metal elements) are the key for the innovations. Details of the design approaches for materials technologies will be discussed. Part of this study was carried under the research activities of ALCA and SIP in JST (Japan Science and Technology Agency).

The IRIS alloy (Ti_49,92Al₄₈W₂B_0,08) has been developed in consistency with the Spark Plasma Sintering, which is a technic of powder metallurgy for which the sample is heated by Joule’s effect. The aim of this paper is to present a study of the microstructure and of the mechanical properties of this alloy.
The IRIS-SPS alloy exhibits a fine near-lamellar microstructure of the IRIS alloy, which was analyzed by scanning and transmission electron microscopies. It is formed by lamellar grains surrounded by extended γ zones containing β₀ precipitates. The size of lamellar grains ranges from 35 to 45µm while the width of the borders remains between 5 and 10 µm. Orientation relationships between γ lamellae and γ grains in the borders, as well as between the γ matrix and the β₀ precipitates are investigated. As a result of these investigations, a formation mechanism of this microstructure is proposed and discussed. The origin of the grain growth limitation during the SPS processing is particularly analyzed.
Tensile tests are performed from room temperature to 1000°C. Creep properties are determined at 700°C/300MPa, 750°C/120MPa, and 750°C/200MPa. The tensile strength and the creep resistance at high temperature are found to be very high compared to the data reported in the current literature while a plastic elongation of 1.6% is preserved at room temperature. A grain size dependence of both ductility and strength is highlighted at room temperature. The deformation mechanisms are studied by post-mortem analyses on deformed samples and by in situ straining experiments, both performed in a transmission electron microscope. In particular, a low mobility of non-screw segments of dislocations at room temperature and the activation of a mixed-climb mechanism during creep are identified.
Finally, it will be shown that the γ borders that surrounded the lamellar colonies play a key role on the control of the size of the grains and on the mechanical properties. During the SPS cycle, the β phase surrounding the α grains limits their growth and prevents their coalescence in addition to the pinning effect of boundaries on borides. They provide more deformable phase during the deformation at room temperature while their optimized size is determinant for limiting the diffusion process occurring at interfaces at high temperature, thus contributing to the enhancement of the creep resistance.

Due to the rapid development of advanced additive manufacturing production routes in recent years, the demand of high quality alloy powders is significantly increased. We studied gas-atomised spherical powders of several Nb-bearing γ-TiAl based alloys, Ti-45Al-10Nb and Ti-45Al-5Nb-xC in at.% (x = 0, 0.5, 0.75, and 1), which were produced in-house by HZG using the plasma melting induction guided gas atomization (PIGA) technique. The phase composition of different powder fractions was determined by synchrotron high-energy X-ray diffraction at the HEMS beamline P07 at PETRA III (DESY), as well as by SEM, EDX, 2D and FIB based 3D EBSD measurements. Due to the high cooling rates in the range of 1*10⁴ K/s, the powder particles mainly consist of hexagonal-close-packed α- and body-centred-cubic β-phase. As the cooling rate depends on the particle size, considerable amounts of the β phase only were found in the small powder fractions. The total β phase amount was generally higher in the alloy with a higher Nb content, and also the effect of carbon, known as a strong α-stabilizer, was observed. Dendritic cauliflower-like structures are more pronounced in bigger powder particles due to the slower solidification and thus a higher Nb segregation in the remaining melt. The absence of preferred misorientation angles between α-grains indicates that α-grains are not formed out of already solidified β-grains by a solid state phase transformation.

The porous Ti based alloys are potential candidates for application in crumple zones used in automobiles due to their high stiffness, deformability and low density. A general route to fabricate the porous metals is sintering metal powder with the space holder powder of NaCl, whereas it is difficult to sinter titanium powder at lower temperature than the melting temperature of NaCl (801 ^oC) as the space holder. The addition of aluminum powder facilitates the sintering of titanium powder even with the NaCl space holder, which indicates a potential route for fabricating the porous Ti-Al intermetallic based alloys. In the present study, we have attempted to produce the porous Ti-Al alloys by pressure-sintering the mixed powder of Ti and Al with the NaCl space holder through a combustion reaction. The powders of Ti (particle size: <45 μm, 99.9% purity) and Al (particle size: <45 μm, 99.99% purity) were mixed to a total composition of Ti-20 at%Al and NaCl (particle size: 300-400 μm, 99.9% purity) was mixed with the Ti/Al powder to give a porosity of 60 vol%. The mixed powder was sintered at 700 ^oC under a constant pressure of 30 MPa to form a cylindrical specimen with a diameter of 20 mm. The soaking the specimens in pure water allows for the removal of NaCl and the formation of the pores to fabricate the porous specimens. Some of the specimens were heat-treated at various temperatures ranging from 850 to 1300 ^oC for 3 h. The compression test was carried out at a constant strain rate of 6.7×10^-3 /s at ambient temperature.
In the as-sintered porous specimen with a composition of Ti-20at%Al, titanium particles were surrounded by the Ti-Al alloy layer (consist of various intermetallics) with a thickness less than 5 μm and connected each other. After the heat-treatment at 1200 ^oC, a single-phase microstructure of Ti₃Al phase appeared in the porous specimen. The compression tests demonstrate that the as-sintered porous specimen exhibits the brittle behavior, whereas the heat-treated porous specimen shows both high plateau-strength level of approximately 100 MPa and high plateau-end strain over 50%, resulting in high absorption energy. These results indicate that the homogenization treatment for the formation of single-phase Ti₃Al intermetallics would be effective to achieve high energy absorption capacity of the porous Ti-Al alloy. The effect of the Al composition on the compressive strength of the porous Ti-Al alloys will be presented as well.

The superior strength of single crystalline Ni-base superalloys is mainly caused by the high volume fraction of the cuboidal L1₂ ordered γ’ hardening phase which is precipitated in a disordered face-centered cubic γ matrix. The strengthening effect is rooted in the difficulty for γ channel dislocations to cut into the γ’ precipitate. The interaction of dislocations with the γ/γ’ interphase boundary (IPB) is an inherently atomistic problem. A quantitative determination of the critical resolved shear stress τ_c required for dislocations in the γ matrix to penetrate into the γ’ phase is therefore difficult to obtain from experiments. Although atomistic simulations are ideally suited to study this process only very few, qualitative, studies exist in the literature. Idealized samples with periodic boundary conditions modelling infinite dislocations are well suited for quantitative studies; however, simplifications like the use of perfectly planar IPBs might artificially suppress important dislocation mechanisms. In addition, the lattice misfit between the γ and γ’ phases results in internal stresses which must be correctly accounted for in any large scale atomistic simulation of deformation mechanisms through the use of appropriate boundary conditions.
Here we report on atomistic studies on dislocation interactions with the γ/γ’ interface. A quantitative determination of τ_c by molecular statics simulations is presented in the case of a coherent planar (100) IPB in a quasi-2D simulation setup using two different embedded atom method (EAM) type potentials and two different types of dislocations. This allows us to relate τ_c to internal misfit stresses and correlate it with potential properties such as planar defect energies. The influence of the local chemical environment on these planar defect energies is also explored. These results can be directly used to parameterize mesoscale simulations such as discrete dislocation dynamics. The importance of the γ/γ’ interface curvature and γ/γ’ misfit stresses is also emphasized through the use of experimentally-informed atomistic simulation setups reconstructed from atom probe tomography data and scanning electron microscopy micrographs, which reveal the underlying fundamental dislocation mechanisms in the deformation of γ/γ’ microstructures.

In this presentation, we will show the effect of energetic ion irradiation on the transformation of lattice structures and mechanical properties for several Ni-based intermetallic compounds. It is known that they show good high temperature strength and oxidation resistance. For these properties, they have been applied for structure materials such as turbine blade.
Bulk specimens used in the present experiment were Ni-25at%X (X=Al, Nb, Ta, Ti, V) intermetallic compounds. They were irradiated with 1-200MeV heavy ions at room temperature. After the irradiation, the lattice structures of the specimens were investigated by using a grazing incidence x-ray diffraction measurement (GIXD). The local atomic arrangements around selective atoms were examined by means of EXAFS at a synchrotron facility. The change for surface hardness was measured by Vickers hardness tester. The microstructure observation was carried out by means of transmission electron microscope (TEM). The thermal annealing effect was also investigated after the irradiation.
Before the irradiation, Ni₃Al and Ni₃V bulk species show fcc based lattice structures at room temperature. On the other hand, Ni₃Ti, Ni₃Nb and Ni₃Ta show more complicated ordered structures. In the ion-irradiated Ni₃Al and Ni₃V alloys, we observed the disordered A1 structure (disordered fcc structure) which is intrinsically the high temperature structure or does not exist in the thermal equilibrium phase diagram. In the ion-irradiated Ni₃Nb and Ni₃Ta alloys, we observed the amorphous state. In the ion-irradiated Ni₃Ti intermetallic compound, we did not observe any lattice structure transformation. In the symposium, the ion beam induced phase transition will be discussed in terms of collective atomic displacements such as elastic thermal spike and sequential replacements of atoms. The recoveries of the irradiation-induced lattice structures and the mechanical properties by the thermal treatments will also be discussed.

Ni₃Al intermetallics-based alloys, such as Ni₃Al single-phase, Ni/Ni₃Al two-phase, and Ni₃Al/Ni₃V dual two-phase alloys, are attractive as high-temperature structural materials, because of their excellent mechanical and physical properties at high temperatures. It is also known that the strength of Ni₃Al-based alloys increases with increasing temperature (referred to as the anomalous temperature dependency of yielding), which is advantageous for high-temperature applications. Thus, there have been numerous investigations on Ni₃Al-based alloys; e.g. it was reported that elemental addition, such as Ti, Nb, and Ta, with Ni-based superalloys led to solid-solution hardening, although it deteriorated the ductility and thermal conductivity of the alloys. Here, age-hardening mechanism must be available to improve the strength without significant degradation of the other properties. Ni₃Al alloys containing V element should be a candidate of age-hardenable systems, based on Ni₃Al-Ni₃V pseudo-binary phase diagram. Therefore, in this study, variations of microstructure and mechanical properties during aging for Ni₃Al-V alloys have been investigated. Alloy ingots with a nominal compositions of Ni-13 at.% Al-12 at.% V were prepared by arc-melting in an argon atmosphere, and then cut into sample pieces. The samples were solution-treated at 1323 K (1050 °C) for 48 h in evacuated quartz capsules, and then quenched immediately into water. The quenched samples were re-encapsulated in a vacuum, and aged at 1123 K (850 °C) for 1 to 48 h. The as-quenched sample exhibited a single phase Ni₃Al (L1₂ structure: lattice parameter, a = 0.373 nm) microstructure, while in the samples aged after 4 h, a number of fine plate-shaped precipitates were found on {001} planes of the Ni₃Al matrix. The plate-shaped precipitates can be identified as Ni₃V (D0₂₂ structure: a = 0.381 nm, c = 0.592 nm), according to the corresponding selected area diffraction pattern by TEM observation. The number and size of the plate-shaped Ni₃V precipitates seemed to be increased gradually with increasing aging time. The Vickers hardness of the as-quenched sample was approximately 260 Hv. During aging the samples, the hardness value increased, and then reached a maximum of 320 Hv, followed by a subsequent decrease and saturation at approximately 290 Hv. The age-hardening behaviour observed for the Ni-13 at.% Al-12 at.% V alloy can be explained in the terms of precipitation of fine plate-shaped Ni₃V phase in Ni₃Al matrix.

The precipitation of γ’-Ni₃Al precipitates is crucial to the strengthening of Ni-base superalloys used in turbine engines. The L1₂ ordered precipitates impede the motion of dislocations through the γ solid solution matrix [1]. The size, shape and distribution of the precipitates determine their effectiveness in allowing the superalloys to be used at high temperature and under extreme load. Understanding the very first stage, the clustering and ordering, of the order-disorder transition which forms the precipitates after solution annealing is therefore key in developing improved superalloys. Studying the formation of clusters is generally a complex problem due to the spatial uncertainty of the position of atoms in any system because of the measurement technique and the thermal fluctuations of atoms around their ideal lattice positions. Based on atomistic simulations, where the spatial uncertainty can be ruled out, we developed a method to distinguish the chemically disordered γ phase from the chemically ordered γ' phase under high noise levels.
In this work we demonstrate how supervised learning and Naïve Bayes can be used to study the chemical order/disorder at the atomic scale based on the local neighbourhoods of individual atoms using bond geometry parameters [2]. We simulate Ni-20at.%Al γ phase systems with high chemical disorder using Molecular Dynamics (MD) and Monte Carlo (MC) techniques at temperatures up to 1500K using an EAM potential [3]. Applying the developed method we observe an increase in γ'-like atoms over time and thus ordering. The increased amount of γ'-like atoms are found to form clusters of increasing size satisfying the Becker- Döhring model. A comparison of the time dependencies of the Al and γ' concentration profiles show that the order-disorder transition begins with ordering while Al concentration changes become visible later on.

[1] R. C. Reed, The Superalloys: Fundamentals and Applications, CUP 2006.
[2] P. Steinhardt, D. Nelson and M. Ronchetti, Bond-Orientational Order in Liquids and Glasses, Phys. Rev. B 28 (1983) 784–805.
[3] Y. Mishin, Atomistic Modeling of the g and g’-phases of the Ni-Al System, Acta Materialia 52 (2004) 1451–1467.

Ni₃Al (L1₂) / Ni₃V (D0₂₂) two-phase intermetallic alloys show high strength and hardness at high temperatures and thereby are expected to be used for high-temperature wear-resistant applications such as tools and dies. For wear-resistant applications, hard coating processes such as thermal spraying and cladding by welding are practically used. Recently, laser cladding becomes widely used as a surface modification processing because this processing offers many advantages over other conventional techniques such as thermal spraying and arc welding. The laser cladding has a high potential for coatings with low substrate dilution, minimal distortion and strong metallurgical bonding with the substrate, leading to refurbishment or improvement of corrosion, wear and other surface related properties. In this study, the fabrication of the Ni base intermetallic alloy coating by laser cladding was attempted by varying the laser power and powder feed rate. Atomized powder of the Ni base intermetallic alloy was laser-cladded on the substrate of stainless steel 304. The hardness and microstructure of the clad layers were investigated by Vickers hardness test, SEM, XRD, EPMA and TEM observations. The hardness of the clad layer was closely dependent on the dilution; it increased with decreasing laser power and increasing powder feed rate. By controlling the dilution (via varying the laser power and powder feed rate ), the clad layer with an almost identical hardness level to that of the Ni base intermetallic alloy fabricated by the ingot metallurgy method was obtained. Although the as-clad layer showed a featureless microstructure, the annealed clad layer exhibited a typical dual two-phase microstructure composed of the cuboidal-shaped primary Ni₃Al (L1₂) particles and the channel consisting of Ni₃V (D0₂₂) and Ni₃Al (L1₂) phases.

Ni₃V categorized to a geometrically close packed (GCP) structure is an order-disorder type alloy in which the order-disorder transformation occurs at T_C ≈ 1045 °C: a disordered phase with a cubic A1 (fcc) structure is stable above T_C while an ordered phase with a tetragonal D0₂₂ structure is stable below T_C. Also, Ni₃V is one of constituent phases composing of dual two-phase Ni₃Al and Ni₃V intermetallic alloys recently developed as high temperature materials by our group. The latest study has shown that the addition of W to the dual two-phase intermetallic alloys enhanced their strength by solid solution hardening and/or precipitation hardening. Accordingly, it is necessary to understand the effect of W addition on the lattice properties, microstructures and mechanical properties for two constituent phases composing of dual two-phase intermetallic alloys. In the present study, the lattice properties, microstructure and hardness of W-added Ni₃V alloys were investigated by means of scanning electron microscopy, X-ray diffraction and Vickers hardness measurement. A various amount of W (up to 5 at.%) was added to a stoichiometric Ni₃V alloy by three substitution manners, i.e., substitution for Ni, V and both Ni and V (Ni/V). Alloys made by arc melting were first homogenized at 1200 °C (above T_C) for 24 h and then annealed at 950 °C (below T_C) for 24 h to ensure the ordering of the D0₂₂ structure. Microstructural observation showed that the solubility limit of W in the Ni₃V phase was approximately 5 at.% for the alloy in which W was substituted for V while it was less than 1 at.% for the alloy in which W was substituted for Ni. The addition of W beyond the solubility limit led to precipitation of particles composing of W solid solution with a bcc structure in the D0₂₂ matrix. The hardening by the addition of W was larger in the alloys in which W was substituted for Ni than for V. From the hardness tests associated with microstructural observation, it was suggested that the moderate hardening observed for the alloys in which W was substituted for V is due to solid solution hardening by W atoms while the prominent hardening observed for the alloys in which W was substituted for Ni is due to precipitation hardening by W solid solution particles.

A Ni base dual two-phase intermetallic alloy shows a unique microstructure composed of primary Ni₃Al (L1₂) precipitates with a cuboidal shape and channel regions of the Ni₃V (D0₂₂) and Ni₃Al (L1₂) eutectoid phases. This intermetallic alloy has high phase and microstructural stability, leading to higher strength than the conventional Ni base superalloys at elevated temperatures. Also, the high-temperature hardness property of this intermetallic alloy is suitable for wear resistant applications: the reduction in hardness with increasing temperature is very small compared with conventional metallic materials such as W-Co cemented carbides and hardened steels. It has been reported that the strength of the dual two-phase intermetallic alloys is enhanced by alloying additions of Ti, Nb and Ta due to solid solution hardening while that is enhanced by alloy additions of Re, Mo and W due to precipitation hardening. However, the phenomenon of the latter strengthening (i.e., precipitation hardening) is complicated and greatly depends on the substitution manner of additional elements. In this study, the microstructural evolution and hardening behavior during aging heat treatment of the dual two-phase intermetallic alloys containing some refractory metals, especially focusing on W, were investigated. W was added to the base alloy by three substitution manners, i.e., substitution for Ni, Al and V. Alloy ingots were made by argon arc-melting and solution-treated at 1553 K for 5 h, followed by aging at 1248 K for various periods. Microstructures were characterized by FE-SEM, EPMA and XRD. Mechanical properties were evaluated by Vickers hardness and tensile tests. Age hardening occurred in all the alloys irrespective of the substitution manners of W although fine precipitation in the channel regions occurred only in the alloy in which W was substituted for Ni. However, the age hardening was most significant in the alloy in which fine precipitation occurred in the channel regions, i.e., W was substituted for Ni. The observed hardening will be discussed considering relative contribution of solid solution hardening, precipitation hardening and hardening by ordering (or stabilization of the eutectoid phases) in the channel regions.

Iron aluminides are currently once more subject of intensive research in view of their potential to replace stainless steels. Specifically at temperatures of about 700 °C in corrosive environments iron aluminides may be a cost-effective alternative to steels, but therefore their inadequate strength has to be largely improved. Among the concepts for increasing the strength of iron aluminides, incoherent precipitation of Laves phase suggests itself, as in many ternary Fe–Al–X systems equilibria between Fe–Al and a Laves phase occur. However, as casting usually results in inhomogeneous and coarse precipitates, which lead to strong but rather brittle alloys, ways to distribute the Laves phase in a more purposeful way in the microstructure have to be found.
It has been demonstrated for Fe–Al–Ta alloys, that the Laves phase can be precipitated either finely dispersed within the Fe–Al matrix or as a film along the Fe–Al grain boundaries by employing the metastable Heusler phase as a precursor. As formation of a metastable L2₁ Heusler phase also precedes the formation of the stable C14 Laves phase in the Fe–Al–Nb system, possibilities for a refinement of the microstructure by alloying with B were investigated.
Four quaternary Fe–Al–Nb–B alloys were produced by vacuum induction melting (VIM). The microstructures were studied by light optical and scanning electron microscopy (LOM, SEM) in the as-cast state and after annealing at 700 °C/100 h or 1000 h. Phases were identified by X-ray diffraction (XRD) and energy and wave-length dispersive spectrometry (EDS, WDS). The ductility of the samples was evaluated by four point bending tests and compressive yield stress and creep behavior were established.

Ordered phases with the B2 structure (β₂) have been widely recognized with their capability in improving the mechanical performance of Ti-Al based alloys, e.g. Ti-Nb-Al. For Ti-Mo-Al ternary system, the existence of β₂ phase with non-stoichiometric composition around Ti₂MoAl was early identified but there have been limited works concerning its mechanical properties. In this study, we investigated the high temperature stress-strain response of Ti-Mo-Al alloys containing β₂ phase, aiming to explore their potentials in engineering applications. Ti-Mo-Al ingots with various compositions were firstly prepared by a conventional arc-melting technique, from which the compression specimens with the dimension of ~2x2x4 mm³ were then cut using electrical discharge machining (EDM). The constituent phases of these alloys were investigated by X-ray diffraction (XRD) while their microstructures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All the compression tests were conducted at 1073 K with a constant strain rate of 2.1x10^-4 s^-1. The compressive strength of Ti-Mo-Al alloys generally increased with the concentration of Mo. Ti₅₀Mo₂₅Al₂₅ (at.%) exhibited strong strain hardening. Through exploiting the strengthening effect of β₂ phase, a significant enhancement of high temperature strength could be expected for Ti-Al based alloys.

Within a large-scale project to evaluate possibilities for application of intermetallic alloys in large diesel engines, the corrosion behavior of two Fe₃Al- and a NiAl-base alloy are investigated. Both classes of alloys do have outstanding oxidation resistance and high wear resistance. The present investigation aims at evaluating their corrosion behavior in oxidizing atmospheres, acids or molten salts.
In this study, the iron aluminide based alloys Fe-Al-Ti-B and Fe-Al-Ti-B with 2 at% Mo and the NiAl-base Ni-Al-Cr-Ta alloy are investigated. A good balance between strength up to 700 °C and sufficient ductility at ambient temperature characterizes the Fe-Al alloys while the NiAl alloy provides high strength at least up to 900 °C. The oxidation resistance of the alloys is established in air using a thermobalance. Hot corrosion is tested at 900 °C for 100 h in molten Na₂SO₄ salt. For wet corrosion, the electrochemical potential is measured in sulphuric acid with a pH of 1.6 at 97 °C. Additionally, NaCl, formic acid as well as nitric acid (all pH = 2) were added as pollutants.

Phase equilibria among β-Ti, α-Ti, α₂-Ti₃Al and γ-TiAl phases at elevated temperatures are significantly important not only for hot workability but also for microstructure control through phase transformations in TiAl base alloys. We revealed that the composition range of the β+α+γ three-phase coexisting region is very sensitive to temperature, and the region shifts toward the Ti-Al binary edge with decreasing temperature in any Ti-Al-M (M: V, Nb, Cr, Mo) ternary systems above 1473 K. In addition, the three-phase coexisting region above 1473 K changes to β+α₂+γ three-phase coexisting with decreasing the temperature through, not the α→α₂ ordering reaction, but a temperature invariant reaction, that is, a transition peritectoid reaction of β+α→α₂+γ. These study revealed an unique transformation pathway of β+α→α→α(α₂)+γ→β+γ in the ternary systems, thereby making it possible to achieve both hot free forging in process and toughness in service temperature by using the high and low temperature β phase. Then, next step is what happens on these phase equilibria and phase transformations if the interstitial element of carbon is added in TiAl alloys. There are a number of studies on TiAl alloys containing carbon, but most of them focused on mechanical properties, carbide identification and the morphology, and very limited studies have been reported on carbon effect on the phase equilibria. One of the reasons for the limitation is the difficulty in quantitative analysis of carbon in solution in each phase of β, α and γ phases by nondestructive method. In this study, thus, effect of carbon on phase equilibria among the phases in multi-component TiAl alloys with carbon up to 0.6 at.% has been examined at elevated temperatures. A particular attention has been paid to the quantitative analysis of carbon in each phase using soft-X-ray emission spectroscopy (SXES) method. This method allows us to detect the second order reflection of the C-Kα emission located at 140 eV in the ultra soft-X-ray region (<200 eV). The energy resolution of the spectrometer is under 0.3 eV, more than an order and two orders of magnitude better than that of the conventional wavelength dispersive spectrometer (WDS) and energy dispersive spectrometer (EDS), respectively. Above 1473 K, no carbides were observed in any phases in the alloys with 0.6 at.% C. The carbon in solution stabilizes the α against β phase, and the addition of 0.2 at.% C raises the phase boundary of β+α/α raises by 80 K, whereas it shows little effect on that of α/α+γ. In contrast, carbides of p-type Ti₃AlC and h-type Ti₂AlC are formed in γ phase below 1273 K. The carbon in solution affects the phase relationships much stronger than that of the substitutional elements M. The details of the change in phase equilibria will be presented in conjunction with the partitioning of carbon among the phases based on thermodynamics. A part of this work was supported by Cross-ministerial Strategic Innovation Promotion Program(SIP).

In the last decades there has been a growing interest in developing new intermetallic families, which would we able to improve the performances of superalloys at high temperature. In particular a new Ti-Al-Nb-Mo family called TNM is being optimized to fulfil the required performances. In the present work we will present an overview of the studies performed in the last decade on several γ-TiAl intermetallics, with special emphasis on the last generation of Ti-Al-Nb-Mo alloys, and the model Mo bearing alloys TiAl-3Mo and TiAl-7Mo.
As a deep understanding of the atomic mechanisms controlling plastic deformation at high temperature is a key point for a final optimization of the alloy, a new approach by mechanical spectroscopy has been considered. Indeed, mechanical spectroscopy has been used to measure the internal friction and the dynamic modulus evolution from 600 K up to 1625 K in the above alloys. Several relaxation processes has been found linked to atomic diffusion mechanism in the studied alloys. In particular a relaxation process has been observed at about 1050 K and characterized as a function of temperature and frequency in order to obtain the activation parameters of the responsible mechanism. The activation enthalpy has been measured as H= 3 eV and attributed to a reorientation of Al-Ti_V-Al atoms in α₂-Ti₃Al phase [1]. On the contrary, in Mo-bearing alloys in which the β-TiAl phase has been stabilized, another relaxation process has been observed with activation enthalpy of H=3.65 eV, attributed to a mechanism involving diffusion β in phase. The measurement of the activation parameters of these relaxations will be presented along the talk, and the corresponding atomic mechanisms will be also discussed.
[1] J. San Juan, P. Simas, T. Schmoelzer, H. Clemens, S. Mayer and M.L. Nó. Acta Mater. 65, 338-350 (2014).

Creep behavior of gamma TiAl alloys having compositions along the transformation pathway of α-Ti (hcp) to α₂-Ti₃Al+γ to β-Ti (bcc)+γ have been examined at 1073 K, in order to identify the microstructural stability on creep resistance. The specimen having an initial microstructure of nearly α₂/γ lamellar microstructure shows a minimum creep rate at a strain of 1 % with no microstructure change. However, the creep rate accelerates rapidly due to the strain induced phase transformation of α₂+γ to β+γ, resulting in the microstructure change to globular β/γ regions along the α₂/γ lamellar colony boundaries. On the one hand, the specimen having an initial microstructure of globular β/γ regions along α₂/γ lamellar colony boundaries by using the phase transformation of α+γ to β+γ by aging at 1473 K, shows higher creep rate to reach the minimum with no microstructure change. The creep rate of this specimen is higher than the specimen with initial microstructure of nearly α₂/γ lamellar microstructure at an initial of acceleration stage, then changes to small, leading to longer creep life. On the other hand, the specimen having the initial microstructure of β/γ lamellar microstructure along the α₂/γ lamellar colony boundaries by using the cellular transformation of α₂+γ to β+γ by aging at 1173 K, shows slowest creep rate to reach the minimum with no microstructure change. In addition, this specimen shows slowest creep rate in accelerating stage, leading to longest rupture life. These results clearly suggest that the existence of bcc β phase is not the factor to deteriorate the creep resistance if the morphology of the β phase is lamellar, while the β phase is the factor to deteriorate the creep resistance if the morphology of β phase is globular. In addition, the microstructural stability is effective in increasing the creep strength. And the detailed quantitative microstructure analysis associated with the dynamics of creep rate / time curves will be presented. A part of this work was supported by Cross-ministerial Strategic Innovation Promotion Program (SIP).

Significant performance improvement of aircraft engines and land-based gas turbine engines requests a new class of heat resistant materials with temperature capability beyond Ni-based superalloys. Alloys based on Nb silicides are considered as one of the promising candidate materials due to their good combination of high temperature strength and room temperature toughness through the proper microstructure control. However, since high temperature strength and room temperature toughness are generally in a trade-off relationship, attempts to further increase the high temperature strength often results in reduced toughness simultaneously. In fact the silicide phases are believed to have practically no deformability at room temperature, and toughness increment is attained by the deformation of the ductile niobium phase to hinder the further microcrack propagation. However our recent study shows that pop-in occurs in the α-Nb₅Si₃ phase during loading of nanoindentation. Note that pop-in is a strain burst phenomenon that occurs as a consequence of abrupt elastic strain energy relief accompanied by the onset of plastic deformation. The occurrence of pop-in indicates that plastic deformation occurs at room temperature while suppressing fracture to occur due to the presence of hydrostatic pressure underneath the indenter tip. We believe this pop-in behavior is quite important since the behavior of dislocation nucleation and multiplication under high stress concentration can be a piece of important knowledge to understand the deformation and stress concentration relief ability at the crack tip. The present study focuses on the dislocation structure developed by ex-situ indentation, i.e., nanoindentation. In-situ observation in TEM was studied by a picoindenter.

The ternary intermetallic compound Mo₅SiB₂, T2 phase with the body-centered tetragonal D8_l structure has been considered as a promising material for ultra-high temperature structural applications owing to its very high melting point (2200 °C), good oxidation resistance and excellent high-temperature strength. Previous studies on the deformation behavior of T2-Mo₅SiB₂ using millimeter-sized single crystals and multiphase Mo-Si-B alloys have revealed that dislocations with Burgers vectors of <100>, <110>, 1/2<111> and [001] are activated at high temperatures above 1200 °C, while brittle fracture in elastic regions occurs at lower temperatures. However, the details of operative deformation modes such as slip planes for these dislocations have not been fully clarified yet, partly because of the serious difficulty in surface slip trace analysis and the frequent occurrence of dislocation climb during high-temperature deformation. Recently, micropillar compression test at room temperature using micron- to submicron-sized single crystalline specimens has been proved to be useful for studying deformation behavior of brittle materials. In the present study, we have applied the micropillar compression technique to T2-Mo₅SiB₂ single crystals in order to clarify the details of operative deformation modes. Micropillar specimens with square cross sections with various size and loading axis orientations have been prepared by focused ion beam technique and tested in compression at room temperature using a nanoindenter equipped with a flat punch diamond tip. Two slip systems (001)<100> and {1-10}<110> are confirmed to be operative at room temperature in [021]- and [010]-oriented micropillars, respectively. For both cases, critical resolved shear stress values are extremely high about 2-4 GPa exhibiting the slight size-scale effect, i.e., ‘smaller is stronger’ phenomena, approximately following power-law relationships with relatively small power-law exponents.

TiC or ZrC-added Mo-Si-B alloys are newly developed as novel Mo-Si-B based alloys. Combining Mo-Si-B based alloys with these light-weight carbides, the density is reduced below 9.0 g/cm³, and both high-temperature strength and room-temperature fracture toughness are well improved. However, the effect of these carbides on the mechanical properties has not been clearly understood yet. In this study, to investigate the effect of ZrC phase on mechanical properties, the microstructure, high-temperature compressive strength and room-temperature fracture toughness of Mo-Si-B-ZrC alloys were investigated systematically. A series of ZrC-added Mo-Si-B alloys consisting of Mo solid solution (Mo_ss), Mo₅SiB₂ (T₂) and ZrC were prepared by arc-melting followed by heat treatment. The microstructure of the alloys was characterized by EBSP-OIM. The high-temperature compressive strength increased with increasing both T₂ and ZrC volume fractions. Meanwhile, the room-temperature fracture toughness increased with increasing Mo_ss and ZrC volume fractions. It was strongly suggested that ZrC phase plays key roles to improve both high-temperature compressive strength and fracture toughness in the Mo-Si-B-based alloys.

MoSi₂/Mo₅Si₃ eutectic composite is one of the candidates for ultra-high temperature structural materials because of its high eutectic temperature of 1900 C and fine script-lamellar microstructure formed simply by directional solidification (DS). However, the poor fracture toughness at room temperature is a drawback to be improved for its practical applications. Recently, we have systematically investigated the effects of ternary additions on the microstructural characteristics of MoSi₂/Mo₅Si₃ eutectic composites. In the present study, microstructure – fracture behavior relationship of DS ingots of binary, ternary and quaternary MoSi₂/Mo₅Si₃–based eutectic composites were investigated. The ternary elements are classified into two types depending on their solubility to the composites. Type-1 elements (Ti, V, Cr, Nb, Ta and W) with relatively large solubility can be used to control lattice misfit at interphase boundaries. Among the type-1 elements, Ta and W are found to be beneficial for promoting delamination at interphase boundaries by increasing the lattice misfit. Fracture toughness evaluation by the Vickers indentation method confirms the higher fracture toughness for Ta-alloyed DS ingots, which is inferred to be caused by the higher propensity of delamination and crack deflection at the interphase boundaries. Type-2 elements (Fe, Co, Ni, Ir, B and C) with very low solubility are found to reduce the lamellar thickness and modifies the morphology of interphase boundaries effectively by very small amount of their addition. In order to utilize these beneficial effects of both type-1 and type-2 elements, the Ta and Ni-alloyed DS ingot was prepared. Preliminary evaluation of fracture toughness of the quaternary DS ingots confirms that the fracture toughness value is improved by increasing a density of interphase boundaries where delamination and deflection of crack propagation frequently occur. These results suggest that the addition of both type-1 and type-2 elements to DS MoSi₂/Mo₅Si₃ eutectic composites is effective in improving fracture toughness by optimizing the lattice misfits and lamellar thickness.

Quite recently, Mo-5Si-10B-10TiC(at.%) alloy, called as the 1st generation MoSiBTiC alloy, has been developed, which possesses the density comparable to that of Ni-based SX superalloys (≤ 9.0 g/cm³) and excellent high-temperature compressive strength. The alloy system has great potential for ultrahigh-temperature applications such as gas turbines and jet engines. In order to safety predict the service lifetime of the alloy in an ultrahigh-temperature environment, the detailed analyses of strength and microstructure development after deformation is essential. This study aims to investigate the tensile creep properties of the 1st generation MoSiBTiC alloy. Some button ingots with the composition of 65Mo-5Si-10B-10TiC (at. %) were prepared by conventional arc-melting in Ar atmosphere, and subsequently annealed at 1800°C for 24h. Tensile creep tests were performed under various stresses including 137MPa at 1500°C. Microstructures of crept specimens were observed by scanning electron microscopy (SEM) and electron backscatter diffraction analysis (EBSD) in order to consider its deformation mechanism. The alloy had good tensile creep strength, e.g. the rupture time of 44 h under 137MPa. EBSD images of the crept specimens indicated the trace of dynamic recrystallization in Mo_ss. Sub-boundaries were observed in some T₂ grains, suggesting that a part of T₂ was also heavily strained at the temperature.

In the Mn-Si-V alloy system, the ν and H phases are present as two neighboring intermetallic compounds around compositions of Mn-18 at.% Si and Mn-20 at.% Si-10 at.% Mn, respectively. According to the previous studies on their crystal structures referred to as the ν and H structures, the ν structure can be characterized by periodic arrays of both dodecagonal and decagonal atomic columns, while only dodecagonal columns are regularly arranged in the H structure. However, a crystallographic relation between the ν and H structures has not been understood sufficiently. To clarify this, we have examined the crystallographic features of prepared Mn-Si-V alloy samples consisting of ν and H regions, mainly by transmission electron microscopy.
In this study, Mn-Si-V samples with compositions of (15-20) at.% Si-(0-5) at.% V were provided for the present experiment. From their x-ray powder-diffraction profiles measured at room temperature, for instance, it was confirmed that a coexisting state consisting of ν and H regions was found in 7 at.% Si-3 at.% Mn samples. We then focused on the crystallographic features of these samples to understand the crystallographic relation between the ν and H structures. Observation made by transmission electron microscopy first indicated that a column axis of a dodecagonal column in the ν structure was parallel to that of the H structure. In high-resolution electron micrographs taken from coexistence regions, furthermore, arrays of bright dots indicating dodecagonal atomic columns in the ν and H structures were found to be coherently connected to each other. Another interesting feature is that line-shaped contrasts due to structural defects were frequently observed in micrographs, particularly in ν regions. As for the origin of the line contrast found in H regions, it was analyzed to be due to atomic columns, which act as an antiphase boundary with a phase shift of π along a column axis. In this study, such a column is referred to as an antiphase-boundary (AB) column. Because a dodecagonal atomic column in the H structure is converted into a decagonal one in the introduction of AB columns, a local structure including a line-shaped defect can be identified as the ν structure. Thus, the ν structure may be regarded as a modulated structure of the H structure, in which AB columns with the phase shift π are periodically introduced into the H structure.

Additive manufacturing (AM or 3D printing) techniques have received a considerable amount of attention as new processes for fabricating near-net-shape parts of various metallic materials. Several types of AM technologies, i.e., selective laser melting (SLM) or electron beam melting (EBM) have been used for metallic parts fabrication. Recently, Das and his co-workers have reported that the single crystal growth of Ni-base superalloys by the SLM method is possible, which suggest the SLM technique can be used for fabricating single crystalline near-net-shape parts of various high-temperature intermetallic compounds. Thus, it is important to investigate the applicability of the AM technique to various high-temperature intermetallic compounds such as transition-metal silicides. Among them, interest has been paid to MoSi₂ with tetragonal C11b structure because of its high melting point (2020 C), good mechanical properties, excellent high-temperature oxidation resistance and relatively low density. However, further improvement of fracture toughness at room temperature and high temperature strength are required for its practical application. One of the possible ways to improve these insufficient mechanical properties is forming composites with other hard materials. Recently, we have systematically studied the microstructure – mechanical property relationship of binary and ternary MoSi₂/Mo₅Si₃ eutectic composites with a well-aligned script-lamellar structure formed simply by directional solidification (DS) process and revealed that the fracture toughness and high temperature strength can be improved by controlling interface properties and lamellar thickness. These results imply that the MoSi₂/Mo₅Si₃ eutectic composite is a good candidate for applying the SLM process to obtain bulk materials with an aligned microstructure.
Using directionally solidified MoSi₂/Mo₅Si₃ as a substrate, the melting of the surface was performed using a SLM machine by changing the printing parameters of laser power, scan speed and scan spacing. Similar script lamellar structure with drastically finer lamellar thickness was confirmed to be obtained by the laser surface melting. The lamellar structure tended to growth perpendicular to the liquid-solid interface during the melting, with some variations depending on the printing parameters. Also, the orientation relationship of the fine script lamellar is different than the usual orientation relationship found in DS ingots of MoSi₂/Mo₅Si₃ eutectic composites. The hardness of the fine script lamellar was measured and it had increased around 30% compared to the one of the substrate. These results confirm the viability of using the SLM method in MoSi₂/Mo₅Si₃ eutectic composites to obtain finer microstructures with improved mechanical properties.

Recently, TiC-added Mo-Si-B alloys have been developed for ultra-high temperature applications. TiC addition to Mo-Si-B ternary alloys improves their fracture toughness and high temperature strength. More recently, we found that ZrC-added alloys show better mechanical properties. On the other hand, the oxidation resistance of these alloys has not been studied. The present study focuses on the oxidation behavior of MoSiBTiC and MoSiBZrC alloys.
ZrC-added alloys showed larger rapid mass loss at the primary stage of oxidation at 1100°C compared with TiC-added alloys due to the vaporization of Mo and B oxides. The mass loss was followed by steady-state mass loss, indicates the formation of protective SiO₂ layer. For this reason, Ti and C are likely to be better additive elements for Mo-Si-B alloys. However, even for the TiC-added alloys, continuous mass loss due to pesting was catastrophic at 700 and 800°C since a protective SiO₂ layer didn’t form at these temperatures.

In this study, the effect of off-stoichiometry on the deformation behavior of Mo₅SiB_{2 (}T₂) at room temperature was studied by micro-compression tests. T₂ single-crystal micro-pillars cut from Mo-Si-B alloys with various compositions were uniaxially compressed. These Mo-Si-B alloys were produced by conventional Ar arc-melting and then annealed at 1900 °C for 150h and 1800 °C for 24 h in an Ar atmosphere. For each composition, the micro-pillars were compressed along [001], [100], [110], [021] and [443], respectively. Before testing, the compression axis was carefully measured by scanning electron microscopy (SEM) with the electron back scattered diffraction pattern (EBSP) method. In the micro compression test, the peak load and loading rate were set to be 0.1 mNs^-1. The deformation and fracture characters of tested pillars were observed by SEM and transmission electron microscopy (TEM) to identify deformation mechanism. Most deformed T₂ single-crystal micro-pillars exhibited a brittle fracture mode. The specimens which compressed along [001] direction have showed the highest fracture stress. Some of the T₂ pillars showed sliding on (001) plane.

Hot-dipped galvannealed (GA) steels are widely used for the chassis of automobiles and building materials because of their high corrosion resistance, weldability, and paintability. The coating layer usually consists of a series of intermetallic phases in the Fe-Zn system, stacked on the steel substrate in the order of Γ (Fe₃Zn₁₀), Γ₁ (Fe₁₁Zn₄₀), δ_1k (FeZn₇), δ_1p (Fe₁₃Zn₁₂₆) and ζ (FeZn₁₃). When the GA steels are deformed under severe conditions such as press forming operation, the coating layer occasionally fails (powdering/flaking), resulting in reduced corrosion resistance and paintability. The coating failure has been understood only phenomenologically, and almost nothing is known about the mechanical properties of each of the intermetallic phases. Previously, our compression experiments of polycrystalline micropillars of each phase prepared from the thin coating layer (~10 μm) of the GA steels have shown that the δ_1p phase with a hexagonal lattice (space group:P6₃/mmc) did not exhibit sufficient plastic deformation before premature fracture similarly to the brittle δ_1k and Γ₁ phases. However, considering that the optimum formability of GA steels is achieved when the coating layer consists mostly of the δ₁ (δ_1k/δ_1p) phase, the δ_1p phase might intrinsically exhibit deformability to some extent. In the present study, we investigated the deformation modes of the δ_1p phase via compression tests of single-crystal micropillar specimens with various sizes (0.5~10 μm with an aspect ratio of ~1:3) machined by the focused ion beam method. The compression tests of the micropillars were performed with a micro hardness testing machine equipped with a flat diamond tip at room temperature. The loading axes were a–axis ([2-1-10]), c–axis ([0001]), and the [33-62] orientation, which is inclined to the c–axis by 45 degrees. For the [33-62] orientation, slip on the (0001) plane was confirmed to operate by slip trace observations on two orthogonal surfaces of specimens. The slip direction for the (0001) slip was found to be <11-20> judging from the change in the specimen shape before and after compression. (0001)<11-20> slip system in the δ_1p phase exhibits a considerably weak size dependence of the critical resolved shear stress.

Transient liquid phase diffusion bonding (TLPDB) is a joining process that is processed at low bonding temperature by high heat resistance, and it has been investigated as a high temperature bonding solution of electric components such as die-bonding. (Cu, Ni)₆Sn₅ is an expected material for TLPDB since the diffusion speed of Sn in Cu-xNi (x = 5～15) is higher than that in Cu. In order to estimate reliability of electric devices, extensive studies on the mechanical properties of (Cu, Ni)₆Sn₅ has been required. Though we have some reports on the hardness of (Cu, Ni)₆Sn₅ thin layers measured by nano-indentation, the studies on mechanical properties of (Cu, Ni)₆Sn₅ bulks have been scarce because of the difficulty of bulk-sample preparation. Therefore, the purpose of this study is to prepare (Cu, Ni)₆Sn₅ bulks and to reveal their compression strength. (Cu, Ni)₆Sn₅ bulks were synthesized above 250°C by transient liquid diffusion process. Large plastic strain over 20% was obtained by constant low flow-stress at 300°C.

Ternary Mg-TM(transition metal)-RE(rare earth) phases with long period stacking ordered (LPSO) structures have attracted considerable attention as a new type of strengthening phases in Mg-alloys exhibiting high strength and high ductility simultaneously. Previous studies on plastic deformation of the LPSO phases using directionally-solidified (DS) ingots have revealed that the basal slip is the easiest deformation mode operative in most crystal orientations, while kink bands are believed to be formed when the grains of the LPSO phases were compressed nearly along the basal plane. However, there are still many unsolved problems on the operative deformation modes in Mg-TM-RE LPSO phases. Recently, we have conducted micropillar compression experiments of single crystalline Mg-Zn-Y LPSO phase as a function of the loading axis orientation and specimen size and confirmed the activation of basal slip, prism slip as well as the formation of a complex deformation-band structure (or kinked structure) depending on the loading axis orientation, specimen size and strain rate. In the present study, we have investigated deformation microstructures of various single crystalline micropillars by scanning transmission electron microscopy (SEM), transmission electron microscopy (TEM) and atomic-resolution scanning transmission electron microscopy (STEM). In addition, in-situ micro-compression tests in SEM and TEM have been conducted in order to investigate the formation process of deformation bands. When the loading axis is parallel to the basal planes, a complex deformation-band structure accompanied by so-called strain burst behavior is observed to be developed very rapidly, which results in serious difficulty in understanding the formation process of the deformation-band structure. By controlling the shape of the micropillar specimen and using in-situ compression equipment, we have successfully terminate the compression tests at the very early stage of the deformation-band formation. Microstructure analysis of the micropillar specimens by TEM/STEM have revealed that the deformation-band structure can be interpreted as a mixture of deformation twins, accommodation kinks, accommodation twins and additional basal slips inside the twinned regions. These observations suggests that the deformation twins are considered to play a key role for the development of the deformation-band structure.

The crystal structural natures of FeCr-σ phase (tP30) in Fe-Cr binary system have been investigated in order to understand the nature of the phase. Two alloys were prepared by arc melting in Ar. One is the stoichiometric composition of Fe-50at.% and the other is Fe-rich composition of Fe-43 at% Cr. These alloys were cold rolled first and equilibrated at 973 K for up to 1000 h with and without homogenization at 1273 K in bcc α-Fe single-phase region. Phase composition analysis and identification were conducted by EPMA and powder XRD technique, respectively. Obtained XRD profiles were further analyzed by means of Rietvelt analysis, to clarify the site occupation change of the elements in the five sub-lattices (M1, M2, M3, M4 and M5) of tP30 structure, together with the change in lattice parameters. It should be noted that ordering to σ phase is extremely sluggish, but it is highly enhanced by introducing strains. Fe-rich s phase has the lattice parameter of a axis smaller than that of the stoichometoric composition, whereas that of c axis becomes larger even though the atomic radius of Fe atom is smaller than that of Cr. Rietvelt analysis revealed that excess Fe atoms tend to preferentially go into the M5 sub-lattice site. Another finding from XRD results is that the lattice planes of {410}_σin Fe-rich s phase exhibits an unusual behavior in both its inter-planar spacing and atomic arrangements on the planes among other crystal planes, with respect to the that of the stoichiometoric σ phase. Observed planarity of 410 planes may also have something to do with preferential site occupation of Fe atoms into the M5 sub-latticesite. The relationship between preferential site occupation and its crystallographic features will be discussed.

Recently, the MAX phases with a general formula M_n+1AX_n (n = 1, 2, 3), where M is a transition metal, A is an A-group element, and X is C or N, have received considerable attention as one of the promising materials for various high temperature applications due to their unique properties combining those of ceramics (high melting points, high stiffness, low density, and good oxidation resistance) and metals (high electrical and thermal conductivity, thermal shock resistance, and excellent machinability). The MAX phases have layered hexagonal crystal structures composed by stacking pure M, A, and X atomic layers, which is expected to results in strong anisotropy of their deformation behavior. Previous studies using polycrystalline materials have revealed that the activation of predominant basal slip dislocations and the formation of kink bands. However, details of the deformation mechanisms and their exact role for attractive mechanical properties have not been fully understood yet, partly because of the lack of detailed studies using oriented single crystals of the MAX phases. Recently, mechanical tests using micropillars of single crystals have been proved to be useful for studying fundamental deformation behavior of various crystalline materials. In the present study, single crystalline micropillars with various loading axis orientations have been prepared from bulk polycrystals of one of the representative MAX phases Ti₃SiC₂, and tested in compression at room temperature using a nanoindenter equipped with a flat diamond tip. Deformation microstructures of Ti₃SiC₂ single crystalline micropillars have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The basal slip are confirmed to be activated in most loading axis orientations except for the cases where the loading axis is parallel or perpendicular to the basal planes. Critical resolved shear stress (CRSS) values of the basal slip is very low about 20-100 MPa exhibiting a trend of “smaller is stronger” approximately following a power-law relationship with a relatively large power-law exponents close to those observed in FCC metals. When the loading axis is parallel or perpendicular to the basal planes, fracture occurs without exhibiting any plastic deformation at extremely high stress levels of ~4 GPa or ~8 GPa. These results clearly indicate the extreme anisotropy in plastic deformation of Ti₃SiC₂ MAX phase.

Niobium and its alloys have attracted as new refractory-based high temperature materials due to their high melting points. However, these alloys show poor oxidation resistance at high temperature. Al₃Nb is known to be a brittle material but its oxidation resistance is good. As the fracture toughness of Al₃Nb single crystal and its dependence on the composition are not obtained, the micro-sized fracture testing proposed by Suzuki et al. is performed. Al₃Nb single crystal micron-order size cantilevers with a chevron notch were fabricated in a grain of the two-phase poly-crystals by using FIB (Focused Ion Beam). In this method, pre-cracks and grain boundaries which may affect the result of poly-crystal samples are eliminated from the cantilevers. Furthermore, this method is beneficial to fabricate the cantilever from a grain in multiphase alloys for evaluating the dependence of the fracture toughness on the composition of even line compound phases. In other words, this method has an advantage to evaluate the fracture toughness of each phase in desired multiphase alloys. From the load-displacement curves during bending by a nano-indenter, the average value of fracture toughness of Nb-rich Al₃Nb, which is made from the Al₃Nb+AlNb₂ alloy, is evaluated to be 2.9 MPam^1/2, while the fracture toughness of Al-rich Al₃Nb made from the Al₃Nb+Al alloy is also evaluated to be 3.3 MPam^1/2. From this result, the fracture toughness of Al₃Nb doesn’t largely depend on its Al/Nb ratio. The fracture toughness evaluated in this work is larger than the value of polycrystalline Al₃Nb of 1.8 MPam^1/2. The difference may be caused by effects of grain boundaries and/or pre-cracks in poly-crystal sample. 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.

Samples of Ni57-xNdxFe18Ga25 with x=2 and 4 were prepared in ribbon form by rapid quenching via melt spinning route. The samples were analyzed by X-ray diffraction (XRD), magnetic measurements and Mossbauer spectroscopy, both in the as-quenched form and after thermal annealing at 900 C for 2 min and 400 C for 2 hours. For x=2 the Nd atoms are completely dissolved in the Ni-Fe-Ga matrix, while for x=4 the additional occurrence of the secondary 2:17 phase could be resolved. These findings were supported by the analysis of hyperfine magnetic field sites and distributions obtained from the non-linear least-squares fitting of the Mossbauer spectra. Thus, for the sample with x=2 annealed at 400 C the Mossbauer spectrum could be resolved considering 2 singlets, with the IS of 0.15 and 0.21 mm/s, consistent with a certain degree of atomic ordering revealed also by the narrow diffraction maxima in the XRD pattern.
The room temperature transmission Mossbauer spectrum of the as-quenched Ni53Nd4Fe18Ga25 sample was fitted with a singlet corresponding to paramagnetic austenite and a hyperfine magnetic field distribution. The hyperfine magnetic field distribution extracted from the Mossbauer spectrum of the as-quenched Ni53Nd4Fe18Ga25 sample is bimodal, with distribution peaks at 7 and 15 T. These peaks are consistent with the presence of a magnetic phase, which could be assigned to the 2:17 secondary phase, in good agreement with the XRD and magnetization results. For the hyperfine magnetic field distribution extracted from the Mossbauer spectrum of the annealed Ni53Nd4Fe18Ga25 sample at 900 C, the distribution has a dominant peak at 10 T, which could be attributed to ferromagnetic austenite and a weaker peak at 15 T, which indicates the presence of the 2:17 secondary phase.
Thus, we found that Nd substitution for Ni in Ni57-xNdxFe18Ga25 alloys decreases the martensite transformation temperatures and exhibits a similar dependence on valence electron concentration to those in other shape-memory alloys.

Alloy design typically requires fine control of chemistry. In ternary and higher alloys, multiple compositional degrees of freedom result in a large potential alloy compositional space. Standard serial alloy synthesis approaches (arc melting, induction melting, etc.) are not conducive to effectively probing this expansive compositional domain. On the other hand, experimental approaches to synthesize metallic combinatorial libraries are generally limited to vacuum-based thin film approaches, which deviate from bulk-like properties due to the dominance of surface effects and residual stress derived in thin films. Here, we demonstrate an alternative approach to depositing combinatorial libraries based on electrochemical deposition of thick metal films. We demonstrate this technique on ternary Ni-Mn-Sn Heusler alloys.
Metallic Ni, Mn, and Sn films are electrodeposited sequentially onto a refractory tungsten substrate from separate acidic aqueous solutions in standard 3-electrode cells. Deposition rates and times are adjusted to control thickness of individual layers, and thus, resulting ternary alloy composition. After deposition, films are annealed under forming gas to allow for interdiffusion of alloy constituents, resulting in ~30 μm thick Ni₅₀Mn₃₇Sn₁₃ Heusler alloy films. Deposition conditions (pH, concentration, deposition rates) have been optimized to result in smooth films and minimize oxidation rates. Annealing times (3 to 7 hr) and temperatures (800 to 900 °C) are optimized to allow for alloy homogenization, while minimizing evaporation and diffusion into the substrate. X-ray diffraction, as well as wavelength dispersive spectroscopy of film cross-sections indicate a single homogeneous phase, and suggest that the technique results in good control over final alloy composition.
The effects of deposition sequence (W//Mn/Sn/Ni, W//Mn/Ni/Sn, and W//Sn/Mn/Ni), as well as the total number of layer repeats (while maintaining a constant overall thickness) on film composition and phase transformation properties are evaluated. Differential scanning calorimetry of Heusler alloy films suggest that phase transformations behave in a similar fashion to those observed in bulk alloys synthesized by traditional techniques. The prospect of this technique for more general alloy design problems is discussed.

Thermoelectric conversion technology is expected as one of the next generation power-generating technologies. Among thermoelectric materials (TE), Magnesium silicide (Mg₂Si) has advantages such as lightweight, low environmental load, abundance of elements, and high power generation efficiency at the temperature range from 600 to 900K. For automotive applications, TE modules are exposed to external force such as mechanical vibration and thermal stress in severe thermal environment. Therefore, it is necessary that especially improvement of the mechanical properties as well as the thermoelectric performance. Among mechanical properties, improvement of fracture toughness is essential because that of Mg₂Si is ~ 1.0 MPa･m^1/2. In this study, we fabricated the SiC-dispersed Mg₂Si by the Plasma Activated Sintering (PAS) method in order to increase toughness. The purpose of this study is to examine the effect of the particle size dependence of the SiC (particle size 2-3μm or 35nm) on the fracture toughness and thermoelectric performance of the Mg₂Si dispersed with x vol% SiC particles (x=0, 1, 5).
The Specimen (Φ15mm) was sintered by the PAS method. The Young’s modulus was measured by the ultrasonic pulse-echo method and the fracture toughness was measured by the indentation fracture (IF) method with the indentation load of 300 and 500 gf. Electrical conductivity, Seebeck coefficient and thermal conductivity were measured by the four-terminal sensing, the thermoelectromotive method and the laser flash method. Dimensionless figure of merit (ZT) was determined using those values.
The relative densities of the all specimens are more than 98.0 % regardless of the particle size. SiC particles are located on the grain boundary of the Mg₂Si. The Young’s modulus of the composite increases with the amount of the SiC particles. The fracture toughness of 1.53 MPa is obtained for 5vol%, 2-3µm SiC addition and 1.68 MPa is obtained for 5vol%, 35nm SiC addition. Crack deflection is caused at interface between Mg₂Si and SiC. Crack pinning on the grain boundary also acts efficiently. These effects are remarkable especially on the nanosized SiC-dispersed Mg₂Si. As for thermoelectric properties, nanosized SiC-dispersed Mg₂Si shows the ZT value comparable to that of Mg₂Si.

Thermoelectric materials can convert energies between heat and electricity. There are several metal chalcogenides such as copper selenides (Cu_xSe_y), copper tellurides (Cu_xTe_y) and copper sulphides (Cu_xS_y) that can be used in a variety of scientific applications such as solar cells, photodetectors, photothermal conversion devices, electroconductive electrodes, among others. There are also a number of methods for the synthesis of metal chalcogenides thin films, but for this work, the electrodeposition technique is being used for the deposition of copper selenide (Cu₂Se) thin films. The deposition conditions were established by doing a cyclic voltammetry of an acidic Cu-Se electrolytic bath with a molar concentration of [2:1] in the range of -0.5 V to 0.5 V vs. SCE. Several experiments were performed for this bath at different temperatures; however it was found that by using a temperature of 40 °C gives better results regarding the quality of the film. We report the formation of the semiconducting material Cu₂Se in thin film on soda-lima glass substrates coated by a 500 nm transparent conductive oxide (SnO₂:F) ~ 10 Ω/�sq. Cyclic voltammogram results gave us the specific potential range for the deposition of Cu₂Se thin films, from 0.1 to -0.2 V vs. SCE. Cu₂Se thin films were obtained at these potential values, all the samples exhibited a shiny purple-like colour. The deposition time ranged from 3 to 5 min to obtain a thickness between 2 – 5 mm. SEM results showed that the growth of Cu₂Se thin films followed the morphology of the substrate, so that the film is very compact and uniform with small particle size ~ 250 - 500 nm. EDS showed that it is necessary to perform a thermal annealing to adjust the chemical composition, because there is an excess of Se, a little bit off the stoichiometry. Thermoelectric properties are being studied and results will be presented.
Acknowledgements: This work was partially supported by CEMIE-Sol P26, DGPI-BUAP (México) and CONACYT grant No. 167993.

Magnesium silicide (Mg₂Si), an intermetallic compound, is a promising candidate for practical thermoelectric (TE) power generation due to its power generation capability and several advantageous attributes: it is nontoxic, cheap, light weight, and composed of elements abundant in nature. These extraordinary properties of Mg₂Si have recently stimulated increasing efforts to develop a high performance and environmentally benign TE power generation system using this material. Mg₂Si has already achieved a ZT value above unity, which exceeds a desirable target for practical use. In order to realize the practical application of this material for a TE generator, a low fabrication cost as well as an adequate lifetime at elevated operating temperatures are needed. The conventional pi-type module structure, which consists of both n- and p-type TE elements, would be thermally unstable over long periods of time at elevated temperatures because of a difference in thermal expansion between the n- and p-type elements. We therefore developed a TE module with a uni-leg structure, which consists of only n-type Mg₂Si elements, because it is advantageous in terms of cost as well as durability. In the present study, we introduced a new soldering alloy to reduce the electrical and thermal contact resistance between parts of the module; e.g., element-element, element-bridge, and the whole device and heat sources. Furthermore, we calculated distributions of temperature and heat current on the TEG module using ANSYS software, which is based on a finite elemental model. In the calculation, we used experimentally obtained transport coefficients of Mg₂Si: the Seebeck coefficient, and the electrical and thermal conductivities. We also performed heat transfer analysis using Flow Designer software to obtain the thermal impedance of our module.

Since Au-based shape-memory alloys exhibit good biocompatibility and excellent X-ray radiography, these alloys have a large potential to exceed Ti-Ni SMAs in the field of biomedical implant devices. Especially AuCuAl-based alloys have attracted attention. We have found that Fe addition to AuCuAl alloys is effective to improve ductility and decreases martensitic transformation temperature. However, since Fe is a magnetic element, the occurrence of artifact in magnetic resonance imaging (MRI) measurements must be concerned. From this viewpoint, ternary AuCuAl alloys composed of nonmagnetic elements are more desirable. However, it is still difficult to acquire both sufficiently low martensitic transformation temperature for superelasticity and good ductility in the ternary AuCuAl alloys judging from the present available data. The possible reasons for the degradation of ductility must be related to the motion of dislocations and grain boundary characteristics. In the case of Cu-based SMAs, the ductility of polycrystalline alloys is known to depend on microstructure strongly; the single crystal alloys as well as so-called bamboo structure alloys are ductile depending on specimen size and grain size. In this study, the deformation behavior of AuCuAl ternary and Fe-added AuCuAl quaternary alloys were investigated using micro-sized specimen and the results obtained were discussed in comparison with the bulk sized specimen both of which were fabricated from the same ingots.
All the alloys were prepared by Ar-arc melting method, and homogenized at 873K for 21.6ks and solution-treated at 773K for 3.6ks followed by water quenching. Microstructures were observed by scanning electron microscopy. Phase constituent and phase transformation were characterized by θ-2θ X-ray diffraction measurement and differential scanning calorimetry, respectively. Mechanical properties were evaluated by compression tests at room temperature using a testing machine specifically designed for micro-sized specimens where the specimens with the size of 20×20×40μm were fabricated by focused ion beam system.
XRD analysis revealed that 2at.%Fe-added AuCuAl alloy contains second phase which is evaluated to be α-Fe (bcc). By SEM observation the second phase was clearly seen at the surface of the polycrystalline Fe-added AuCuAl micropillar specimen. It was also found by compression tests that the Fe-added AuCuAl micropillar shows yield stress being 50MPa lower than that of the bulk specimen. Further deformation of the micropillar, the applied stress was almost kept constant around 100MPa regardless of strain, but large work hardening behavior was again seen when the strain exceeded 5%. This work hardening must be due to the elastic deformation of stress-induced martensite. However, during unloading, pseudoelastic shape recovery which appeared in the bulk specimen was not observed for the micropillar. This difference must be caused by the difference in the amount of accumulated dislocations.

Elastocaloric cooling (EC), also known as thermoelastic cooling, was recently recognized by the US Department of Energy as a promising alternative to state-of-the-art vapor compression (VC) cooling system. While VC is a mature, dominant technology currently meeting all refrigeration and cooling needs globally, the efficiency of its modern compressors are approaching the theoretical limit as well as contributing to an environmental footprint through the use of refrigerants with global warming potential (GWP). Hence, there is a need to develop an alternative, environmentally friendly high-efficiency cooling technology. EC utilizes shape memory alloys (SMA) that have a latent heat associated with the shape memory effect (SME) to generate heating and cooling. In order to prove the scalability of EC, a 400 W prototype system based on a smaller 100 W studied earlier, using nitinol tubes as functioning material and compression drive mechanism has been designed, constructed, and tested in this study. Four beds of nitinol tubes, and a heat-exchange system make up the components of the prototype. The material requirements for optimal performance and development of improved shape memory alloys for elastocaloric cooling will be discussed. This work is supported by ARPA-E.

The martensite microstructure of Ti-39Ni-11Pd shape memory alloy was investigated by electron backscattering diffraction (EBSD) analysis, transmission electron microscopy (TEM) and the geometrically nonlinear theory of martensite (GNTM) to reveal the place of incompatibility in the microstructure. Our previous study on a beta-titanium shape memory alloy revealed that the compatibility at martensite/martensite interface clearly depends on the kind of twin orientation relationship (OR) at the interface; most of the interface with {111} type I twin OR is compatible whereas most of the interfaces with <211> type II twin OR is incompatible. The nature of the difference between these two kinds of martensite/martensite interfaces is not clear at present. In this study, the martensite microstructure of Ti-39Ni-11Pd is investigated to reveal whether morphology of microstructure and the place of the incompatibility depend on the alloy system or not.
Ti-39Ni-11Pd (at.%) has cubic-orthorhombic (B2-B19) martensitic transformation that is similar to the transformation in the beta-titanium shape memory alloys. The alloy is fully martensite phase at room temperature and there are twelve habit plane variants (HVs). An ingot was fabricated by Ar arc-melting method and was homogenized at 1373K for 21.6ks. For TEM and EBSD observations, 3mm disks were cut by electro-discharge machining and electro-polished by a twin-jet method with an electrolyte of 80% CH3OH + 20% H2SO4. The lattice parameters were determined by θ-2θ X-ray diffraction (XRD) analysis.
GNTM showed that the kinematic compatibility at HV/HV interface can be held among all pairs of HVs if and only if non-zero additional rotation is applied to one HV. This means that the compatibilities at habit plane and HV/HV interface are competitive. In the EBSD observation at a lower magnification, twelve HVs and 1426 HV/HV interfaces were detected. The HV/HV interfaces were characterized by three kinds of twin OR: {111} type I (26%), <211> type II (70%) and {011} compound (3%). Irregular ORs were also observed (1%). The twin ORs at the HV/HV interfaces were precisely analyzed by the Kikuchi line method in TEM. The {111} type I twin OR was exactly held (within 0.1degs) whereas <211> type II twin OR had a deviation about 0.5degs; the incompatibility existed only at the HV/HV interface with <211> type II twin OR. Comparing with the previous results, the formations of the compatible HV/HV interface with exact {111} type I twin OR and the incompatible HV/HV interface with slightly misoriented <211> type II twin OR seem to be universal in the cubic-orthorhombic transformation.

The Co-Pt binary system exhibits a narrow two-phase region comprised of the ordered alloy phases L1₀ and L1₂. These connect to the high-temperature A1 parent phase by a eutectoid: A1 –> L1₀ + L1₂, at a nominal composition of 60 at% Pt. By continuously cooling an A1 alloy down through the eutectoid, it is possible to produce the unique nanochessboard structure via pseudo-spinodal decomposition. The nanochessboard is a self-assembled 2+1D-periodic tiling of magnetically-hard L1₀ nanorods, coherently embedded in a soft L1₂ matrix. Lateral lengthscales of the chessboard are of order 20 nm, and while we are exploring exchange-coupled magnetism in this structure, the emphasis of this presentation is on the phase transformation and phase equilibria. We combine x-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) to examine the eutectoid transformation and to verify the published binary phase diagram, in particular, the temperature of the eutectoid isotherm. We obtain some significant differences with the published diagram in the vicinity of the eutectoid. TEM and XRD help establish the lateral (compositional) extent of the two-phase region, while DSC was used to establish the vertical (temperature) extent. The study required reconciliation of seemingly contradictory results. For example, a continuous slow-cooling regimen (-40 ^oC per day) results in a well-structured chessboard where there is roughly equal mole fractions of L1₀ and L1₂. However, extended, isothermal annealing of the same sample, in order to approach equilibrium, produces a large shift in the phase fractions, where L1₂ instead makes up about 90% of the sample. We suggest that this result implies the existence of at least one re-entrant solidi bounding the two phase region. DSC heating cycles place the eutectoid isotherm 18 K higher than found by others. However, the DSC results require careful interpretation, since the thermal scan rates are nearly three orders of magnitude larger than those used in creating well-formed chessboards. For example, we consistently observe a significant and extended heat release feature prior to the main disordering peak, which complicates the analysis of the onset temperature for disordering. This early heat absorption may result from the temperature dependence of the order parameter, but the magnitude of the enthalpy is surprisingly large. Furthermore, cyclic heating and cooling cycles produce roughly identical enthalpies of transformation, despite the large scan rates leading to incomplete ordering upon cooling. The role of coherency strain will be discussed. JF, EV and PG gratefully acknowledge support of the National Science Foundation through grant DMR-1105336.

Microstructural changes and interdiffusion in Cu – 48 mass% Zn specimens prepared by ultrasonic powder consolidation (UPC) and ball milling were investigated by optical microscopy, SEM and X-ray diffraction. UPC produced disc-shaped specimens 4 mm in diameter and about 1 mm in thickness.
Ball milling yielded composite powders 1 - 2 mm in diameter exhibiting alternate layers of Cu and Zn characteristic of ball-milled powders. In both processes, significant interdiffusion occurred between Cu and Zn. Ball milling producing beta brass between Cu and Zn, which was identified from its characteristic yellowish color. Using a few seconds of UPC combined after a short duration of ball milling or heat treatment, significant beta brass formation occurred in in the sample, whereas it took a few hours of ball milling to observe it to a comparable degree. The effects of ultrasonic deformation on the interdiffusion in UPC are discussed.

This paper discusses the concept, microstructural evolutions and reliability of bismuth-nickel transient liquid phase (TLP) bonding as a joining method for the components in high temperature electronics as a substitute for lead-based solders. TLP bonding can be made by melting pure Bi (that melts at 271°C) on the surface of Ni, which reacts to form a uniform structure of intermetallic compounds (IMC) that have much higher temperature capability (no decomposition up to 470°C) than pure Bi. Such conversion of pure Bi layer into the IMC occurs in the liquid phase via isothermal solidification. Ni diffuses into molten Bi resulting in the consumption of the Ni layer and the formation of Bi₃Ni intermetallic phase at the solid-liquid interface. Studies of TLP bond microstructures with increasing time for isothermal portion of the bonding process indicates an increase in the amount of Bi₃Ni phase and shows evidence of the formation of BiNi that has even higher temperature capability (614°C). The nucleation and growth kinetics of both intermetallic phases are further studied in this work. The shear strength test was performed on the sandwiched coupons joined by the TLP bonds to evaluate their performance at various processing conditions and the corresponding microstructures. The shear strength was not degraded when tested at high temperatures until 350°C, which indicates the good potential to use the TLP bonds for high temperature operation conditions. High temperature aging and thermal shock tests were also carried out to evaluate the reliability and microstructural stability of the bonds.

Microstructural and electrical properties of Ni(Ti)-germanide formed by using a co-sputtering of Ni and Ti on epitaxial Ge film grown on Si (100) substrate (Ge-on-Si) have been investigated as a function of germanide formation temperatures. During co-sputtering, the DC power of Ni target was fixed at 100 W, while that of Ti target was varied from 0 to 50 W to adjust the amount of Ti atoms incorporated in Ni(Ti) films. For the Ni(Ti)-germanidation process, an rapid thermal annealing (RTA) was carried out at 300 – 700 °C for for 30 s under N₂ ambient. For all samples, the sheet resistance gradually decreased with increasing RTA temperature up to 500 and 600 °C, above which the resistances increase due to the agglomeration of the germanide film. The sheet resistance of the as-deposited samples increased with increasing the input DC power of Ti target. This implies that an increase in the input DC power of Ti target led to the increase in the amount of Ti atoms incorporated in Ni(Ti) films when considering that the resistivity of Ti is higher than that of Ni. Similarly, for the samples annealed at low temperatures (300 and 400 °C), an increase in the input DC power of Ti target led to the increase in the sheet resistance, which could be associated with the presence of Ti-germanide phase being more resistive than Ni-germanide one. However, after RTA processes at high temperatures (500 - 700 °C), the degradation of sheet resistance was relatively less significant for the sample co-sputtered with higher input DC power of Ti target, indicating the enhancement of thermal stability of germanide film. Furthermore, as for two-step annealing process (sample was first rapid-thermal-annealed at 500 °C, followed by annealing again at temperatures of 500, 550, and 600 °C for 30 min in a flowing N₂ atmosphere), the increase in the input DC power of Ti target resulted in wider temperature windows of 2^nd step annealing process showing the insignificant change of sheet resistance. Such an improved thermal stability of Ni(Ti)-germanide film could be attributed to the delay of the agglomeration of germanide caused by the presence of Ti atoms in the films, which was confirmed by scanning electron microscope (SEM) and transmission electron microscope (TEM) examinations. Therefore, the Ni and Ti co-sputtering technique demonstrated here could be promising for the formation of thermally stable Ni(Ti) germanide film which is applicable for high performance Ge-on-Si based electronic devices.

Gold is the most electronegative metallic element whilst the alkali metals are highly electropositive; therefore the properties of the AuM (M = Li, Na, K, Rb and Cs) intermetallic compounds are quite interesting. In alkali gold compounds Au tends to act as an electron acceptor. Among those, AuLi and AuNa intermetallic compounds are of interest for plasmonic or microbattery applications due to their metallic nature. According to the Au-Na phase diagram, Au and Na form Au₂Na, AuNa and AuNa₂ [3]. However, Na’s high affinity for oxygen and reactive nature has previously limited its use in intermetallic compound research. Au is well known for its chemical stability and good performance as a plasmon resonator, however Au suffers from optical losses due to interband transitions at some important regions of the electromagnetic spectrum. Therefore, a considerable effort has been made to find other low loss materials for plasmonic applications. Theoretical considerations suggest that intermetallic compounds of alkali-noble materials are also of interest in this regard, in particular AuK and AuNa are attractive due to a good ratio of band edge to plasma frequency [5]. In fact, based on this ratio, theoretical studies predicts that AuNa could produce high quality plasmon resonances into the IR region [5].
Fabrication of alkali-noble compounds is certainly challenging due to the very different vapor pressures as well as surface energies of these elements. Here we report a synthesis method that exploits the fairly low melting point of Na. We use a vacuum furnace to evaporate Na which is then reacted with a gold substrate. The crystal structures of synthesized films were confirmed through X-ray diffraction studies which provided evidence for the formation of Au₂Na. Optical characterization of films was carried out using ellipsometry and a spectrophotometer, and an estimate of the dielectric function obtained.


References:

[1] M. Jansen, The chemistry of gold as an anion, Chemical Society Reviews, 37 (2008) 1826-1835.
[2] J.E. Ellis, Adventures with Substances Containing Metals in Negative Oxidation States, Inorganic Chemistry, 45 (2006) 3167-3186.
[3] A.D. Pelton, The Au−Na (Gold-Sodium) system, Bulletin of Alloy Phase Diagrams, 7 (1986) 4.
[4] K. Takemura, H. Fujihisa, Na-Au intermetallic compounds formed under high pressure at room temperature, Physical Review B, 84 (2011) 014117.
[5] M.G. Blaber, M.D. Arnold, M.J. Ford, Optical properties of intermetallic compounds from first principles calculations: a search for the ideal plasmonic material, Journal of Physics: Condensed Matter, 21 (2009) 144211.

For the past ten years Metalysis have produced tantalum, titanium and titanium alloy powders for high performance applications using their solid state salt electrolysis process. This low energy and environmentally friendly process is now being used to manufacture the next generation of High Entropy Alloys (HEAs).

In most cases the manufacture of HEAs involves high temperatures which put all of the alloying elements into the liquid phase. This can lead to numerous problems and restrict the number of HEAs which can be made, particularly the alloys where one needs to combine low melting point elements with refractory elements and also where there are significant liquid density differences between the constituents causing melt segregation.

The aim is to present the preliminary work carried out by Metalysis and to show how the solid state diffusion process based on molten salt electrolysis lends itself to the industrial scale manufacture of the next generation of HEAs. This study will focus on the HEAs whose constituent alloying elements have large differences in both their melting points and liquid densities, for example, chromium, niobium, tantalum, titanium and aluminium.

The ferromagnetic metal Co₂MnGa is grouped in a class of materials known as Heusler alloys [1], a number of which show great promise for high performance electronic devices due to their strong electron spin polarisation and high Curie temperatures. Theoretical and experimental investigations suggest that Co₂MnGa, and related Heusler alloy materials, are potential candidates for important applications in spintronics and magnetic device concepts such as anomalous Hall effect sensors [2]. Despite this potential, few attempts have been made to produce thin films of Co₂MnGa suitable for exploring these device possibilities [3–5].

Our research primarily focuses on producing optimised thin films of Co₂MnGa by pulsed laser deposition [6] on MgO and GaAs substrates, with magnetotransport properties suitable for device fabrication. Films grown using this technique have magnetic moments up to 3.6 μ_B/formula unit, approaching the bulk moment of ~4 μ_B/formula unit. Recent theory predicts a high spin polarisation of the Hall current [7]. In agreement, the films have giant anomalous Hall conductivities up to 1600 Scm^-1, an order of magnitude larger than in transition metal ferromagnets.

In our recent work Co₂MnGa films were subject to device processing techniques and reactive ion etching. We will present these results and the subsequent effect of device fabrication on both film quality and magnetotransport properties of the material.

References:
[1] T. Graf, C. Felser, and S.S.P. Parkin, Prog. Solid State Chem. 39, 1 (2011).
[2] E.V. Vidal, G. Stryganyuk, H. Schneider, C. Felser, and G. Jakob, Appl. Phys. Lett. 99, 132509 (2011).
[3] S.N. Holmes and M. Pepper, Appl. Phys. Lett. 81, 1651 (2002).
[4] M.J. Pechan, C. Yu, D. Carr, and C.J. Palmstrøm, J. Magn. Magn. Mater. 286, 340 (2005).
[5] R.J. Kim, Y.J. Yoo, K.K. Yu, T.-U. Nahm, Y.P. Lee, Y.V. Kudryavtsev, V.A. Oksenenko, J.Y. Rhee, and K.W. Kim, J. Korean Phys. Soc. 49, 996 (2005).
[6] E. Valerio, C. Grigorescu, S.A. Manea, F. Guinneton, W.R. Branford, and M. Autric, Appl. Surf. Sci. 247, 151 (2005).
[7] J.-C. Tung, and G.-Y. Guo New J. Phys. 15, 033014 (2013).

The existence of the ordered L1₂ Co₃(Al,X) phase in ternary and quaternary systems provides a pathway for the design of a new class of high temperature structural alloys. Single crystal compositions with creep properties equivalent to current Ni-base single crystal are presented. Superlattice intrinsic stacking faults are a primary feature of the creep deformation process. Solute segregation at these faults is examined by HAADF-STEM, atom probe tomography and first principles simulations. The implications for the design of new Co-base materials are discussed.

Co-base superalloys strengthened through γ' precipitates with a composition of Co₃(Al,W) have attracted much attention since their discovery in 2006. Due to the higher melting point of Co, they have a higher solidus and liquidus temperature and show less segregation than Ni-base superalloys. Already very simple ternary Co-Al-W alloys show creep properties which are similar to well developed 1st generation Ni-base superalloys. The new Co-base superalloys share many common characteristics with Ni-base superalloys, however, distinct differences also exist. In Ni-base superalloys Re is known to be a very important alloying element, since it partitions very strongly to the γ phase where it leads to strong solid-solution strengthening. Furthermore, the slow diffusivity of Re retards diffusional creep processes. In this paper it is discussed whether Re leads to a similar strengthening influence in Co-base superalloys as in Ni-base alloys.
Atom probe tomography investigations show that Re partitions in Co-base alloys also to the γ phase, but not as distinct as in Nickel-base superalloys. Nanoindentation and micropillar compression tests of the pure γ' phase indicate an increase of the hardness and the critical resolved shear stress of the γ' phase caused by a considerable concentration of Re. Compressional creep tests show that the positive effect of Re on the creep strength is by far not as pronounced as in Ni-base superalloys.

Given the novel nature of γ/γ' cobalt-based superalloys, little is known about their microstructural evolution under service conditions. Advances in numerical frameworks, greater computational resources, and a strong collaboration with other theoretical researchers and experimentalists allow us to study precipitate evolution in this novel material system. Here, we present a mesoscale model of γ' precipitate coarsening in cobalt-based superalloys that is informed by atomistic and experimental data. The elastic stiffnesses of the γ and γ' phases, γ/γ' misfit strain, interfacial energy, and applied stress are incorporated to predict microstructural evolution and help guide the development and testing of new alloy compositions. A simplified phase field model was developed to determine equilibrium precipitate shapes governed by interfacial and elastic energy, with the next step being the incorporation of CALPHAD-based thermodynamic information and realistic atomic mobilities to model compositional inhomogeneities and diffusion. Three-dimensional simulations were performed to study the effect of temperature, precipitate size, applied stress, and spatial distribution on precipitate morphology and spacing. This approach should help shorten the development cycle of new γ/γ' cobalt-based superalloys by predicting microstructure evolution before performing time-consuming experimental testing of new compositions.

This work was performed under financial assistance award 70NANB14H012 from U.S. Department of Commerce, National Institute of Standards and Technology as part of the Center for Hierarchical Material Design (CHiMaD). We gratefully acknowledge the computing resources provided on Blues and Fission, high-performance computing clusters operated by the Laboratory Computing Resource Center at Argonne National Laboratory and the High Performance Computing Center at Idaho National Laboratory, respectively.

Recently, a new ternary L1₂ (γ′) phase Co₃(Al,W), which can coexist with a fcc solid-solution phase (γ) based on Co, has been discovered. We have investigated the compression deformation behavior in polycrystals of the L1₂-Co₃(Al,W) and found that Co₃(Al,W) exhibits a positive yield stress-temperature dependence (yield stress anomaly: YSA) as in the case of Ni₃Al and many other L1₂ compounds. However, our previous study of micropillar single crystals of L1₂-Co₃(Al,W) has demonstrated that, the high-temperature strength of Co₃(Al,W) is considerably lower than that of Ni₃Al-based L1₂ compounds, due to a narrow temperature range of YSA in Co₃(Al,W) (950-1,100 K). It is obvious that Co₃(Al,W) in the ternary form cannot provide excellent high-temperature strength for Co-base superalloys. Hence, the high onset temperature for YSA (∼950 K) in Co₃(Al,W) should be decreased and the peak temperature for YSA (1,100 K) should be increased to widen the anomalous temperature range so as to increase the high-temperature strength. Our transmission electron microscopy analyses have indicated that the considerably high onset temperature for YSA in Co₃(Al,W) is attributed to the low energy (137 mJ/m²) of the complex stacking fault (CSF), which is formed by the sub-dissociation of the superpartial dislocation with b = 1/2<01> into Shockley partials with b = 1/6<112>. On the other hand, the peak temperature for YSA in Co₃(Al,W) corresponds to the solvus temperature of the γ’ phase. Thus, it is believed that the high-temperature strength of ternary Co₃(Al,W) can be improved by increasing the CSF energy as well as the γ’ solvus temperature upon addition of alloying elements. In the present study, we investigate the effects of alloying elements on the anomalous temperature range as well as the deformation mechanisms in Co₃(Al,W). Compression tests were conducted from 298 to 1,423 K in vacuum. The onset as well as peak temperatures of YSA were determined from yield strength-temperature curves. Four-fold dissociation of dislocation was observed in the anomalous temperature range by transmission electron microscopy. The CSF energy was calculated from the dissociation widths. We confirmed that alloying elements, such as Ni, which increase the CSF energy, lower the onset temperature for YSA in Co₃(Al,W). It is considered that the CSF energy is a dominant parameter of determining the onset temperature for YSA in both ternary and alloyed Co₃(Al,W). On the other hand, alloying elements, such as Ni and Ta, which increase the solvus temperature, increase the peak temperature for YSA although the peak temperature does not always correspond to the solvus temperature.

A Ni₃(Si,Ti) based intermetallic compound with an L1₂ structure has been attracting considerable attention as a high temperature structural material because of their high temperature strength due to positive temperature dependence of yield stress. Although high temperature ductility in high strain rate condition of Ni₃(Si,Ti) based alloys is important for the industrial processing and the practical application, it has never been reported yet. In the present study, microstructure and mechanical properties of the Ni₃(Si,Ti) based alloy castings were therefore investigated.
20kg ingots of Ni-11at%Si-9.5at%Ti alloy (ternary alloy), Ni-11at%Si-7.5at%Ti-2at%Nb alloy (Nb-added alloy) and Ni-11at%Si-5.5at%Ti-4at%Mo alloy (Mo-added alloy), were melted by vacuum induction melting under an argon atmosphere, and poured in a sand-mold. The specimens for tensile test were prepared from the alloy ingots after homogenization heat treatment at 1323K for 100 hours. Microstructural observation was carried out using an optical microscope and a field emission scanning electron microscope with electron backscattering diffraction analyzer.
Ternary alloy exhibited an L1₂ single-phase microstructure, while Nb-added alloy and Mo-added alloy showed two-phase (L1₂ and G phases) and multi-phase (L1₂, Ni solid solution (Ni_ss), and some other intermetallic compound phases) microstructures, respectively.
0.2% proof strength and tensile strength below 1073K were increased by the addition of Mo. Furthermore, tensile ductility of Mo-added alloy above 1073K was drastically improved. From the results of microstructural observation correlated with mechanical properties, it was concluded that the dispersion of Ni_ss phase is efficient in improving high temperature ductility of the Ni₃(Si,Ti) based alloys.

Deformation in polycrystalline materials is a multiscale process dependent on interactions of dislocations at the atomic-, micro-, and meso-length scales. The predictability of computational deformation models, such as crystal plasticity, for engineered materials rely on validation from experimental results acquired at these pertinent length scales. In particular, experimental analysis of scanning transmission electron microscopy foils indicates that polycrystalline nickel-based superalloys subjected to tensile creep conditions exhibit single slip system activation in grain interiors, but near grain boundaries the activation of multiple slip systems is common.

This presentation will focus on data-analytic techniques for correlating mesoscale experimental data collected by electron microscopy and x-ray micro-Laue diffraction to study the creep deformation mechanisms in a polycrystalline nickel-based superalloy. The combination of a focused beam of (500nm x 500nm spot size) high intensity, white (7-30keV) x-rays at the Advanced Photon Source at Argonne National Laboratory, and a sample translational stage allows for collection of Laue diffraction patterns for the analysis of 3D micrographs in a non-destructive manner. 2D large-area crystal orientation (EBSD) and plastic strain measurements (DIC) guided the selection of volumes for the 3D micro-Laue experiments. It is observed that single slip system activation manifests in the diffraction spots as systematic elongation of the spots from spherical to elliptical with the primary ellipse axis parallel to the projection of the active burgers vector into the detector space, while multiple slip system activation manifests as a generalized broadening of the diffraction spots. Techniques for measuring the aspect ratio of the diffraction spots were used to assess if the size range of the multi-slip regions near grain boundaries is a function of observed slip transfer/accumulation and grain boundary sliding observed in 2D measurements.

Modern turbine engines rely heavily on Ni-based superalloys, the strength of which is controlled by the detailed distribution of γ'-Ni₃Al precipitates. The L1₂ ordered precipitates form significant obstacles to the motion of dislocations through the γ solid solution matrix [1]. The size, shape and distribution of the precipitates determine their effectiveness in allowing the superalloys to be used at high temperature and under extreme load. In the case of polycrystals it has been shown that the growth of γ' precipitates is linked to the behaviour of grain boundaries forming dendritic-like structures [2]. Understanding how the order-disorder transition underlying precipitate formation is impacted by the presence of grain boundaries (and vice versa) is crucial to the microstructural design of superalloys and their mechanical properties. Experimental interpretation of atomic arrangements near grain boundaries is complicated by thermal noise, spatial and chemical uncertainties, extrinsic defects and intrinsic strain fields. To rule out some of these uncertainties we base our study on Molecular Dynamics (MD) simulations. Using MD we extend a method we developed earlier to infer distinct atomic arrangements and automatically produce classifiers. This allows us to study local arrangements in terms of their chemical ordering as well as their general neighbourhood under conditions of high thermal and chemical noise.
In this work we demonstrate how unsupervised learning and Naïve Bayes can be used to study the kinetic behaviour and chemical order/disorder of symmetric tilt and twist grain boundaries at the atomic scale based on the local neighbourhood of individual atoms using bond geometry parameters [3]. We simulate a set of Ni-Al systems containing a range of solute concentrations with and without pre-existing γ' precipitates using MD and an EAM potential [4]. Applying our method we find significant correlations between order-disorder transitions on the atomic scale and the movement of grain boundaries which themselves do not obey simple Brownian motion. Furthermore we find that increased solute content in the supersaturated state does not pin the grain boundaries but rather the opposite, and that in some cases grain boundary motion leaves behind a region rich in γ' atoms.

[1] R. C. Reed, The Superalloys: Fundamentals and Applications, CUP 2006.
[2] R. J. Mitchell, H. Y. Li and Z. W. Huang, On the formation of serrated grain boundaries and fan type structures in an advanced polycrystalline nickel-base superalloy, Journal of Materials Processing Technoloy 2 (2009), 1011-1017.
[3] P. Steinhardt, D. Nelson and M. Ronchetti, Bond-Orientational Order in Liquids and Glasses, Phys. Rev. B 28 (1983) 784–805.
[4] Y. Mishin, Atomistic Modeling of the γ and γ’-phases of the Ni-Al System, Acta Materialia 52 (2004) 1451–1467.

Ni-based superalloys, such as Ni₃Al single phase, Ni/Ni₃Al two-phase, and Ni₃Al/Ni₃V dual two-phase alloys, yields excellent mechanical properties and good phase stability at high temperature. Because of these desirable features, they are promising for usage as novel high-temperature materials for turbine blades, thermal engine and so on. For these applications, it is also important to understand the variation of thermal conductivity and heat capacity as a function of temperature. In this study, we systematically investigated the influence of microstructure and composition on the thermal conductivity for Ni-Ni₃Al-Ni₃V pseudo ternary system. Alloy ingots with nominal compositions of Ni-(25- x) at.% Al- (25- y) at.% V (x, y = 0 to 25) were prepared by arc-melting in an argon atmosphere, and then heat-treated at 1553 K in vacuum for homogenization. Thus, twelve alloys with Ni solid-solution single phase, six with Ni₃Al single phase, six with Ni₃V single phase, three with Ni/Ni₃Al two-phase, and six with Ni₃Al/Ni₃V two-phase were obtained. The thermal conductivity for pure Ni, stoichiometric Ni₃Al and Ni₃V intermetallics was 96 W/mK, 37 W/mK and 34 W/mK, respectively, at room temperature. In the single-phase alloys of Ni solid solution, Ni₃Al, and Ni₃V, the thermal conductivities (λ) decreased as a content of second or third additional elements (C_i) increased, which was well-fitted by Nordheim’s rule, 1/λ = 1/λo + AC_i, where lo is thermal conductivity of pure Ni, or stoichiometric Ni₃Al or Ni₃V, and A is constant. For the two-phase alloys, the thermal conductivity was verified primarily as a function of the volume fraction and thermal conductivity of constituent phases, following Landauer’s composite rule: for example of two-phase alloys composing of Ni₃Al phase with 12 at.% Al and Ni₃V phase with 2 at.% Al, the thermal conductivity of the former was analyzed to be 13 W/mK, which was lower than that of the latter (32 W/mK). Therefore, the thermal conductivity of the Ni₃Al/Ni₃V two-phase alloys increased with increasing V content of the two-phase alloys, because the volume fraction of Ni₃Al phase decreased with increasing V content. It was also found that influences of the alloy microstructure and composition on the thermal conductivity above mentioned were common at temperatures ranging from room temperature to 1073 K. Therefore, based on the findings obtained in this study, the thermal conductivity of any alloys in the Ni-Ni₃Al-Ni₃V pseudo ternary system at room temperature to 1073 K can be estimated.

Nickel-base superalloys, consisting of the γ' Ni₃Al phase with a L1₂ crystal structure embedded in the γ phase with an A1 crystal structure, are characterized by a remarkable high temperature strength.
Recent results have shown that also in the ordered γ' phase small disordered particles can be formed. This current work aims to investigate the effect of γ particles within the γ' phase, on the mechanical properties of a ternary Ni_86.1Al_8.5Ti_5.4 model alloy. For this purpose a specific heat treatment, consisting of slow cooling from above the γ' solvus temperature to 940 °C, was conducted. A subsequent annealing at 750 °C allows to generate γ particles within the γ' phase. The γ particles grow with proceeding heat treatment duration. This specific heat treatment allows to assess the influence of a different γ particle size in the γ' phase by nanoindentation and micropillar compression testing. The hardness as well as the critical resolved shear stress is found to increase up to an optimal γ particle size, before the hardness starts to drop again. Furthermore, an industrial relevant, typical γ/γ' microstructure where also unordered particles form inside the γ' phase was investigated by nanoindentation and compression testing. Here also, an increase of the strength can be achieved by choosing an optimal γ particle size. Complementary investigations in the TEM were carried out to determine the ideal γ particle size and to clarify the underlying mechanism responsible for the strengthening effect induced by γ particles.
Overall it was found, that the γ' phase can be strengthened by unordered γ particles in the ternary Ni-Al-Ti superalloy.

Ni-based superalloys strengthened by gamma prime are widely used because of their proper combination of resistance to degradation in corrosive and oxidizing environment, toughness and better creep properties. Gamma prime particles also suppress the grain growth of the matrix during hot deformation. Thus, the behaviors of gamma prime particles during hot deformation need to be investigated to better understand its effect on the mechanism of dynamic recrystallization. Solution heat treatment of as-forged AD730 was carried out at 1150 °C for 4 hours to dissolve all the gamma prime particles, followed by precipitation heat treatment at subsolvous temperatures. The coarsening kinetics of gamma prime at 1060 °C, 1000 °C and 940 °C were investigated. The gamma prime size distribution was becoming broader when the annealing time increased, which showed the characteristics of Ostwald ripening. The coherent interfacial energy was evaluated from the coarsening rate assuming the growth is limited by aluminum diffusion. The effect of gamma prime distribution on the flow stress at high temperatures of the specimens precipitation-heat-treated at 1060 °C were analyzed at strain rate of 5/s. Annealing time did not show great effect on peak strain and peak stress, although there was ignorable decrease. In contrast, steady state stress decreased about 100 MPa when the precipitation time increased from 1.5 hours to 36 hours. The power relationship between steady state grain size and steady state stress was assessed. Microstructure observations were also carried out using scanning and transmission electron microscopy techniques. The microstructure before and after dynamic recrystallization of the alloy suggested the interaction between gamma prime particles and grain boundaries movement greatly affects microstructure after deformation. When a high angle grain boundary passes, smaller gamma prime particles will dissolve into matrix due to increase in interfacial energy during deformation and re-precipitate coherently behind grain boundaries during hot deformation and cooling. On the other hand, larger gamma prime particles will partly dissolve but remain undissolved and the gamma primes become incoherent. There was a gamma prime free zone around the remained particles after re-precipitation. The coherency of gamma prime particles was verified using both electron backscatter diffraction and atomic resolution high angle annular dark field. The critical mean size of gamma prime dissolution during dynamic recrystallization was estimated to be less than ~200 nm in radius.

A wide range of microstructures and mechanical properties occur in the FeNiMnAl system. Here we summarize our observations on four different types of microstructures: 1) ultrafine microstructures (5-50 nm) with the phases aligned along <100> consisting of either (Fe, Mn)-rich B2 and (Ni, Al)-rich L2₁ phases present in Fe₃₀Ni₂₀Mn₂₀Al₃₀, or (Ni, Al)-rich B2 and (Fe, Mn)-rich b.c.c. phases present in Fe₃₀Ni₂₀Mn₂₅Al₂₅; 2) fine microstructures (50-70 nm) consisting of alternating (Fe, Mn)-rich f.c.c and (Ni, Al)-rich B2-ordered plates with an orientation relationship close to f.c.c.(002)//B2(002); f.c.c.(011)//B2(001) present in Fe₂₈Ni₁₈Mn₃₃Al₂₁; 3) coarser (0.5-1.5 µm) lamellar microstructures consisting of alternating (Fe, Mn)-rich f.c.c and (Ni, Al)-rich B2-ordered phases with a Kurdjumov-Sachs orientation relationship between the phases present in Fe₃₀Ni₂₀Mn₃₅Al₁₅; and 4) a f.c.c. high-entropy Fe_40.4Ni_11.3Mn_34.8Al_7.5Cr₆ alloy. The microstructures and mechanical properties in these alloys have been determined as a function of annealing time, testing temperature and strain rate. Some of the unusual mechanical behavior that has been observed will be emphasized.

This research was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences grant DE-FG02-07ER46392.

Fe-23Al-6Ni (at%) alloys are composed of the bcc Fe-Al matrix and the B2-type NiAl precipitates. Difference in primary slip system between the bcc matrix and the B2 precipitates is responsible for strong precipitation hardening which is called slip frustration hardening. Addition of small amount of V to the alloys led to the formation of the L2₁-type Ni₂AlV phase in the bcc matrix. Fe-Al based single crystals containing the L2₁ precipitates were prepared by a floating zone method and the deformation behavior was examined. In V-doped crystals, fine Ni₂AlV phase was precipitated in the bcc matrix with small misfit strain. V-doped crystals at <149> orientation exhibited high strength above 900 MPa up to 823 K. Further increase in deformation temperature resulted in a decrease in yield stress. On the other hand, at <011> orientation, the yield stress at room temperature is approximately 1.2 GPa while it decreased gradually with increasing temperature. At <149> orientation favorable for <111> slip of the bcc matrix, paired 1/2<111> dislocations cut the Ni₂AlV precipitates below 823 K. <111> slip was unfavorable for the Ni₂AlV precipitates, resulting in high strength at the orientation. On the other hand, <001> dislocations were mainly observed after deformation at and above 823 K even at <149> orientation. In addition, at <011> orientation, <001> dislocations favorable for the Ni₂AlV precipitates moved in the crystals in the temperature range between room temperature and 923 K. In general, <001> dislocations hardly move in the bcc phase, which also led to high strength. Thus, slip frustration hardening took place in V-doped crystals, which is similar to undoped Fe-23Al-6Ni alloys. Moreover, a shear stress increase by the Ni₂AlV precipitates at <149> orientation showed quantitatively good agreement with the theoretical model for the slip frustration hardening.

Iron aluminides are possible low-density, oxidation-resistant alternatives for steels in warm-temperature application. However, reduced ductility at room temperature and an intrinsically low creep resistance limit their application. A lamellar microstructure can improve these properties simultaneously as previously demonstrated for TiAl alloys. In the Fe-Al system, a lamellar microstructure can be obtained through a eutectoid transformation in the composition range of 55 – 65 at.% Al. Specifically, the high-temperature ε-phase, Fe₅Al₈ decomposes into B2-ordered FeAl and triclinic FeAl₂.
The high temperature mechanical properties of these two-phase intermetallic alloys have not been investigated in detail to date. Thus, in this study, the constant-load compressive creep response of a binary, eutectoid Fe-Al alloy was examined. The resulting creep curve did not exhibit a steady state regime; instead following a characteristic minimum, creep rate increased with increasing time and strain. The minimum creep rate is temperature- and applied stress-dependent. At 700 °C and 100 MPa, it is at ~10^-7 s^-1 and, thus, roughly two orders of magnitude lower than that for single phase B2-ordered FeAl. Quite often an increasing creep rate is indicative of a microstructural instability. Therefore, the lamellar spacing was measured after different times and strains and significant coarsening of the lamellae was noted. Additionally, it is possible that the dominant creep mechanism changes as well. As the material did not exhibit any indications for failure (e.g. cracking) during compressive creep, it may be assumed that both phases have to deform plastically. While it is known that the creep behavior of FeAl in a large composition range is primarily based on dislocation slip, knowledge on high temperature plastic deformation mechanisms of triclinic FeAl₂ phase is scarce. To determine the deformation behavior of the FeAl₂ phase, the creep behavior of this single phase was studied. Various crept specimens were examined in the TEM to understand the underlying mechanisms of deformation of both phases during creep. In the early stages of the constant load creep test, plastic deformation by slip appears to be restricted to the FeAl lamellae; the interface dislocation content increases as well to accommodate this plastic strain in FeAl, signaling dislocation-mediated sliding. Subsequently, signs of plastic deformation are noted in FeAl₂ and at even longer times, the lamellar morphology starts to destabilize.

Formation of brittle intermetallics in otherwise ductile alloys during synthesis and processing, or subsequent service, can be deleterious to mechanical properties. The driving force for such phase transformations is the strong formation enthalpies of many intermetallic compounds. An intriguing possibility has been raised in the literature that, in high-entropy alloys (HEAs) containing multiple principal elements, configurational entropy can stabilize simple solid solutions against the formation of intermetallic phases. The equiatomic HEA, CrMnFeCoNi, was generally believed to be one such alloy that is stable as a solid solution. However, recent studies have shown that it decomposes into multiple phases at intermediate temperatures. In this presentation, we will review our recent results on the kinetics of its phase decomposition and effects of grain size. The influence of phase transformations on the mechanical properties will be examined with attention paid to embrittling effects. Our results provide motivation to study the long-term stability of other HEAs as well as complex intermetallic-based alloys being considered for elevated-temperature applications. Funding is acknowledged from the German Research Foundation (DFG) through projects GE 2736/1-1 (FF and EPG) and LA 3607/1-1 (GL) and from the Austrian Science Fund (FWF) through project P26729-N19 (AH).

Additive manufacturing (AM) is a quickly evolving processing technology by which near net-shape parts are produced by melting consecutive layers of powders by a laser or an electron beam. Specifically for intermetallic phases, which are often hard and brittle, this technology offers the possibility to produce parts which require no or little machining and the high cooling rates involved in the processing may in addition lead to grain refinement.
By selective laser melting (SLM) and laser metal deposition (LMD) defect free and dense (>99.5%) parts from binary, ternary and quaternary iron aluminide alloys were produced. It could be shown that alloying concepts developed for cast alloys can be successfully employed for AM, to strengthen the iron aluminide alloys by increasing the ordering temperature T_c ^D03↔B2, precipitation of incoherent boride precipitates or generation of coherent A2 + L2₁ microstructures.
The ductility in 4-point bending, compressive and tensile yield stress, and the compressive creep behavior of the SLM and LMD samples were evaluated. Though a marked grain refinement can be achieved, this did not in general lead to an improved ductility. XRD measurements indicate that ductility may be more affected by internal stresses. Compared to the as-cast condition, the yield stress of the AM processed alloys shows marked variations up to about 500 °C. At higher temperatures, yield stress and creep resistance match that of the excellent values for the as-cast alloys.
Finally, chemically graded steel/iron aluminide samples were produced by LMD and evolution of microstructures and concentration profiles were studied after processing and long-term annealing.

Intermetallic compounds in general are brittle at low temperatures due to insufficient independent slip systems or low grain boundary cohesion. The present paper reports on the deformation of YCu and YAg showing that despite of violating the von Mises criterion these B2-structured intermetallic transition metal compounds are extremely ductile down to 130 K and 4 K, respectively. Based on a thorough thermal activation analysis the reasons for this unexpected behavior are discussed, like low elastic anisotropy, low Peierls stress and martensitic transformations.

Advanced ultrasupercritical (A-USC) steam power plants are designed to operate at temperatures and pressures over which conventional steels require additional corrosion protection. The fireside tubes undergo extensive corrosion while the internal tubes suffer from excessive steam oxidation, in particular when ferritic-martensitic (FM) P92 is employed. This paper overviews the formation of Al_xFe_y-based diffusion coatings on FM P92 and on austenitic stainless steels (AAS) from aqueous slurries containing Al microparticles. The temperature and time of annealing will be shown to clearly influence the formation of different intermetallic Al_xFe_y coating phases through concomitant oxidation reactions of the Al microspheres, melting of Al and dissolution of Fe (and alloying elements) in the melt resulting in self-propagating high temperature synthesis (SHS) followed by final solid state diffusion. Sharp interfaces between the intermetallic layers appear when the coatings are elaborated in the AAS (> 20 wt.% Cr) due to segregation of Cr and/or formation of Al_xCr_y that act as diffusion barriers to Al. In contrast, Cr dissolves in the Al_xFe_y phases when the coating grows in P92 (<9 wt.%Cr).

The behaviour of such B2-AlFe based slurry coatings was also evaluated from the metallurgical and physicochemical standpoints under thermal cycling in air, isothermal oxidation in steam and oxyfuel conditions at 650 and 700°C for 2000h under 1 bar. The intermetallic coatings allowed to form majorly protective alumina scales that hurdled the corrosive attack of contaminants. Therefore, the kinetics of degradation fitted better with a double logarithmic scale instead of a typical parabolic regime governed by solid state diffusion. The intermetallic phases of the coating remained stable over the time although stabilization of Ni-rich AlFe phases occurred at the diffusion coating/substrate interface and Cr dissolved in the external AlFe layers with exposure time. Therefore, the intermetallic Al_xFe_y slurry coatings displayed a great stability with temperature over time.

Novel austenitic heat resistant steel Fe-20Cr-35Ni-2.5Nb (at.%, model steel) is strengthened by two intermetallic phases: Fe₂Nb-ε Laves phase and Ni₃Nb-γ'' phase. The Ni₃Nb-γ'' phase of D0₂₂ structure precipitates coherently within grain interior for short-term age hardening, while the Fe₂Nb-ε phase of C14 structure preferentially precipitates on grain boundary and significantly improves the long-term creep strength through the “grain boundary precipitation strengthening (GBPS)”. To further improve the creep strength, the key is to increase the coverage of Fe₂Nb-ε phase on grain boundary (so-called the “area fraction ρ”). In the case of the competitive precipitation of two phases over the common supersaturation, it’s essential to understand the “partial supersaturation” used for the precipitation of each phase, so that the precipitation could be controlled by the partial supersaturation. In this study, thus, the partial Nb supersaturation for grain-boundary Fe₂Nb-ε phase under γ/Fe₂Nb/Ni₃Nb three-phase equilibrium is evaluated and altered, as an attempt to control the precipitations of Fe₂Nb phase and Ni₃Nb phase under their competitive precipitation over the common Nb supersaturation. Besides the model steel, steels of Fe-20Cr-35Ni-3Nb (Nb+ steel) and Fe-20Cr-36Ni-2.5Nb (Ni+ steel) are also studied to intentionally change the partial Nb supersaturations for Fe₂Nb phase and Ni₃Nb phase.
The microstructures of all the steels after aging for 3600 h at 1073 K show Fe₂Nb-ε phase on grain boundary, Fe₂Nb-ε within grain interior and Ni₃Nb-δ phase (D0_a structure, transformed from γ'' phase) within grain interior. The area fraction in model steel is 60%, which increases to 80% in the Nb+ steel and remains unchanged in the Ni+ steel. The partial Nb supersaturation for grain-boundary Fe₂Nb-ε phase is derived as the product of its molar fraction and its Nb content difference with the γ matrix. According to the calculation with the experimentally obtained phase fractions and compositions, while the total Nb supersaturation in the model steel is 1.50%, the partial Nb supersaturation for grain-boundary Fe₂Nb-ε phase is 0.35%. This value increases to 0.55% in the Nb+ steel but stays constant in the Ni+ steel. The correspondence with the area fraction clearly demonstrates that the partial Nb supersaturation is the controlling factor of the precipitation of Fe₂Nb-ε phase on grain boundary. In the meantime, partial Nb supersaturation calculation also reveals that the Nb addition increases the partial Nb supersaturations for both Fe₂Nb-ε phase and Ni₃Nb-δ phase within grain interior, while Ni addition strongly transforms the partial Nb supersaturation for Fe₂Nb-ε phase to that for Ni₃Nb-δ phase within grain interior. Based on these results, a microstructure control concept which utilizes the Nb addition for increasing the area fraction and Ni addition for adjusting the amount of Ni₃Nb-δ phase within grain interior is proposed.

Medical devices, particularly endovascular stents, manufactured from the superelastic alloy Nitinol, are subjected to complex mixed-mode physiological loading. Fatigue lifetime-prediction methodologies for Nitinol, however, are based on uniaxial loading and thus fall short of accurately predicting safe lifetimes of stents in vivo. While there is a considerable body of research documented on the fatigue of Nitinol in uniaxial tension/bending, there remains an almost total lack of comprehensive fatigue lifetime data for other loading conditions, such as torsion and tension/torsion. Here, thin-walled Nitinol tubes were examined in torsion at various mean/alternating strains and results compared to corresponding data under uniaxial tensile/bending loads. Using an equivalent (Lagrangian) strain approach, a strategy for normalizing these data is presented. Based on this, a fatigue lifetime-prediction model for the multiaxial loading of Nitinol is presented utilizing a modified Coffin-Manson approach where the number of cycles to failure is related to the equivalent alternating transformation strain.

NiTi-based shape memory alloys (SMAs) are of significant interest to the automotive and aerospace industries for potential light-weight solid-state actuator applications. There is currently a drive toward developing SMAs which can be used in high temperature environments, for applications such as fuel control valves within jet engines. A thorough understanding of the complex microstructural mechanisms occurring is needed to maximize transformation temperatures and work output, while minimizing the detrimental cyclic instability leading to functional fatigue.

Two multiphase systems are at the core of this effort: NiTiHf and NiTiAu. The addition of hafnium (Hf) and gold (Au) dramatically increases the viable operating temperature window in these alloys. NiTiHf can be tailored to achieve a favorable balance of properties, including high strength, stability, and work output at temperatures approaching 300°C. This behavior is strongly influenced by the formation of nano-scale precipitates, known as H-phase. These precipitates raise transformation temperatures and enhance shape memory behavior, while improving stability by suppressing plasticity during transformation. Less is known about the NiTiAu system, however recent constant-force thermal cycling (CFTC) experiments have demonstrated work output at temperatures above 400^oC, as well as notable compositional insensitivity.

In order to advance the reliability of these alloys to where commercialization is viable, further research is necessary to reveal the important microstructure-property relationships involved. Advanced scanning transmission electron microscopy (STEM) based characterization techniques are being used to explore the mechanisms responsible for the unusual behavior seen in these alloy systems, complemented by atom probe tomography (APT) and transmission Kikuchi diffraction (TKD) efforts.

Investigations have highlighted the powerful effects of temperature, composition, and aging on critical alloy properties in the NiTiHf system. Structural analysis of the H-phase precipitate has been completed in previous work and recent efforts are focused on understanding the precipitate-matrix interactions which lead to a near-perfectly accommodated martensitic transformation in some cases. STEM-based tomography of nano-scale needle samples provide much needed H-phase volume fraction information at challenging length-scales.

In the NiTiAu system, microstructural investigations show the presence of two types of secondary phases in these alloys, which have been characterized using selected area diffraction (SAD) analysis together with HAADF-STEM observations. Phase fraction and X-ray energy dispersive spectroscopy (XEDS) experiments provide evidence that they have a moderating effect on matrix composition. The resulting minimized variation in properties may make these alloys good candidates for fabrication techniques where precise compositional control is difficult, such as additive manufacturing.

AuCuAl has been in general expected as a high temperature shape memory alloy which has a thermoelastic martensitic transformation from L2₁ parent b phase to typically doubled B19 martensite phase at 240~450 K. On the other hand, our group has proposed that AuCuAl alloy can be a candidate as a novel biomedical shape memory alloy (SMA) owing to its good biocompatibility and high X-ray radiography, in the case that AuCuAl exhibits the martensitic transformation temperature being close to the body temperature. However, polycrystalline AuCuAl possesses limited room temperature ductility when the martensitic transformation temperature is close to and/or lower than room temperature. Then, enhancement of ductility of AuCuAl without raising martensitic transformation temperature is required for the practical biomedical applications. In this study, we focus on the effect of quaternary additional element Co to AuCuAl. Co-Cr alloy is widely used as a practical biomaterial and it is typically known as Vitallium (Co-Cr-Mo alloy). In this paper, martensitic transformation, microstructure, phase constitution and mechanical properties of AuCuAl containing Co are reported.
The alloy compositions selected were Au-27at.%Cu-18at.%Al containing 0 (Co-free), 1 (1Co), 2 (2Co) and 3at.%Co (3Co). The alloys were synthesized by arc melting method using Ar-1H₂ atmosphere. They were hot-forged at 873K and solution-treated at 773K for 3.6ks followed by water quenching. Martensitic transformation temperatures were determined by differential scanning calorimetry (DSC). Microstructures and chemical compositions were analyzed by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX). Phase constituent was determined by θ-2θ X-ray diffractometry (XRD). Mechanical properties were evaluated by cyclic loading-unloading tensile tests at room temperature.
It was found by DSC measurement that the martensitic transformation temperature is not significantly affected by Co addition. Judging from the XRD analysis and SEM-EDX observation, the maximum solid solubility of Co in AuCuAl alloy is approximately 1at.%, and CoAl precipitate exists in 2Co and 3Co. Besides, it was found that the fracture strain and ultimate tensile strength dramatically increases with increasing Co content. By SEM fractography, Co-free alloy showing limited ductility exhibits intergranular fracture, but Co-added alloys exhibit transgranular fracture. These phenomena are similar to Fe addition to AuCuAl.

Shape Memory Alloys (SMAs) have the remarkable capability to switch between two "programmed” geometries upon the application and removal of stimuli. Their shape memory properties result from a diffusionless phase transformation between two intermetallic phases. Many polycrystalline SMAs, such as Cu-based and Co-based SMAS, are limited by their inherent brittleness caused by severe stress concentration at grain boundaries when the high-symmetry austenite phase transforms to lower-symmetry martensite. We focus on a dual-phase SMA in which a ductile non-transforming second phase is precipitated to plastically accommodate transformation strain in its adjacent volume and alleviate stress concentration. We study strain recovery at austenite/precipitate interfaces using nanoindentation and resolve orientation influence using EBSD mapping. Austenite volume adjacent to its interface with precipitate has the highest strain recovery of all regions tested. These preliminary results suggest that optimizing austenite/precipitate morphology is promising to enable applications of polycrystalline SMAs with high superelastic recovery and enhanced transformation ductility.

This talk will discuss an experimental investigation into the relationship between detwinning and damage resulting from phase transformation of the functional material Nickel-Titanium. A custom methodology combining SEM and high-resolution, distortion-corrected digital image correlation was used to examine microscale phase transformation. Methodology development will be briefly discussed, including new patterning techniques for reduced length scales and corrections for the complex distortions inherent in SEM imaging. Using the DIC-calculated displacements, the progression of phase transformation and its relation to underlying crystallography was examined at the grain level in mechanically loaded tensile samples. The effect of parameters including grain size and orientation was investigated. It was found that similarly oriented grains do not necessarily transform similarly, in contrast to a common assumption in mean-field theories. Specifically, grains with similar orientation (as determined by the misorientation of the grain and specimen axes) showed variation in both the mean strain of the grain as well as the range (heterogeneity) of strain across the grain, as determined from surface measurements. Additionally, neither grain size nor degree of misorientation (of common crystal axes from the loading axis) affected the mean strain and strain range. Finally, experimentally-measured strains were correlated to theoretically-calculated transformation strains to determine habit plane variant / correspondence variant solutions. A number of findings with regard to this analysis will also be discussed, including the finding that sub-grain areas that initially transform with a high CV fraction tend to accumulate more residual strain, and that sub-grain detwinning occurs prior to the end of the macroscopic stress plateau.

Superelastic Nitinol (Ni-(50-51)%Ti) is used in self-expanding stents and heart valves. After deployment, the material is in a two-phase condition (austenite + stress-induced martensite). Blood flow during cardiac cycles subjects this two-phase composite to high cycle fatigue (4 x 10⁸ cycles in 10 years). Fundamental understanding of fatigue damage evolution of Nitinol in this two-phase condition and its relationship to the complex underlying microstructure is of importance for patient safety and reliability of nitinol medical devices.
To this end, we have systematically characterized the fatigue response and associated microstructural changes accompanying cyclic loading under conditions resembling those in service. Specifically, dog-bone-shaped specimens were preloaded using a procedure simulating deployment in service, whereby the gage section underwent full transformation to martensite and a partial reverse transformation to austenite. The heterogeneous strain distribution due to the phase transformation was tracked by digital image correlation (DIC) and the mean and cyclic displacement amplitudes for the fatigue tests were chosen based on DIC-based strain analysis. The specimens were then subjected to displacement-controlled fatigue loading up to 10⁷ cycles. TEM samples were extracted from volumes that were austenitic and martensitic during fatigue to follow the microstructural evolution. These results will be presented and discussed.

Superelastic behaviors and thermodynamic properties of Ni-rich Ti-Ni alloys were studied in terms of equilibrium between the B2-parent and B19’-martensite phases under various temperatures and stresses. We confirmed that the stress-induced martensitic transformation (i.e. superelasticity) can be introduced down to 40 K in the alloys that do not exhibit the thermal-induced transformation. Screening temperature dependence of the stresses equilibrating the parent and martensite phases for some alloys ensured that such equilibrium is described by the Clausius-Clapeyron equation in terms of the transformation entropy change ΔS^MT.
With respect to the micromodulation prior to the martensitic transformation, in-situ observations under a change of temperature using Ω-filter-equipped transmission electron microscope located the temperature where the period of the <110>{11(－)0} transverse-type lattice modulations exactly reached to 3. In vicinity of this temperature (namely, the commensurate(C)-incommensurate(IC) temperature T^C-IC), physical anomalies were detected as a small, wide hamp in specific heat and a change of Young’s modulus in stress-strain curve, respectively. We successfully evaluated composition dependences of the associated entropy change ΔS^C-IC and T^C-IC. The C state arose in alloys whose Ni content is greater than 51.0 at.% where the thermal-induced transformation suddenly vanishes and could not become discerned as a sign of finite ΔS^C-IC at about 52.0 at.%Ni. It was thus supposed that an occurrence of the C-IC transition is notably transitional and becomes poor in volume as the Ni content diverges from 51.0 at.%.
Conversions among intensive variables in the Clausius-Clapeyron relation allow us to speculate the equilibrium in the composition-temperature system. In terms of this analogy, we successfully speculate the phase equilibria in Ti-Ni Ni-rich portion via the transformation entropy changes. The findings in experiments offer the reasons for a sudden vanishment of the thermally-induced martensitic transformation and an occurrence of the isothermal martensitic transformations in the Ni-rich TiNi section, with taking into account the contributions of the C-IC transition and the hysteresis of the martensitic transformation.

Shape Memory Alloys (SMAs) show a rich thermo-mechanical response stemming from an austenite to martensite phase transformation and the availability of multiple inelastic deformation mechanisms- twinning, reorientation, slip. The evolution of microstructure and deformation modes in SMAs has been extensively studied through a variety of empirical and modeling efforts due to its connection to the work output and durability of SMA applications like actuators. However, two challenges remain. First, relatively few efforts have obtained an in-situ, 3D reconstruction of microstructural change during thermo-mechanical loading. Second, few empirical studies have attempted to quantify and partition into deformation modes the 3D deformation at the microstructural length scale. Recent advances in high-energy X-ray diffraction present an opportunity to address these challenges.

We present a framework coupling high-energy X-ray diffraction microscopy (HEDM) with forward modeling to obtain a 3D reconstruction of austenite-martensite microstructures in SMAs during mechanical loading. The forward modeling component, which uses a crystallographic theory of martensite transformation, acts as a predictive component to determine likely martensite microstructures when a direct reconstruction using HEDM is challenging. Additionally it provides the contribution to deformation from phase transformation, allowing the strain to be partitioned into various deformation modes.

We utilize this framework to examine two hypotheses related to the micro-mechanics of phase transformation in single crystal and oligocrystalline NiTi SMA. First, we hypothesize that in bulk bi-crystals and even in bulk single crystals, the martensite microstructure induced by stress is affected by boundary constraints and interaction with other deformation mechanisms. This manifests in the formation of habit plane variants different from the prediction of a Schmid-type or maximum work-type criterion. Second, we hypothesize that localized deformation due to slip as well as the presence of inclusions plays a role in determining the type of martensite microstructure formed during loading. We test these hypotheses by tracking the deformation and microstructure in two oligocrystals of NiTi SMA with different crystal orientations. This integrated diffraction-modeling approach provides a novel tool to elucidate the micro-mechanics of phase transformation in SMAs.

The actual tendency to miniaturization of engineering devices has led to a growing interest in the development of shape memory alloy (SMA) thin films for micro electromechanical systems (MEMS). This development has been focused on NiTi based alloys, from which many actuation devices have been produced. Nevertheless, in recent years it has been observed that SMA based on CuAlNi exhibit notable properties at micro and nano-scale, opening new promising possibilities in this area.
In the present work, thin films of CuAlNi shape memory alloys (SMA) have been produced by multilayers deposition, using the electron beam evaporation technique, and they have been further thermally treated to produce the alloy by solid solution diffusion. The microstructure of the alloy has been characterized by Scanning and Transmission electron microscopy techniques (BSE, EBSD, micro-diffraction, HAADF, EDX) in a JEOL-JSM 7000F, a Philips CM-200 and a FEI-Titan Cubed with Super-X. TEM samples were prepared by Focus Ion Beam in a FEI-Helios Nanolab. Thin films show small grains with a diameter under 2μm, which are fully transformed to martensite. Between the two kinds of possible martensites, only the β’₃ (C2/m) martensite has been observed and the orientation relationships between variants have been determined. At the same time very small precipitates (smaller than 20 nm), rich on Cu or Al, have been found in or near grain boundaries. The precipitation and growing of the stable phases has been observed by In Situ TEM heating until 673K. The characterization of the stable precipitates has been realized at RT by STEM in Titan Cubed, using the HAADF detector and the super-EDX quantitative maps. Intermetallic precipitates, smaller than 150 nm, have been determined to be NiAl B2 (Pm-3m) and CuAl α₂ (Fm-3m) stable phases, preferentially formed at the grain boundaries, which co-exist with the β’₃ martensite variants.

Thermoelectric conversion is one of clean technologies for the power generation with no emission of greenhouse gas. The β-FeSi₂ is an ecofriendly thermoelectric material which is applicable at high temperatures, however, the performance of β-FeSi₂ is not very good when compared with other high potential thermoelectric materials. Since the β-FeSi₂ is an intrinsic semiconductor, it can be controlled as both p- and n-type by doping elements, which is advantageous for assemble of thermoelectric modules. We fabricated β-FeSi₂ based composite alloys consisting of β matrix with the dispersion of SiO₂ particles including complex oxides. We proposed so-called combined reactions sintering process; (1) starting form α-Fe₂Si₅ powder and Fe₃O₄ powder, (2) α decomposes into β and Si by the eutectoid reaction, (3) Si is oxidized to produce SiO₂ particles while Fe₃O₄ is reduced, and (4) β matrix grows larger due to the solid phase reaction between reduced Fe and eutectoid Si. The details of these combined reactions process remain unclarified. The objective of the present work is to understand the formation mechanism of composite microstructure during the combined reactions process by the characterization of microstructure, and to evaluate the phase equilibria regarding the partitioning behavior of doping elements from α phase to β phase during the process. We reported that the formed SiO₂, residual eutectoid Si, and byproduct Fe₂SiO₄ particles were confirmed in composite alloys using STEM. In this work, reduced Fe particles were found in the vicinity of additionally grown β region using STEM HAADF images. High density planar faults were observed forming on (100) planes of the orthorhombic β-FeSi₂ matrix in composite alloys. The existence but the details of some planar faults was reported in papers. On the other hand, we evaluated the solubility limit of doping elements not only in β phase but also in α phase since the partitioning behavior affects thermoelectric properties. Isotherms of Fe-Si-X ternary phase diagrams, where X is Mn and Co, were evaluated at 1073 K for β phase equilibrium and at 1173 K for α phase equilibrium, respectively. The solubility of p-type Mn is larger than about 10 at% in both α and β phases. In the case of n-type Co, the solubility in α phase, about 12 at%, is larger than in β phase, about 9 at%. The suitable compositional range of both p-type Mn and n-type Co is roughly from 1.5 to 3 at% for composite alloys, which can be controled properly within the solubility limit. Thermoelectric properties of composite type β-FeSi₂ alloys with doping Mn and Co were evaluated in the temperature range from 300 K to 1073 K by the measurements of Seebeck coefficient, electrical resistivity, and thermal conductivity.

The defect types in TiNiSn Half-Heusler thermoelectrics are studied with a combination of theoretical (DFT and Calphad) calculation and experimental phase diagram study. Through temperature dependent isothermal section mapping, the solubility region of TiNiSn was determined together with measured hall carrier concentration as indicator of defect types. DFT calculations are applied to calculate the formation energy of each type of defect. Calphad calculation based on DFT results and existing thermodynamic parameters are performed to compare with experimentally determined phase diagram. All evidence leads to an explanation as for why single phase TiNiSn is hard to synthesize. This also leads to thermoelectric performance optimization through defect engineering with the aid of phase diagram road map.

β-FeSi₂ is one of the few semiconducting transition metal silicides, and has appealing thermoelectric properties – it has a high Seebeck coefficient, low thermal conductivity, inexpensive, abundant constituents, non-toxic, and oxidation resistant. However, its poor electrical conductivity significantly reduces ZT, and requires materials engineering to become a viable thermoelectric material. Performance can be improved by creating a nanocomposite of β-FeSi₂ and diamond cubic (Si-Ge) phases, as the latter might improve carrier transport while increased interface density will enhance boundary scattering of phonons. Microstructure can be hierarchically controlled by exploiting the eutectic solidification (L → α-FeSi₂ + SiGe) in the Fe-Si-Ge phase diagram and the subsequent eutectoid decomposition (α-FeSi₂ → β-FeSi₂ + Si) in the Fe-Si phase diagram. Processing conditions are able to control lengthscales of both eutectic (solidification rate) and eutectoid (aging temperature) diamond cubic microconstituents, as well as the α and β grain sizes.
Ambient cooling from an arc-melted liquid produces large ~50 μm wide SiGe lamellae. Rapid solidifications techniques such as melt-spinning and pulse laser annealing (PLA) have successfully reduced these lamellae into the nanoscale, ~130 nm and ~30 nm wide respectively. The finer-scale α grains and SiGe lamella enhance heterogeneous β nucleation while restricting their overall growth. This changes the b-phase grain size distribution from 16.0 ± 15.3 μm (binary slow-cool) to 800 ± 490 nm (ternary melt-spun) when aged at 567 °C to promote eutectoid decomposition. The lengthscale of the eutectoid Si, as well as β grain size, is directly related to its isothermal ageing temperature. Prolonged aging at 910 °C produces coarse Si precipitates, with a broad distribution of diameters and spacings. Aging at 567 °C for 56 hrs produces periodic and evenly spaced Si nanowires with diameters ~20-30 nm. This reduced isothermal aging significantly increased the β-FeSi₂/Si interface density and reduced thermal conductivity by 2x. By utilizing both rapid solidification and low temperature agings, we have reduced thermal conductivity by 4x. Support from the II-VI Foundation is gratefully acknowledged.

It has been theoretically predicted that ThMn₁₂-type compounds like NdFe₁₂ and NdFe₁₂N exhibit high magnetization and magnetocrystalline anisotropy energy comparable to the current Nd₂Fe₁₄B permanent magnet material [1–3]. However, experimentally synthesized the actual materials are very difficult due to the thermodynamic instability of the compounds themselves. The substitution of Ti atom to the Fe atom seems to stabilize the ThMn₁₂-type compound[4] but significantly reduces the magnetization of the compound. We attempt to investigate the origin of instability in the NdFe₁₂ and NdFe₁₂N structures and the stabilization effects of both structures by Ti-doping from first-principles calculations. The electronic structure calculations, based on density functional theory (DFT), with an onsite Coulomb interaction correction (+U) are performed. Cluster expansion method is also performed to investigate the mixing behavior between Fe and Ti atoms on Fe site in both NdFe₁₂ and NdFe₁₂N structures.

The calculation results revealed that Ti-doping is indeed reduced the electronic instability in the density of states of both NdFe₁₂ and NdFe₁₂N structures. The thermodynamic stability of the NdFe₁₂ compound is increased when substitute Fe with Ti atom which is also consistent with the experimental investigation on the ThMn₁₂-type compound[4]. Apart of thermodynamic instability, NdFe₁₂ compound is also dynamically unstable due to the present of imaginary phonon frequency. According to the calculation results, the imaginary frequency disappears when the structure subjected to external pressure.

[1] T. Miyake, K. Terakura, Y. Harashima, H. Kino, S. Ishibashi, J. Phys. Soc. Japan 83, 043702 (2014).
[2] Y. Harashima, K. Terakura, H. Kino, S. Ishibashi, T. Miyake, Proc. Comput. Sci. Workshop 2014, pp. 1–8 (2015).
[3] Y. Hirayama, T. Miyake, K. Hono, JOM 67, 1344 (2015).
[4] R. Coehoorn, Phys. Rev. B 41, 11790 (1990).

The recent rare earth crisis spurred a large amount of research worldwide in the search for new rare-earth-free intermetallic compounds with uniaxial structures that have a large magnetization, anisotropy and Curie temperature which can be used for the development of advanced permanent magnets (PM). Recently, the Fe₅SiB₂ compound with the tetragonal Cr₅B₃ structure has been reported to exhibit good saturation magnetization, a Curie temperature in excess of 700 K but a relatively low anisotropy comparable to oxide ferrite materials [1]. Furthermore, this reported compound was formed by solid state reaction and required significant effort to be prepared into a single phase. In this work, we report a more simple approach to prepare this compound in cast and melt-spun samples with small Ga substitutions for Fe [(Fe_4.5Ga_0.5)SiB₂] and Ge for Si [Fe₅(Si_0.75Ge_0.25)B₂] with the objective to increase its magnetic anisotropy and investigate its potential as a new PM. Fe₅(Si_0.75Ge_0.25)B₂ samples were melt-spun with a wheel speed of 30 m/s and found to primarily consist of an amorphous phase; calorimetry reveals a large exothermic peak with an onset temperature of 800 K. Samples heat-treated above this temperature consisted of the tetragonal Cr₅B₃ structure with lattice parameters a = 5.3662(17) Å and c = 10.2622(68) Å, with a c/a ratio of 1.912. Arc-melted ingots of Fe₅(Si_0.75Ge_0.25)B₂ homogenized at 1273 K for 10 days and (Fe_4.5Ga_0.5)SiB₂ annealed at 1073 K for 7 days were also found to form in the Cr₅B₃ structure. The lattice parameters of these latter samples are a = 5.3709(18) Å and c = 10.2586(70) Å (c/a = 1.910) for the Ge-containing sample and a = 5.3642(23) Å and c = 10.2615(90) Å (c/a = 1.913) for the Ga-containing sample. The lattice parameters of these chemically modified compounds are lower than those of the parent unmodified compound. Both the Ge-substituted and Ga-substituted samples exhibit a Curie temperature of approximately 800 K, which is higher than that of the unmodified phase (T_C = 784 K). The Ga-substituted sample also has a saturation magnetization (153.6 emu/g @ T = 300 K) comparable to that of the parent unmodified Fe₅SiB₂ compound. The magnetic properties of these chemically modified samples will be examined over a range of temperatures to further assess their potential as a new intermetallic permanent magnet.

[1] M. A McGuire and D. S. Parker, JAP 118, 163903 (2015)

Work supported by DOE Grant DE-FGO2-90ER45413 and Northeastern University

The great impact of CO₂ emissions on the environment makes the alternative use of CO₂ as a feedstock for the production of both fuels and chemicals immensely important. Particularly the hydrogenation of CO₂ to methanol, in combination with H₂ from renewable sources, is an attractive process as methanol can be used directly as a liquid energy carrier. Pd-Ga intermetallic compounds, in particular the Pd₂Ga phase, have become especially interesting over the past decade, even rivalling or outperforming the catalytic activity of the successful and widely used Cu/ZnO system. A crucial question concerning Pd₂Ga catalysts is the influence of secondary phases, e.g. Ga₂O₃, on the overall behaviour of the catalyst and the interplay between these phases and the alloy itself.

X-ray photoelectron spectroscopy (XPS) is a method used widely in the solid-state and surface sciences as well as in catalysis. It provides a unique combination of qualitative and quantitative results concerning the chemical environment and elemental ratios whilst being very surface-sensitive and non-destructive. The surface sensitivity of XPS depends on the kinetic energy of the excited photoelectrons which is directly coupled to the X-rays in use. This close correlation between information depth and experimental setup can be exploited to study structures and multi-layered systems.

In this study nanoparticles of Pd₂Ga catalysts with varying Pd:Ga ratios and preparation steps, have been studied by XPS using air-sensitive techniques. Core level spectra, including Pd 3d, Ga 2p and Ga 3d, were used to study the oxidation states of the metals and the elemental composition of the samples. The combination of Ga 2p and Ga 3d core levels, which lie about 1 keV apart in energy, allows the differentiation of species at the surface and in the bulk of the nanoparticles. Whilst the information depth of the Ga 2p core line (quantified by the inelastic mean free path) is only 1.1 nm, it is 3 times higher for the Ga 3d core line. Therefore, whilst the Ga 2p spectra provide a truly surface sensitive image of the catalyst, the Ga 3d core level is dominated by the bulk of the nanoparticles. Using this analysis, Ga₂O₃ is found to be present at the surface of the nanoparticles and the ratio between it and the intermetallic Pd-Ga phase varies with Pd:Ga ratio. Furthermore, valence band spectra are used to study the electronic structure of the Pd₂Ga nanoparticles. A clear change in the valence band structure and in particular in the onset of the valence band is observed upon the formation of the alloy phase.

Ultimately, this work shows the capability of XPS measurements coupled with high level analysis to study the chemical and electronic structure of nanostructured intermetallic alloy systems. In addition, depth sensitive information is available from XPS and can be used to identify specific surface species which can influence the overall behaviour of the alloy, e.g. when used as a catalyst.

Independently manipulating the size and aspect ratio of grains in nanostructured films/coatings would enable us to have significant control over the mechanical behavior of such materials. Here, we developed a synthesis process to tailor the microstructure of sputter deposited intermetallic films. We achieved this by altering the recrystallization process from an amorphous phase by employing thin crystalline seed layers.

Amorphous TiNi and TiAl thin films were deposited on Si/SiN_x substrates using DC magnetron sputtering at room temperature. Thin, crystalline layers of Ti and Al (thickness <2 nm) were deposited in between the amorphous layers to act as preferential grain nucleation sites (seeds). The films were subsequently annealed in vacuum to obtain a crystalline microstructure. Transmission electron microscopy and x-ray diffraction were used to study the effects of the seeds on the microstructural evolution and crystallization kinetics of these films.

The seeds were found to aid crystallization, with seeded films crystallizing faster than non-seeded films for the same annealing temperature. Furthermore, the final microstructure of the seeded films was in the nanocrystalline regime up to a temperature of 700^oC, whereas the unseeded films underwent rapid recrystallization to form large microcrystalline grains. Moreover, cross-sectional TEM analysis revealed that the grain height scaled with the seed spacing along the thickness of the film. In situ TEM heating experiments showed that the seeds significantly increased the number of nucleation sites in the amorphous matrix, which led to a refinement in the final grain size. In contrast, unseeded films crystallized by the formation and coalescence of large single crystal islands.

Particle accelerators (colliders; synchrotrons;…) are key research tools, but are evermore costly to build and operate as the performance horizon is pushed back. The heart of a particle accelerator is the string of resonant microwave cavities, in advanced machines made of solid niobium superconducting at ~ 1.8 K. Providing the cavities with interior coating of the A15 compound Nb₃Sn can permit operation at 4.2 K instead, potentially a power consumption reduction of nearly a third. R&D interest focuses on tin vapor thermal diffusion coating of solid niobium cavities. Much is being learned and significant progress has been achieved, but the goals of full understanding and operational cavities are still ahead. Modern materials characterization tools are playing a critical role.

Work at William & Mary supported Office of High Energy Physics, U.S. Department of Energy under grant DE-SC0014475. Partially authored by Jefferson Science Associates, LLC, under U.S. DOE contract DE-AC05-06OR23177

In contrast to conventional Al-Cr coatings elaborated by CVD-related techniques, this work reports on the mechanisms of formation of Al-Cr slurry diffusion coatings on pure nickel. The reactivity of Al and Cr microparticles was first evaluated by DSC. Decreasing the Al-Cr ratio and increasing the Cr microparticle size resulted in less exothermic reactions. Appropriate Al-Cr mixtures (100Al, 67Al33Cr, 44Al56Cr, and 20Al80Cr, wt.%) were then admixed to the slurry binder and sprayed onto nickel. Upon subsequent annealing in argon at intermediate temperature (650°C), Al reacted preferentially with Cr in the top coat to result in Al-rich intermetallic phases (Al₁₁Cr₂ and/or Al₁₁Cr₄). The derived decrease of the Al activity fostered the direct formation of AlNi/AlNi₃ coating within the substrate, which was very thin (~5 µm) and discontinuous, from the three Al-Cr mixed slurries. Slight thickening of the coatings occurred by Ni outward diffusion by further annealing at 1000°C. Al₈Cr₅ and AlCr₂ phases were identified by XRD at the top coat but no substantial Cr enrichment was observed in the coatings themselves. Thicker Cr-containing AlNi coatings could only be achieved by fostering the annealing to 1080°C for 6 h like in conventional out-of-pack aluminizing processes. In all cases, the reduction of the Al activity by Cr was not only due to Al_xCr_y formation but also to peripheral oxidation of the Cr particles. In contrast, a thick (~40 µm) multilayered Al₃Ni₂/AlNi/AlNi₃ coating formed from the Cr-free Al slurry following melting of Al and dissolution of Ni in the melt at 650°C. Full conversion into AlNi was almost achieved after annealing at 1000°C.

Ordered intermetallics have attractive properties and behavior at medium to high temperatures, including excellent corrosion and erosion resistance, creep resistance, high strength and low density. Generally, these alloy systems exhibit limited ductility in polycrystalline form. However, there is significant development in their use as coatings in order to enhance high-temperature properties of less robust structural materials. Thermal spray processes are widely used in the application of surface coatings, but porosity and poor adhesion can lead to coating delamination or premature failure of the substrate. In this work, we demonstrate a general strategy for the synthesis of intermetallic coatings using plasma spray to deposit a precursor coating that will react with the substrate to produce a nanostructured intermetallic coating.
In our particular example, we apply a titanium alloy coating to a nickel-based superalloy substrate. Electron microscopy, XRD, and Vickers indentation confirm that the as-sprayed coating is highly porous, heavily oxidized, and brittle. Heat treatment at an elevated temperature allows the Ni and Ti to interdiffuse and react, forming an intermetallic compound. The remaining components in both the coating and substrate do not undergo this interdiffusion process, and instead segregate into additional alloy phases with complex nanostructures and well-defined lengthscales. The processed intermetallic coating is fully dense, and the discrete oxide domains left by the plasma spray process are no longer present. In addition, the NiTi phase is continuous from the coating to the substrate which results in excellent adhesion. The coating is extremely strong, exhibiting a 10 GPa Hardness using a Vickers indenter with a 2 N load, and a 20 GPa hardness using nanoindentation with a 45 mN load, and the indentations display no cracking associated with brittle fracture.
This work demonstrates the scalable fabrication of an extremely strong coating for Ni alloy systems and provides a framework for the development of mechanically robust coatings for other structural metals.

Recent band structure calculations have indicated that a number of Heusler alloys are spin gapless semiconductors, i.e., magnetic semiconductors with a high Curie temperature and 100% polarization of the charge carriers. A spin gapless semiconductor manifests itself in a high electrical resistivity and a small anomalous Hall effect, despite of a large magnetic moment [1]. First principle calculations [2] suggest that Fe₂MnAl has a semiconducting ground state with vanishing electronic density of states for the minority spins. Experimental studies of polycrystalline Fe₂MnAl Heusler alloy revealed [3,4] that it orders ferromagnetically at T_C ~ 150 K and demonstrates uncommon for metals temperature dependence of the electrical resistivity. To further explore physical properties of this alloy, we prepared thin films of Fe₂MnAl epitaxially grown on a MgO(110) substrate and characterized their magnetic and transport properties. Results of our measurements showed that the Fe₂MnAl thin films order magnetically at ~ 65 K. Moreover, magnetization curves measured upon heating up and cooling down exhibit a temperature hysteresis in the interval from 40 K to 65 K, which is suggestive of a structural transformation. Magnetic relaxation study performed in this temperature interval showed anomalous time dependence of the magnetization which first increases and then decreases exponentially. Results of the transport properties measurements revealed that the Fe₂MnAl thin films demonstrate temperature dependence of the electrical resistivity typical for semiconductors which agrees with that reported for the polycrystalline samples. Studying the influence of stoichiometry on magnetic and transport properties of Fe₂MnAl thin films is in progress.
Work at Argonne was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02- 06CH11357.
References
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The MnBi alloy is an important high-energy density magnetic material useful for devising small magnets and devices for various electronic applications. It is a rare-earth free alloy which is being explored to tailor magnetic properties with high magnetocrystalline anisotropy in the low-temperature phase and excellent magneto-optical properties in the high-temperature phase. In this investigation, we report synthesis of the alloys of varied compositions Mn_xBi_100-x, x = 30, 40, 50, 60 and 70, using a standard vacuum arc melting and casting method of the Mn and Bi metals in the predetermined compositions. A single crystalline MnBi phase is formed over the series in terms of the X-ray diffraction patterns. An analysis of the EDX spectra with elemental mapping demonstrates the Mn and Bi atoms are distributed uniformly. The FESEM images show a granular nanostructure of the alloys of average grain sizes varied in the range 100-200 nm. The Vickers hardness (H_v) and Young modulus (Y) were measured on the various alloys using a nanoindenter. The H_v and Y values were measured using two different loads of 490 mN and 981 mN. At 490 mN load, the Mn₄₀Bi₆₀ alloy exhibits the highest values of H_v = 0.8534 GPa and Y = 57.50 GPa. At the higher 981 mN load, both the values fall down owing to sliding of the planes. The H_v and Y values vary non-linearly in a function of the x-values. Thermal conductivity (k), thermal diffusivity (α) and specific heat capacity (C_v) were measured of the alloys over 35 - 150 °C temperatures. The Mn₆₀Bi₄₀ alloy exhibits the highest values k = 5.09 W/m-K, α = 0.52 mm²/s and C_v = 9.74 MJ/m³K at room temperature. These values arise linearly upon the heating in this regime. The results demonstrate a simple technique of producing a rare-earth free alloy with tailored magnetic and thermal properties as demanded for fabricating the permanent magnets and small magnet devices.