Symposium PM06 : Advances in Intermetallic-Based Alloys for Structural and Functional Applications

Intermetallic Titanium Aluminid based alloys are considered for high-temperature aero and automotive engine applications. The advantage of this class of innovative high-temperature materials is their low density in combination with good strength and creep properties up to 850°C. A drawback, however, is their limited damage tolerance at room temperature, which is reflected in a low plastic fracture strain and fracture toughness. Advanced engineering Titanium Aluminid alloys are complex multi-phase materials which can be processed by ingot or powder metallurgy. Engine components can be manufactured by casting as well as additive manufacturing, e.g. electron beam melting. Each ingot production process leads to specific microstructures which can be optimized by thermo-mechanical processing, e.g. isothermal or hot die forging and subsequent heat treatments. Thermo-mechanical processing can provide balanced mechanical properties, i.e. a minimum ductility at room temperature as well as sufficient creep strength at elevated temperature. In order to achieve this goal, the knowledge of the occurring solidification processes and phase transformation sequences is essential. Therefore, thermodynamic calculations were conducted to predict the phase diagram of engineering TiAl alloys. After experimental verification, these phase diagrams provided the basis for the development of heat-treatments. To account the influence of deformation and kinetic aspects sophisticated ex- and in-situ methods have been employed to investigate the evolution of the microstructure during thermo-mechanical processing. For example, in-situ high-energy X-ray diffraction was conducted to study dynamic recovery and recrystallization processes during hot-deformation tests. Novel high-strength Titanium Aluminid based alloys, such as TNM alloys have been developed in the last decade to meet the advanced requirements of aero engines.These alloys are characterized by a high content of β-stabilizing alloying elements, such as Nb and Mo. Because Nb and Mo represent the decisive alloying elements, this alloy family, based on the γ-TiAl phase, has been named “TNM alloys” in order to distinguish them from the well-known and even stronger “TNB alloys” which rely on a high Nb concentration and small additions of B and C. At room temperature, strength levels > 1000 MPa can be achieved in advanced TiAl alloys by appropriate thermo-mechanical processing and subsequent heat treatments. It is important to note that also high temperature properties, such as creep resistance, were considerably improved, e.g. by implementation of precipitation hardening, which further extend the application range of Titanium Aluminid based alloys.

Innovation of structural materials is urgently being required for contribution to worldwide issues on energy, environment, and high performance jet engine development with larger thrust-to-weight ratio is one of them, since more than 30,000 new airplanes are to be produced by 2030.
A five-year National project of “Structural Materials for Innovation (SM⁴I)” in Cross-ministerial Strategic Innovation Promotion Program (SIP) starting from 2014 is currently going on in Japan. In this project of SM⁴I, a focus is placed on innovative structural materials applicable for jet engines, where author at Tokyo Tech is committed to as a technical leader and take responsibility for alloy design and development of TiAl alloys in collaboration with other universities (Hokkaido Univ, Osaka Univ.) and industries (Kobe Steel, Ltd and IHI Co.). In this talk, design approaches and achievements for the development of wrought TiAl alloys to be used for LPT and HPC blades are presented. First, we built up a new database for phase diagram calculations in multi-component systems of the alloys, including both substitutional and interstitial elements. In case of Ti-Al-M₁-M₂ quaternary systems, for example, we optimize the interaction parameters to calculate the phase diagram in good agreement with experimentally determined phase diagram, by taking into considerations of temperature, aluminum and M₁/M₂ concentration dependencies in the four phases of β-Ti, α_2,-Ti₃Al, α-Ti and γ-TiAl phases. Based on the phase diagram calculations, we successfully proposed model alloys with excellent hot workability even at higher strain rate and room temperature ductility of more than 1%. It should be noted that an introduction of bcc b-Ti phase and microstructure design using a unique phase transformation pathway of β+α→α→β+γ in the multi-component systems makes it possible to have excellent properties in both process and service temperatures.
We revealed the alloy with lamellar microstructure decorated by β/γ duplex microstructure at the lamellar colony boundaries show better crack initiation and propagation resistance than that with fully lamellar microstructure. The detailed microstructure control method using the phase transformations and toughening mechanism will be presented.
Part of this study was carried under the research of SIP in JST (Japan Science and Technology Agency).

Intermetallic γ-TiAl based alloys provide promising engineering properties for lightweight high-temperature applications. Hierarchical structures that comprise microstructural constituents on the nanometer scale have received much attention in recent years, as research suggests that they can further improve the performance of these alloys. In the present work, a ternary Ti-43Al-7Mo (at.%) model alloy is investigated. This β/γ alloy can be homogenized in the β single-phase region at 1450 °C, while water quenching prevents further large-scale phase transformations. Subsequent reheating to temperatures above 500 °C provokes the formation of fine γ particles on the sub-micron level. As these particles appear homogeneously distributed throughout the β_o matrix and form a continuous network after long annealing times, the microstructure of the alloy can, thus, be substantially refined. However, the exact mechanisms of this phase transformation are still not fully understood. The present work addresses these mechanisms through the application of a variety of characterization techniques. In a dilatometer setup at a synchrotron radiation source, β-homogenized specimens were heated at various constant rates ranging from 10 to 400 K/min. In situ high-energy X-ray diffraction was used to study the structural changes in the prevalent phases. The combination of in situ small-angle X-ray scattering and ex situ atom probe tomography allowed the analysis of the early stages of the precipitation process by characterizing the nanometer-scale precipitates in terms of size, shape and chemical composition. The final microstructure of selected heat-treated specimens was analyzed using scanning electron microscopy. In sum, the experiments offered deep insights into the functioning and the kinetics of the thermally activated γ growth sequence in a β-homogenized Ti-Al-Mo alloy. The detailed understanding of this phase transformation is an essential prerequisite for the design of refined microstructures in β/γ alloys without the necessity for prior hot working.

TiAl alloys are candidate materials for low pressure turbine blades of a jet engine due to their low density and excellent strength at high temperatures. Electron beam melting (EBM) is one of the suitable manufacturing processes for the TiAl alloys. Because it is possible to fabricate 3D products easily by adding layer-on-layer of materials. We found the TiAl alloys fabricated by EBM have unique layered microstructure consisting of equiaxed gamma grains regions (gamma bands) and duplex-like regions perpendicular to building direction. This layered microstructure is formed by repeated thermal effect from the melting pool.
In this study, the influence of the layered microstructure on mechanical properties of the TiAl alloys was investigated focusing on the morphology of the gamma bands. To control the layered microstructure, the alloys were fabricated by EBM at various conditions. Moreover, some of the as built alloys were subjected to heat treatments including hot isostatic pressing (HIP) treatments at various temperatures. The width of the gamma bands was affected not just by the electron beam scan speed but also by the heat treatments. The room temperature ductility of the alloys increased with increasing the width of the gamma bands and reached a maximum at approximately 3%. The alloys with suitable morphology of the gamma bands show also excellent fatigue strength. These results indicate that the alloys with the unique layered microstructure have a great potential for aerospace applications.

The present study applies a strategy of grain refinement on a γ-based Ti-45Al-7.5Nb intermetallic compound through high-pressure torsion (HPT) processing for 5 and 10 turns under a compressive pressure of 6.0 GPa at room temperature. Successful grain refinement was introduced in the duplex microstructure to have ultrafine laths with thicknesses of 40-100 nm after 10 HPT turns. Exceptional hardness is recorded after the severe plastic deformation and the X-ray diffraction analysis confirmed the occurrence of phase transformation from γ-TiAl to α₂-Ti₃Al in the alloy. The close investigation demonstrated that hardness and texture evolution vary gradually from the sample surfaces to the core, thereby exhibiting a heterogeneous microstructure. Macroscopic plastic flow is estimated thorough the micro-mechanical responses by the nanoindentation technique for the TiAl alloy. This study demonstrates the promising feasibility of HPT processing for improving essential mechanical properties in the TiAl.

Low Pressure Turbine blades made of TiAl alloys are being used in three civil aircraft engine families (GEnx, PW1000GTF, LEAP). The production process of TiAl semi-finished parts is based on three technological pathways:
VAR ingot metallurgy, VAR Skull Melter homogenization and subsequent investment casting to oversized components followed by mechanical machining
VAR ingot metallurgy, VAR Skull Melter homogenization and subsequent centrifugal casting in permanent moulds to small sized feed stocks for either forging and mechanical machining or direct mechanical machining
Plasma Arc Cold Hearth Melting to small sized ingots for direct mechanical machining
In both processing routes b) and c), a substantial amount of valuable TiAl revert is being generated during the different processing steps. An industrial recycling pocess based on Vacuum Induction Skull Melting with subsequent centrifugal casting according to the technological pathway b) has been developed and commissioned at GfE. Appropriate revert preparation technologies prevent a detectable impurity pick-up from previous processing steps even for multiple use of revert. The corresponding revert preparation and revert conversion technology is approved and validated for the production of pre-materials for components for aircraft engines. The presentation addresses the origin of revert and the appropriate revert preparation technology. The recycling process via Induction Skull Melting is being introduced. Resulting products are indistinguishable from products resulting from the virgin production route via VAR Skull Melting with regard to chemical composition and microstructure.

To evaluate forgeability for wrought TiAl alloy (YSK alloy : Ti-43Al-9(V, Nb)-0.2B), greeble test and compression test were carried out. As a result, it was confirmed that deformation at (β+α) two-phase region promoted recrystallization of α phase and recrystallization was easily developed as grain size was smaller. In addition, (β+α) two-phase region in YSK alloy was larger than the others. Hot-forging was performed on the condition revealed by above tests, and then mechanical properties of heat-treated samples were investigated. Tensile strength and creep strength of YSK alloy were equal to or higher than the other alloys reported so far.
DKth at R.T was about 10 MPa√m. As to forgeability, extrusion, upset, and hot-die-forging at high speed succeeded with existing facilities. At last, more than 200 blades were manufactured by the established method.

Two different families of γ-TiAl intermetallics are already flying as blades of the low-pressure turbine in some engines of commercial aircraft, and considerable effort is being devoted to improve their performances. Here we focus on the family named TNM⁺, which is based on the TNM alloy but containing specifically designed amounts of C and Si in order to optimize the performances. The goal is to increase the creep resistance allowing to extent the working temperature range, and to this purpose a fully nano-lamellar microstructure was developed through thermal treatments.
In the present work we study at high temperature the mobility of defects controlling the mechanisms responsible for creep. However, the study is approached through mechanical spectroscopy by measuring the internal friction (IF) spectra between 300 and 1350^oC. On one hand, the high temperature background (HTB) of the IF is closely related to creep behavior [1], and on the other hand, the relaxation peaks of the IF spectra offer valuable information about the atomic diffusion mechanisms involved in creep [2,3]. In particular, the developed nano-lamellar microstructure allow us to discover and analyze a new relaxation peak, which remains hidden in between the IF peak associated to Ti diffusion in α₂-Ti₃Al phase [2] and the HTB. We have measured the IF spectra at different frequencies and performed a deep analysis to decompose the IF spectra into the different contributions and isolate the relaxation corresponding to each individual atomic mechanism. This way we measured the activation energy of the hidden relaxation, E_a=3.7 eV, which is attributed to the Aluminum diffusion in γ-TiAl phase. Moreover, a new atomic relaxation mechanism is proposed to explain the characteristic of the observed IF peak. Finally, the importance of the discovered relaxation is discussed in terms of its relationship with the atomic diffusion processes involved in creep.

Pipe diffusion along dislocations in metals can occur at faster rates than in bulk regions, expediting creep and contributing to device failure. We use density functional theory methods to calculate the vacancy-mediated diffusion coefficient along a <1 -1 0> screw dislocation in FCC nickel. A lattice Green’s function technique is used to accurately configure the atomic structure surrounding the dislocation core. Vacancy hop rates within this region are calculated using principles of transition state theory with activation energy barriers and vibrational properties obtained from our ab initio framework. A kinetic Monte Carlo model then uses these rates to describe mass transport within the region on representative timescales at various temperatures.

Refractory-metal based materials have attracted attentions for many years because of the increasing demand for high temperature components. In order to obtain such materials having superior properties for high temperature use, multicomponent-multiphase alloys are required. Authors have conducted studies on the mechanical and physical properties of BCC solid-solution phase, M₅Si₃-T₂ silicide phase and B2 aluminide phase, together with phase equilibrium among these constituent phases.
Authors have conducted the investigation on the effects of various additive elements on the strength and deformability of BCC solid solution. As (Nb, Mo)₅(Si, B)₃ phase dispersion in Nb-Mo BCC solid-solution matrix has been studied by various researchers, we also try to understand the phase stability of T₂ phase in alloys. Although silicide phases including (Nb, Mo)₅(Si, B)₃ show superior oxidation resistance, we have focused on B2-aluminide coating because B2-NiAl coatings have been used for commercial Ni-based superalloys. To realize a three-phase alloys composed of refractory BCC, T₂ silicide and B2 aluminide, phase equilibrium in ternary, quaternary and higher-order phase diagrams are experimentally investigated. Among them, brittle compounds such as Nb(Ni, Al)₂ Laves phase form at the interphase boundary between NiAl and Nb-based alloys during high temperature heat-treatment. To avoid the formation of Laves phase, both the B2 phase composition and the BCC alloy composition were optimized based on the phase diagrams such as Nb-Al-Pd and Nb-Mo-NiAl, then a composition area at which BCC solid-solution phase equilibrates with B2 aluminide was found in Nb-Mo-Ni-Al-Pd quinary system.
By the systematic investigation, it was found that the atomic size ratio of constituent elements is still an important key to understand the stability of ternary Laves phases including Al. For a further understanding of the substitution behavior of elements in the Laves phase, electronic structure calculations based on the density functional theory (DFT) was performed on the alloy system including Si.
This work was supported by the Advanced Low Carbon Technology R&D (ALCA) program of the Japan Science and Technology Agency (JST).

A Mo-Si-B-based alloy reinforced by TiC (65Mo-10Ti-5Si-10C-10B (at.%)) shows high creep resistance estimated over the rupture time of 1000 h under 137 MPa at 1350°C and relatively good room-temperature fracture toughness over 15 MPa(m)^1/2. The excellent mechanical properties arise from the high strength of Mo₅SiB₂ (T₂) and TiC, the good ductility of Mo solid solution, and the microstructural configuration of these phases. Under creep deformation, the resistance of Mo solid solution would be one of key factors controlling the overall creep strain of the material and the interfacial sliding between these phases would be the other factor. In this paper, the role of Mo solid solution on ultrahigh-temperature tensile creep deformation is addressed for the MoSiBTiC alloy. The microstructural continuity of Mo solid solution was analyzed in terms of the percolation probability. It was clarified by the percolation probability analysis that both the Mo solid solution and brittle phases (T₂ and TiC) were not 100%-continuous in the microstructure. EBSD and TEM observations for ruptured specimens presented dynamic recrystallization in Mo solid solution due to heavy plastic deformation during creep but much less plastic deformation in the brittle phases. These results represent that the creep strain was given by the dislocation creep of Mo solid solution and the interfacial sliding between the phases. The apparent activation energy of creep was estimated to be about 560 kJ/mol from the Arrhenius plot of the logarithm of the minimum creep rate against inverse temperature. The value is much higher than the activation energy of the self-diffusion of Mo. This strongly suggests that the rate-controlling process of the creep is governed by the bulk diffusion in Mo solid solution rather than interfacial diffusion. However, it is unlikely to be caused by Mo self-diffusion because of its lower activation energy, but it might be caused by impurity diffusion, for example, of interstitial (I)-substitutional (S) pairs. The role of Mo solid solution on ultrahigh-temperature tensile creep deformation of the MoSiBTiC alloy will be further discussed with experimental data.

In terms of preserving resources and reducing environmental impacts, improving the efficiency of turbines for power plants and aircraft engines is an increasingly important research subject. Potential high performance materials are Mo-Si-B alloys. Consisting of a molybdenum solid solution (Mo_ss) phase and two intermetallic phases Mo₅SiB₂ (T2) and Mo₃Si those alloys present balanced room temperature fracture toughness, high temperature creep strength and oxidation performance. In this work vanadium as a lightweight element with a density of 6.1 g/cm³ has been identified as a potential alloying partner which can entirely be solved in the Mo_ss phase as well as in the Mo₃Si and Mo₅SiB₂ phase. To identify the role of vanadium in terms of strengthening the solid solution phase different Mo-XV (X = 5…50 at.%) alloy compositions were produced and evaluated by means of microhardness measurements. Additionally, quantitative values for solid solution hardening were determined by the approach of Labusch. Compared to other alloying concepts, e.g. Mo-Ti, vanadium affects a more balanced strength – ductility relation at room temperature, i.e. a slightly reduced strength but increased plastic deformability.
In the next step, potential Mo-V-Si-B materials which provide a reduced density by about 20% as compared to reference alloy Mo-9Si-8B were investigated. Different alloy compositions were produced by powder metallurgy to observe the effects of V as a solute in the respective phases. The microstructure of the bulk Mo-40V-9Si-8B was characterized via SEM and XRD. From Rietveld refinements the preferred V sites in the lattices of the present phases were derived. The mechanical behavior was determined by microhardness measurements as well as constant displacement tests in the compressive mode between room temperature and 1100 °C. Three point-bending with notched samples as well as compressive creep tests reveal a high fracture toughness and acceptable creep strength of this new type of alloys. Therefore, the first results show a high potential as a lightweight version of Mo-Si-B alloys for structural applications.

MoSi₂-based alloys and composites have been considered as promising candidates for high temperature structural application. Directionally solidified (DS) MoSi₂/Mo₅Si₃ composites have a script pattern in which discontinuous Mo₅Si₃ rods inclined to the growth direction are embedded within MoSi₂ matrix. Since a deeper understanding of pattern formation is crucial to the microstructure design for property optimization, a phase field model based on Multiphase-Field framework has been constructed to elucidate the responsibility of continuous nucleation on solid-liquid interface for the discontinuity of this pattern, which has a close relation to material toughness. Under solidification conditions with various growth rates, three dimensional computation results of microstructures reproduced the characteristics of script pattern observed by scanning electron micrography (SEM). In addition, the simulation results show good agreement in length scale of lamellar spacing with experimental images and analytical solutions obtained by Jackson-Hunt approach.

Current research on high temperature metallic materials focuses on Mo-Si-B alloys which are candidates for novel turbine materials. For structural applications, an important phase field in the ternary Mo-Si-B system is located between the Mo solid solution phase (Mo_ss) and the silicides Mo₅SiB₂ (T2) and Mo₃Si (A15), which is known as the so-called ‘‘Berczik-triangle’’. Near-eutectic Mo-Si-B alloys from this three-phase region exhibit outstanding creep properties, even above temperatures of 1100 °C, as well as a good oxidation resistance. However, ingot processing (IM) of this class of materials is challenging due to the high melting point of Mo-Si-B materials being typically > 2000 °C (eutectic Mo_ss-Mo₃Si-Mo₅SiB₂ alloys ~ 2000 °C). Different multi-step powder metallurgical processes (PM) were typically used in the past to produce dense Mo-Si-B samples under laboratory conditions.
The introduction of a one-step processing route for this type of material via additive manufacturing (AM) or 3D printing represents an important innovation that will allow the production of complex bulk materials with net shape geometries (e.g. turbine blades). This work shows the feasibility of printing pre-alloyed near-eutectic Mo-Si-B powder materials via laser metal deposition (LMD). Therefore, Mo-Si-B powder was manufactured via gas atomization (GA) process out of solid raw materials meeting the requirements for AM regarding flowability and particle size. The specific challenge is the ultra-high melting point of this type of alloys, accompanied by problems of interlayer bonding and defects that may occur during cooling. Compact multiphase Mo_ss-Mo₃Si-Mo₅SiB₂ builts containing low porosity could be manufactured. For further understanding of the microstructural evolution powder particles after GA were investigated and detailed analyses of the resulting microstructure were carried out. For purpose of comparison with PM and IM Mo-Si-B alloys first results of mechanical tests, e.g. hardness, compressive strength and creep response, are presented.

Transition-metal (TM) silicides have received considerable attention as promising structural materials for ultra-high temperature applications that can replace the currently used Ni-based superalloys because of their very high melting temperature above 2000 °C, good mechanical properties and good oxidation resistance. Extensive studies using bulk single- and poly-crystals have revealed that most TM silicides can plastically deform only at high temperatures above 1000 °C. The activation of various deformation modes have been reported so far, however, some ambiguity in the identification of operative deformation modes still remains mostly because of severe oxide formation on the specimen surface as well as dislocation climb, which is inevitable in the case of high temperature experiments. Recently, the micropillar compression method first introduced by Uchic et al. has been widely recognized as a new attractive technique to investigate the mechanical response of not only pure metals but also various hard and brittle materials at size-scales of tens of micrometers or less. Recently, we have applied the micropillar compression method to single crystals of various TM silicides such as TMSi₂ (TM=Mo, Nb, Ta, V, Cr), TM₅Si₃ (TM=Mo, Nb, Ti) and T2-Mo₅SiB₂ as a function of specimen size and loading axis orientation. For most of the TM silicides tested, plastic flow was observed if the specimen size is reduced to micron meter size. For T2-Mo₅SiB₂ phase, three different slip systems were identified to be operative at room temperature. The values of critical resolved shear stress (CRSS) for the three slip systems in T2-Mo₅SiB₂ are extremely high all exceeding 2 GPa. The CRSS value for each slip system increases with the decrease in the specimen size, following the inverse power-law relationship with an exponent much smaller than those reported for FCC and BCC metals. The dissociation scheme and glide plane (actual atomic layers) of the identified dislocations in T2-Mo₅SiB₂ were investigated both experimentally through atomic-resolution scanning transmission electron microscopy imaging of their core structures and theoretically by first-principles calculations of the relevant generalized stacking fault energy curves.

Application of the single-crystal Ni base superalloys for turbine blades at temperatures of up to 90 % of their melting point already reached the limit of their development. Because of the beneficial physical and mechanical properties at high temperatures, Mo base alloys are very promising to substitute Ni base alloys. These materials possess high temperature strength and excellent creep resistance as well as acceptable fracture toughness. Nevertheless, the replacing of Ni base superalloys in turbine applications is still a difficult problem. This study is focused on FE calculations of the deformation of a simple turbine blade made of Mo based alloys under the typical loading conditions in comparison to conventional turbine blade materials, aimed to evaluate the feasibility of Mo base alloys.
The investigations of Mo base alloys displayed the best combination of the high temperature properties for alloys containing Si and B, specifically for the alloy family with a three-phase microstructure: ductile molybdenum solid solution phase and the two intermetallic phases Mo₃Si and Mo₅SiB₂. However, an extensive knowledge about the physical and mechanical properties of each phase is not achieved, yet. In this study, the creep properties of the individual phases are determined and applied for the estimation of the creep properties of the alloy in general. This provides a possibility for the advanced alloy design.

C11_b-MoSi₂/ D₈m-Mo₅Si₃ eutectic alloys with script lamellar structure [1] have been proposed as one of the candidates for materials of next-generation gas turbine operated at extremely high temperatures above 1700 °C. The refinement of script lamellar structure and the modification the properties of lamellar interface are believed to improve the toughness at room temperature and creep resistance at high temperature, which is the key to the practical application. In this study, we have developed a phase-field model for examining the effects of various factors on the geometry of the script lamellar structure formed during directional solidification on the basis of the previously developed model for simulating lamellar structure formation in C11_b-MoSi₂/C40-NbSi₂ [2, 3]. The model can take into account the elastic strain energy and the relaxation of strain by structural ledges. It is also possible to include interfacial segregation of ternary elements to the model. When spontaneous decomposition from liquid to C11_b-phase and D₈m-phase was assumed, inclined lamellar structures were formed under limited conditions, and the lamellar spacing decreased with increasing cooling rate as expected. It is suggested that the nucleation of D8_m-Mo₅Si₃ is important for more quantitatively precise reproduction of script lamellar structure. The effects of nucleation will be presented by Zhu et al. elsewhere. Reference: [1] K. Fujiwara et al. Intermetallics 52 (2014) 72-85. [2] T. Yamazaki et al. Intermetallics 54 (2014) 232-241, [3] T. Yamazaki et al. Comp. Mat. Sci. 108 (2015) 358–366.

Niobium silicide intermetallic alloys are good candidates for applications as low-pressure turbine blades in aircraft engines for service temperatures between 800°C and 1000°C. The Nb-Si based alloys are multiphase materials constituted by a niobium solid solution Nb_ss, silicide phases of the Nb₅Si₃-α and / or Nb₅Si₃-β (tetragonal structure) and / or Nb₅Si₃-γ (hexagonal) type, sometimes Nb₃Si type, depending on the heat treatments applied or the alloying elements added. The solidification structure is generally dendritic with more or less complex eutectic cells. Microstructure control during solidification and processing steps is essential to obtain good mechanical properties. This is difficult by conventional casting, knowing that forming processes (forging, etc.) require high temperatures for these materials which often contain more than 50 vol.% of intermetallic phases. Powder Metallurgy (PM), thanks to the microstructural homogeneity it offers, and to easier shaping processes, is a manufacturing process that deserves to be studied.

This communication reports the study of an alloy with composition 43Nb-25Ti-3Mo-3Cr-6Al-20Si (at.%). The microstructure of this alloy has been followed throughout a typical PM cycle. It is detailed for the following steps: (1) cast ingot to be atomized; (2) powders produced by inert gas atomization, of [40-100] μm and [100-200] μm particle size; (3) bulk samples obtained by Spark Plasma Sintering (SPS) at 1385°C, 1520°C and intermediate temperatures; (4) bulk SPS samples with additional heat treatment (1500°C, 100 h). Quasi-static compression tests as well as creep tests, at 800°C and 1000°C, are presented.

The microstructures are compared with respect to the nature and morphology of the phases, the chemical homogeneity and the porosity. The main detected phases are Nb_ss (containing Ti, Al, Mo, Cr), two forms of the Nb₅Si₃ silicide (α or β, and γ; Nb is mainly substituted by Ti) and TiN, probably formed during ingot melting. Depending on the state of the material, the observations show, for the silicides, different sizes, chemical compositions and morphologies, as well as a variable microstructural homogeneity. The local silicon content can create hypoeutectic zones and lead to the primary solidification of Nb_ss. The microstructure of partially densified samples shows the sintering mechanism by Nb_ss diffusion at the interparticle contact points. The SPS process for niobium silicide based alloys offers good prospects for obtaining a fine and homogeneous microstructure, conferring good mechanical properties on this type of intermetallic material.

This work has benefited from State support managed by the ANR under the "Investissements d'Avenir" program through the MATMECA project (reference ANR-10-EQPX-37) and from ANR funding through the SYNOPSIS project (reference ANR-15-CE08-0042).

Silicon based alloys with good thermoelectric property such Si-Ge, Mg_-Si and Mn-Si system have been widely investigated not only bulk form but also thin film form. α-SrSi₂ is also a promising candidate as a thermoelectric material because it consists of abundant nontoxic elements and a good thermoelectric power factor near the room temperature was reported by Hashimoto et al [1]. However, the number of researches is limited compared with former widely investigated silicide and there are no reports in film form. In this study, we firstly prepared α-SrSi₂ thin films on insulating substrates and measured their thermoelectric properties.
Thin films of Sr-Si system were deposited on c-Al₂O₃ substrate by using RF magnetron sputtering method at various deposition temperature and total pressure.
Constituent phases strongly depend on the deposition temperature. The films deposited below 400^oC consisted of amorphous phase. Metastable SrSi₂ phase with CaSi₂ structure was obtained between 500 and 600^oC and finally stable α-SrSi₂ above 700^oC.
Metastable SrSi₂ phase with CaSi₂ structure showed low power factor below 10 μW/(m K²) for the temperature range of 100-400^oC. On the other hand, α-SrSi₂ show good thermoelectric power factor beyond 700 μW(m K²) at room temperature. This much value is larger than observed value of Mg₂Si (111) one-axis-oriented films prepared by the same deposition process, maximum 130 μW/(m K²) at 300^oC. The present result shows that α-SrSi₂ is one of the promising candidates as thin film thermoelectric materials.
[1]K. Hashimoto et al., J. Appl.Phys. 102 (2017) 063703.

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. This talk provides an overview of recent progress, productionization challenges, and opportunities for improvements.

This presentation aims at describing the creep properties at high temperatures 700°C-850°C of the IRIS-TiAl alloy (Ti-48Al-2W-0.08B) densified by Spark Plasma Sintering (SPS). SPS is a technic of powder metallurgy for which the densification is due to the simultaneous application of a direct pulsed electric courant of high intensity and of a uniaxial pressure on a graphite assembly containing the powder. This process allows achieving original microstructures with enhanced properties due to its rapid processing cycle as well as non-textured, homogeneous alloys resulting from the use of the powder metallurgy route.
The as-spsed IRIS-TiAl alloy exhibits a fine microstructure made of small lamellar colonies (40 µm) which are surrounded by single phased borders made of γ grains and containing some precipitates of β₀ phase. Creep curves and creep results obtained between 700°C and 850°C under various stress levels will be presented. The microstructures of crept samples were studied by transmission electron microscopy. Firstly, it will be shown the activation of pure climb of [001] dislocations in (001) planes which was never reported in the literature. Secondly, the displacement mode of ordinary dislocations will be investigated with a special attention to the role of climb.
These results will be discussed to determine what is controlling the creep properties of the IRIS alloy and to explain the origin of the very high creep resistance of this alloy.

TiAl alloys are required high toughness and ductility for jet-engine applications. In previous studies, we clarified that introduction of the β-Ti / γ-TiAl duplex (DP) microstructure at the α₂-Ti₃Al/γ lamellar colony boundaries improves the stress intensity factor range threshold ΔK and decreases the Paris exponent m. Microstructure analysis revealed many slip lines exist in γ phase in DP region. This suggested that it is possible to enhance room-temperature ductility through controlling DP microstructure. Therefore, in this study, microstructure factors to enhance room-temperature ductility was investigated in multi-component TiAl alloys.
Multi-component TiAl alloys were used in this study. The microstructure was controlled by phase transformation of α → β + γ based on our phase diagram studies. Room-temperature tensile tests were conducted with a strain rate of 3 × 10^-4 s^-1 using an Instron-type universal testing machine. In order to removed surface strain that was introduced during machining, tensile test specimens were firstly electrical-polished. Microstructures observation was carried out using field-emission scanning electron microscopy.
The room-temperature ductility changes from 0.1 % to 1.4 % with increasing of volume fraction of γ phase from 5 % to 80 %. However, γ single phase alloy did not show any ductility, less than 0.1 %. Microstructure analysis revealed that the key of factors for enhancement of room-temperature ductility are the large amount of γ phase (>70%) in DP region and the existence of β phase. The other techniques and mechanisms will be discussed in presentation. This study was supported by Strategic Innovation promotion Program (SIP) in Japan.

Titanium aluminides based on the intermetallic γ-TiAl phase are well-established lightweight structural materials within the temperature range from 600 to 800°C due to their high specific (creep) strength, low density and sufficient oxidation resistance. Increasing the aluminum content lowers the density even further while improving the oxidation behavior and leads to the formation of Al-rich intermetallic phases, such as r-TiAl₂. Although for an alloy with a given chemical composition of Ti-60at.%Al desired lamellar microstructures of γ-TiAl and r-TiAl₂ can be adjusted, the occurrence of metastable phases, such as h-TiAl₂ and Ti₃Al₅, impairs the formation of an equilibrium state and embrittles the alloy. The effects of additional alloying elements, like Mo and Nb, on the microstructural evolution of Ti-60Al are mostly unknown and were investigated in the course of this work for 1 and 3 at.% Mo, respectively, and 1 at.% Mo and 4 at.% Nb. The sample material was manufactured by vacuum arc remelting. Several heat treatments were carried out to make a proper comparison to the binary alloy system. Samples were investigated via scanning electron microscopy to visualize the evolving microstructures. X-ray diffraction was performed to identify the occurring phases. Electron probe micro analysis was executed to measure phase compositions and solubility limits. To investigate transition temperatures of stable as well as metastable phases differential thermal analysis and differential scanning calorimetry was performed. The as-cast state revealed process-related defects and cracks. Mo has segregated in all specimens during solidification, whereas Nb had not. Homogenization was possible via holding the samples at 1400°C for one hour followed by furnace cooling. Subsequently, heat treatments of 200 hours at 1000°C and 500 hours at 800°C were performed, followed by water quenching. The addition of Mo and Nb impaired the microstructural evolution and lowered the amount of r-TiAl₂, but lamellar structures of γ-TiAl and r-TiAl₂ were still obtainable in case of 1 at.% Mo. However, none of the specimens reached an equilibrium state. The range of the γ-TiAl + r-TiAl₂ two-phase field region decreased with increasing Mo content as well as the stability of the embrittling Ti₃Al₅-phase. Transition temperatures were determined and a quasi-binary phase diagram was generated, which visualizes the γ-stabilizing effect of Mo by increasing the range of the γ single phase field region.

Due to their peerless combination of material properties at high temperatures, intermetallic titanium aluminides have already started to be implemented as a turbine blade material in the low pressure section of the turbine engine.

As turbine blades are attached to the turbine disk by a specific dovetail connection, metallic surfaces are inevitably in direct contact under harsh mechanical loads as well as thermal conditions. In this regard, friction and wear issues always occur and can strongly affect the overall lifetime of the components.

Within the scope of this study a basic analysis of the friction and wear properties of two γ–based TiAl alloys, namely TNM-B1 and GE 48-2-2, has been conducted using a high temperature pin-on-disk apparatus. Hereby, tests were performed covering a wide temperature range (up to 800°C) under variation of the counterpart material to simulate conditions that are relevant for todays turbine engines.

As oxidation plays a vital role in terms of high temperature sliding wear conditions being the decisive factor for the specific wear mechanism and rate, the basic oxidation behavior of the chosen TiAl alloys has to be additionally investigated. Isothermal exposures in air were conducted to determine the respective influence of the exposure time (up to 1000h) as well as the temperature.

Conclusively, wear losses/rates were calculated and changes in the occurring wear mechanisms were identified using a profilometer as well as different metallographic techniques to depict differences in the overall tribological performance of both substrates.

The thermodynamic modeling has been performed in order to reproduce the change in phase equilibria among β-Ti, α-Ti, α₂-Ti₃Al and γ-TiAl phases within the temperature range from 1573 K to 1073 K in Ti-Al-Nb-Cr quaternary system. Two ternary subsystems of Ti-Al-Nb and Ti-Al-Cr have been assessed with special attention to the α₂ stabilizing effect against α in Nb and α stabilizing effect against α₂ in Cr. Then those two systems have been combined to calculate quaternary phase diagram. In all the temperature investigated in the present study, the negative interaction exists among three elements of Al-Cr-Nb and Cr-Nb-Ti to stabilize the β against the α(α₂) and γ phases. Thus the ternary interaction parameters have been introduced and its Nb/Cr ratio, Al content and temperature dependence were optimized. The experimental validation of the thermodynamic database at the lower temperature of 1273K ~ 1073 K will be discussed both in terms of the phase transformation pathway and the change in volume fractions of the constituent phases.

The microstructural evolution and transformation behavior of titanium aluminides is a complex interplay in alloy design, process development and under operating conditions. In-situ neutron and synchrotron X-ray diffraction deliver unique and complementary insight into the material’s response to high temperature, deformation and extreme conditions. Neutrons illuminate a larger bulk volume and reveal quantitative phase abundance, bulk texture, lattice parameter changes and other ensemble averaged quantities. In contrast, fine-bundled high-energy X-rays deliver reflections from a number of individual grains. For each constituting phase, their statistics and behavior in time reveal information about grain growth or refinement, subgrain formation, static and dynamic recovery and recrystallization, slip systems, twinning, etc. Features will be presented on selected examinations on titanium aluminides, especially to characterize phase evolution and crystallographic changes upon heating and during plastic deformation, and severe conditions.

γ-TiAl alloys for aero engine blades can be produced by forging but the inherent brittleness of TiAl makes this difficult. A means to facilitate forging of γ-TiAl alloys is to stabilise the ductile soft disordered β phase at forging temperature. This is achieved by the addition of β stabilising elements as for example Nb, Mo or V and was successfully implemented in the TNM alloy. In the ideal case the β phase may be present at forging temperature but can be dissolved fully at service temperature where its presence is unwanted. Unfortunately, often not only the two phases γ and α₂ are found at service temperature but additional stable or metastable phases are present. In an experimental Ti-42Al-8.5Nb alloy an orthorhombic phase constituent was identified which had the crystal structure of the O-phase and is a transformation product of α₂-phase at temperatures below 650 °C. To unambiguously identify the phase and its transformation path high-energy X-ray diffraction (HEXRD) measurements at varying temperatures using in-situ specimen environments were performed. Based on literature knowledge the occurrence of O-phase is only expected for significantly higher niobium and lower aluminum contents.
Initially it was supposed that this O-phase is a feature of this very special experimental alloy composition. Nevertheless, screening tests over a wide variety of alloy compositions proofed that the O-phase can be found in a number of commercial as well as experimental alloys depending on composition and heat treatment history. In general it can be stated that alloy compositions with a combined content of β stabilising elements of 6 at.% or more and Al contents of 46 at.% or lower form O-phase to a greater or lesser extent. Minor alloying elements, such as boron or carbon exhibited no measureable effect on O-phase formation.
The results can be valuable for the development of β stabilised TiAl alloys and the understanding of the long term behaviour of TiAl parts as the possibility of O-phase formation in the medium temperature range was until now disregarded for such alloys.

Designing of wrought TiAl alloys opens the window for a wide range of applications and it will not be exclusive to be applied as low-pressure turbine materials but also as high-pressure compressor materials. β-Ti phase in TiAl alloys allows excellent hot workability during processing and excellent mechanical properties in service temperatures. In other words, microstructure control using β-phase is a key to develop high-toughness wrought γ TiAl alloys. This can be accomplished by understanding the phase transformations involving β-phase. Effects of group V^th (V, Nb) and VI^th (Cr, Mo) elements as β-stabilizers on phase equilibria of TiAl alloys above 1473 K were extensively studied by our group. It was found that V and Nb stabilize α₂ against α. Also, the change in three-phase coexisting region β+α+γ that exists above 1473 K to that of β+α₂+γ occurs not just by the ordering transformation α→α₂ (2^nd order phase transformation) but by a transition peritectoid reaction β+α→α₂+γ (1^st order phase transformation) at a temperature between 1453 K (the congruent temperature of α→α₂) and 1400 K (the eutectoid reaction temperature of α→α₂+γ) in Ti-Al binary system with decreasing temperature. Thus this phase transformation allows a unique transformation pathway for α→α₂+β in the ternary systems. However, it has not been clarified yet that the addition of group VI^th elements (Cr, Mo) stabilizes either α or α₂. Thus, in this study, the effect of Cr addition to TiAl alloy on the phase equilibria among the four phases and phase transformation pathways within the temperature range of 1473 K~1073 K were investigated using several alloys in the composition of interest. In between 1473 K-1373 K, the slope of β/α tie-line in the three-phase coexisting region of β+α+γ remains basically unchanged where the Al content in β-phase is much lower than that in α-phase. However, this slope drastically rotates in a clockwise direction and the Al content between the two phases becomes nearly equal, in between 1373 K and 1273 K and below the eutectoid reaction temperature in the binary system (1400 K). This is a strong indication that thermodynamically α-phase exists even below the 1400 K, i.e. addition of Cr stabilizes α against α₂ and the three-phase coexisting region of β+α₂+γ at lower temperatures is formed through a ternary eutectoid reaction (α→β+α₂+γ) with decreasing temperature. This three-phase tie-triangle moves towards lower Al content in phase diagram. This suggests that Cr addition results in increase of the volume fraction of γ-phase with decreasing temperature even in alloys with low Al content. In between 1173 K and 1073 K, no further shift was observed in the tie-triangle meanwhile it expands towards high Cr content in β-phase. Based on these information, a novel technique for developing a new wrought γ-TiAl with excellent workability during processing at elevated temperatures and excellent toughness in service conditions, could be proposed.

Developing new materials and understanding how they deform is the main challenge of engineers in order to follow and predict the fast evolutions of our society. For instance, in a framework of energetic cost reductions, titanium aluminide (TiAl) alloys have attracted considerable attention due to their unique combination of properties such as high specific strength and stiffness, good creep properties and resistance against oxidation and corrosion, which make them suitable candidate materials for high temperature applications [1]. However, TiAl alloys are brittle at Room Temperature (RT), i.e. below their brittle-to-ductile transition temperature, which lies between 800°C and 1000°C [2]. Furthermore, their complex microstructures (multiphase, different types of microstructures, specific dislocation mechanisms…) with several impacts at different scales are puzzling the materials science community. Despite intense research, literature suffers from a lack of understanding of their elementary deformation mechanisms and the precise role of microstructures [2].

In order to address these questions, we report here, an original and an innovative approach bringing the necessary information, thus allowing linking the multiscale aspects of the mechanical behavior of TiAl alloys at RT. Particularly, we bring new breakthrough on the evolution of deformation microstructures at RT in the vicinity of interfaces in γ phase of a dual-phase bulk TiAl alloy. Plastic deformation is induced locally by µN-nanoindentation. The evolution of the microstructures is characterized comprehensively by accurate Electron Channeling Contrast Imaging (aECCI) before and after deformation [3]. aECCI is a non-destructive groundbreaking procedure offering the ability to provide, inside a SEM, TEM-like diffraction contrast imaging of sub-surface defects (at a depth of about one hundred of nanometers) on centimetric bulk specimen with still unsurpassed resolutions [4]. Defects, such as dislocations, can be characterized by applying the TEM extinction criteria [5]. All features help to explain the poor ductility of the TiAl-based alloys at RT. Accommodation of the deformation is reported and a scenario is proposed [3].

References
[1] – Y. Kim, D. Dimiduk, JOM 43, (1991).
[2] – C. Zambaldi, PhD thesis, Aachen (2010).
[3] – A. Guitton, H. Kriaa, E. Bouzy, J. Guyon, N. Maloufi, Materials 11, 2 (2018)
[4] – H. Kriaa, A. Guitton, N. Maloufi, Scientific reports 7, (2017).
[5] – H. Kriaa, A. Guitton, N. Maloufi, Submitted

Engineering structural intermetallics exhibit outstanding stability at high temperature. Among the different families of intermetallics, γ-TiAl alloys are good candidates for propulsion systems in aero and automotive industries. Recently a third and a fourth generation bearing Nb and Mo in well-balanced quantities and small amounts of C and Si (TNM and TNM+ alloys) have been developed. Adequate thermal treatments of these alloys give a lamellar arrangement of α₂/γ colonies that exhibits a good creep resistance at high temperatures, which has been studied by tensile creep experiments and mechanical spectroscopy /1,2/. A microstructural analysis before and after the mechanical spectroscopy experiments should give important information about the mechanisms responsible for the observed behaviour and this is the approach used along this study.
In the present work two lamellar alloys were studied: Ti-43Al-4Nb-1Mo-0.1B (at%) (TNM alloy) and Ti-43Al-4Nb-1Mo-0.1B-0.3C-0.3Si (at%) (TNM⁺ alloy) with a final aging thermal treatment at 1123K and 1173K respectively. Mechanical spectroscopy measurements between 850K and 1225K show the diffusion of several solute atoms at different temperatures. The activation parameters were determined by tensile creep and mechanical spectroscopy experiments.
The regions of discontinuous precipitation surrounding the lamellar colonies were characterized by scanning electron microscopy (BSE, EDX, EBSD) technique. The lamellar α₂/γ colonies and the small precipitates observed inside the lamellae were characterized by transmission electron microscopy (BF, DF, microdiffraction, nanodiffraction, STEM-HAADF, EDX) with a Titan Cubed 80-300KV working a 200KV. A good correlation between the mechanical spectroscopy results and the small precipitates characterized by TEM-STEM-EDX has been established through the corresponding models of precipitation and mechanical properties.
/1/ J. San Juan, P. Simas, T. Schmoelzer, H. Clemens, S. Mayer, M.L. Nó. Acta Mater. 65 (2014) 338-350
/2/ T. Klein, L. Usategui, B. Rashkova, M.L. Nó, J. San Juan. H. Clemens, S. Mayer. Acta Mater. 128 (2017) 440-450
Acknowledgements:
This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO), CONSOLIDER-INGENIO 2010 CSD2009-00013 project, as well as by the Consolidated Research GroupGIU17/071 from UPV/EHU. This work made use of the FIB and the TITAN Cubed microscope facilities of SGIKER from the UPV/EHU.

Titaniumaluminides had been in the focus of research for more than three decades before they finally took off as part of modern aircraft engines in 2011. Due to their density of about 4 gcm^-3 they offer a high specific strength since they weigh only the about the half in comparison to nickel-based alloys. Thus, a favorable advantage in efficiency, noise reduction and fuel consumption is achieved. Because of their properties they gained interest also for application temperatures beyond the present range of ~750°C. This limit is defined by the strong degradation of their oxidation resistance above that temperature. Beside oxidation resistance the mechanical properties are also affect, however, the origin of this phenomenon is still under debate. In addition, several authors reported an increase in oxygen and nitrogen concentration in the surface region of TiAl-alloys. It was shown that the loss of ductility can be restored by removal of the surface and subsurface layer following high temperature exposure. One issue related to that is, that the oxygen and nitrogen uptake within the alloy are not easy accessible.
Recently developed TiAl alloys and applications such as TMB® and TMB®+ aim for even higher application temperatures and improved hotworkability. Beside γ-TiAl and α₂-Ti₃Al, such alloys additionally contain the β/β_o-phase, whose impact on the high temperature oxidation and mechanical properties after exposure is hardly investigated and shows the requirement of a deeper understanding of the subsurface changes in such alloys due to dissolved nitrogen and oxygen.
In this work, the impact of high temperature exposure on the subsurface microstructure of TMB as well as the resulting effect on the mechanical properties at room temperature are investigated. For the latter, 4-point bending tests are conducted on samples before and after oxidation. Oxidation tests are carried out at 900°C for varying times (24 h – 1000 h) in air. The evolution of the microstructural change (based on changes in phase fractions) in the subsurface zone is investigated over time. Based on EPMA and SEM the change of the microstructure in dependence of the distance from the surface is shown, quantified, and could be correlated to oxygen uptake. A mechanism for phase transition phenomena is proposed.

Due to the low density in combination with a high melting point, vanadium demonstrates a great lightweight potential for turbines in aircrafts or energy industry. Since vanadium as a structural material is in focus of research only recently, the effects of several alloying elements on the materials properties are not or insufficiently examined yet. Therefore, various binary V-X and ternary V-Si-X-systems, that frequently contain intermetallic phases, have been studied. By means of ingot metallurgy (arc-melting process), vanadium samples with different concentrations of alloying elements were manufactured. Resulting from this, single phase vanadium solid solutions (V_ss), two-phase and three-phase alloys were produced. Microhardness measurements and compression tests were carried out to determine the mechanical properties in dependence on the alloying components. The combination between mechanical characteristics and microstructural investigations enables conclusions concerning the materials behavior and the efficiency of solid solution strengthening and second phase strengthening. Therefore, SEM (Scanning Electron Microscopy) and XRD (X-ray Diffraction) methods were used to examine the microstructure, to identify phases and to measure elements concentration in the respective phases. Results achieved within this study may help to assess the potential of novel vanadium-based structural materials regarding to high temperature applications.

Recently, a new ternary L1₂ (γ′) phase Co₃(Al,W), coexisting with a fcc solid-solution phase (γ) based on Co, has been discovered. With precipitation strengthening by the γ′ phase, this class of Co-base superalloy exhibits an improved high-temperature strength, compared to that of conventional Co-based superalloy. However, the creep property in ternary alloy is still insufficient, due to the insufficient high-temperature strength and the lack of γ′ in volume fraction. Besides, the oxidation resistance is also an unresolved problem. In previous studies, the effect of some alloying elements on Co-base superalloys has been investigated. Ni, Ta and Ti are effective in improving the creep property by either increasing the γ′ high-temperature strength or the γ′ volume fraction. On the other hand, Cr and Si are proved to be effective in improving oxidation resistance, but providing a negative effect on creep property, by decreasing the γ′ solvus temperature, unfortunately. Thus, in the present study, we investigate the effect of co-addition of alloying elements Ni, Ta, Ti, Cr, Si on Co-base superalloys, attempting to find out a composition exhibiting both excellent creep property and sufficient oxidation resistance, simultaneously.
Ingots of (Co_0.8, Ni_0.2)-aAl-bW-xTa-yTi-8Cr-Si (at.%; a, b, x, y≥0) were prepared by arc melting. These ingots were homogenized at 1200 ^oC for 24 h in vacuum, followed by heat treatment at a sub-solvus temperature for 96 h. The γ′ solvus temperatures were determined by differential scanning calorimetry (DSC), and the microstructures were examined by scanning electron microscope (SEM). Single crystals with selected compositions exhibiting exclusive γ/γ′ two-phase cuboidal structure and reasonable γ′ solvus temperature were prepared by modified Bridgman technique, followed by heat treatment at 900 ^oC for 96 h. Creep tests were performed in tension under the conditions of 137 MPa/1000 ^oC and 428 MPa/900 ^oC. Oxidation resistance behavior at 1000 ^oC were investigated with cyclic oxidation test for 200 h (20 cycles).
Although Cr and Si alloying decrease γ′ solvus temperature drastically, substituting W (and Al) with Ta (or Ti) is an effective way to increase γ′ solvus temperature without precipitating any secondary phases (such as D0₁₉). Oxidation resistance in multicomponent alloys are indeed to be improved, compared to non-Cr, Si alloyed Co-7Al-8W-Ta-4Ti. However, Rupture time of tensile creep at the condition of 900 ^oC /428 MPa in 4Ta and 8Ti are extremely short, only few hours. 2Ta6Ti exhibits relatively high creep property at the condition of 1000 ^oC /137 MPa (103h in rupture), but still not sufficient (approximately one-third to Co-7Al-8W-Ta-4Ti). It seems that, unfortunately, improving oxidation resistance and creep property simultaneously in Co-based superalloy to a utility-level may come out to be difficult, unless new type of alloying elements that increase γ′ solvus temperature more effective than Ta would be discovered.

Due to their high melting points Cr-base alloys are future candidates for materials for high temperature applications beyond Ni-base superalloys. In addition, high Cr content alloys offer lower densities compared to the commonly used Ni-base superalloys. However, for a future successful application still an optimal alloy composition has to be defined with respect to the improvement of nitridation, oxidation, and mechanical properties at high temperatures. In this work, the influence of Si, Ge, Mo, and Pt alloying on the oxidation and nitridation resistance of Cr-rich Cr_ss-Cr₃Si alloys was investigated using thermogravimetric analysis at temperatures from 1050°C – 1350°C. The samples were additionally analyzed using EPMA, SEM, and XRD analysis. Based on the binary Cr₉₁Si₉ alloy [at.%] around 2 at.% Si were substituted by ternary elements (Ge, Mo, Pt). Before oxidation, all investigated alloys had a two phase microstructure consisting of Cr_ss and Cr₃Si A15 phase. During oxidation an oxide scale of Cr₂O₃ and SiO₂ formed and nitrides were found underneath the scale. However, alloying showed a significant influence on the oxide scale formation, nitridation, and oxidation kinetics. Ge substitution decreased the spallation of the oxide scale, reduced the parabolic weight gain (by oxidation and nitridation), and interestingly, also the linear weight loss induced by the formation of volatile species. In the ternary Cr-Si-X (X = alloying element) systems, Ge and Mo alloying both were enriched at the subsurface zone during oxidation and remarkably decreased nitridation and the formation of brittle Cr₂N compared to the binary Cr-Si system. Their combination in a quaternary alloy, in turn, neutralizes these positive effects. Pt acted as nitrogen getter in a ternary Cr-Si-Pt alloy by forming an antiperowskite phase which simultaneously led to increased nitrogen uptake by the sample when Pt is present at the substrate surface. In order to further optimize the alloy composition, the mechanisms of the respective elements on oxidation and nitridation behavior are proposed for the ternary and quaternary alloys.

Two Co-based superalloy subsystems, Co-Al-Cr and the quasi-ternary system Co-Al-Cr-W with a constant amount of 10 at. % W, were deposited as thin-film materials libraries and analyzed in terms of phase formation an oxidation behavior at 500 °C in air. By combining energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy high-throughput composition measurements, a detailed evaluation of the dependence between the initial multinary metal composition and the oxide scale composition which is forming upon oxidation on the surface of the thin film is established. Phase maps for both materials libraries are provided by high-throughput X-ray diffraction. In addition, the oxidation of a Co-Al-Cr-W bulk sample was analyzed and compared to a corresponding film in the library.

Micropillar compression method has received a considerable amount of attention as a new method to investigate plastic deformation behavior of various crystalline materials in sub-micron scale. Previous studies mainly on single crystals of conventional FCC or BCC metals have revealed that various interesting features in micropillar compression experiments such as strain-burst behavior and size-dependent strength with a trend of “smaller is stronger”, which is generally described with an inverse power-law relationship between the strength and specimen size. Although various models to describe the size-dependent strength have been proposed, the validity of the proposed models and their applicability to other crystalline materials with lower symmetry are still controversial. In order to understand the characteristic deformation behavior of single crystalline micropillars, systematic studies on various crystalline materials including those with lower crystal symmetry are considered to be important. In the present study, we focused on an intermetallic phase Ti₃Al with the hcp-based ordered structure of the D0₁₉ type and its parent hcp-Ti in order to investigate how the size-dependent strength varies depending on the operative deformation modes and atomic ordering. Single crystal rods of the stoichiometric Ti₃Al and hcp-Ti were grown by directional solidification using an optical floating zone furnace. Micropillar compression tests of Ti₃Al and hcp-Ti single crystals were carried out as a function of loading axis orientation and specimen size. When the loading axis is parallel to a-axis, {1-100}<11-20> prism slip was confirmed to be activated in micropillars of both Ti₃Al and hcp-Ti. The values of critical resolved shear stress (CRSS) of the prism slip for both Ti₃Al and hcp-Ti were higher than those obtained for bulk single crystals. The CRSS value of the prism slip for each material exhibited a “smaller-is-stronger” trend approximately following an inverse power-law relationship. The power-law exponent for Ti₃Al was found to be much lower than that of hcp-Ti. Possible influences of the ordered structure on the difference in the size-dependent strength for the prism slip will be discussed based on the TEM observations of dislocations.

γ-TiAl based alloys recently have started to replace Ni-based superalloys as a material for turbine blades in the low pressure turbine section of aircraft engines. γ-TiAl based alloys are characterized by low density (4 g per cm³), good oxidation and corrosion resistance, and high specific tensile and creep strength. The presence of the γ and α₂ lamellar colonies is desirable for good mechanical properties. Cubic phase being in its disordered state (A2 structure) improves the forging properties of the alloys due to its high plasticity. The presence of ordered β_o phase (B2 structure) on the other site should be prevented because it increases the alloys brittleness at the working temperatures of about 700-800°C and has bad creep strength. Therefore, knowledge about the presence of β phase and of its ordering/disordering temperature is of high importance for the development of alloys with improved processing properties and for a prolongation of the turbine blades working time. Currently the presence of ordered β_o phase in the binary phase diagram is still under discussion and the influence of the different β stabilizing elements on the ordering/disordering temperature is not systematically investigated.
We studied the ordering/disordering transformation with in-situ synchrotron and neutron diffraction techniques. A good contrast of neutron diffraction between ordered and disordered β_o/β was used to confidently determine the presence of ordered β_o. 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 investigated. Three ternary alloys with 2 at. % of Fe, Cr, and Mo contain correspondingly 12, 8, and 18 vol.% of the ordered β_o phase. By synchrotron investigations we determined the degree of ordering via both site occupancy calculations and correlation between superstructural and fundamental reflections for a and b phases. Alloys with 2 at.% of Cr and of Fe have degrees of ordering of about 65% and with Mo of about 58%. The evolution of the phase composition during the experiments was calculated by Rietveld refinement using the MAUD programm.

γ-TiAl based alloys are promising materials for high temperature applications up to 800°C because of their light weight and high creep strength. However, the application of this material to jet engines is limited to the last stages of low pressure turbine blades due to low ductility and toughness for now. In order to apply this material to other parts such as high pressure compressor, toughening is essential. We performed fatigue crack growth tests on wrought TiAl alloys and revealed that introduction of β-Ti/γ duplex (DP) along α₂-Ti₃Al/γ lamellar colony boundaries increased the fatigue threshold, ΔK_th and decreased the Paris exponent, m. We proposed that the improvement of ΔK_th is partly caused by the deformation of the DP microstructure. In order to verify this proposed mechanism, the quantification of deformation in each phase is important. In the present study, in-situ observation of tensile deformation in TiAl based alloys under an SEM at room and high temperatures is performed, and strain localization is measured by digital image correlation (DIC) technique. Three dimensional observation of the un-deformed and deformed materials is also conducted to observe the morphology of the microstructure and the crack stop points. The alloy with nearly lamellar (NL) microstructure, in which the volume fraction of DP is 7%, shows elongation of 0.14% at room temperature. Deformation is observed in both lamellar colonies and DP region in NL microstructure as a result of the strain localization measurement by DIC technique. In some favorably oriented lamellar colonies, strain localization along lamellae is more than five times as much as the macroscopic strain. The amount of deformation in each phase and the crack initiation and growth behavior during tensile will be discussed. This study was supported by Strategic Innovation promotion Program (SIP) in Japan.

MoSi₂ with the tetragonal C11_b structure has been considered as a promising material for ultra-high temperature structural applications because of its high melting point (2020 °C), excellent oxidation resistance, and relatively low density. However, poor fracture toughness at room temperature and insufficient high-temperature strength are still drawbacks to be improved for its practical applications. One possible way to solve these drawbacks is to form an in-situ composite with one or two strengthening phases. Among various candidates, we have recently focused on MoSi₂/Mo₅Si₃ eutectic composites because of their high eutectic temperature (1900 °C for the binary alloy) and fine microstructures of the so-called script lamellar type formed simply by directional solidification (DS) and have studied the influences of ternary additions on the microstructure and mechanical properties of DS MoSi₂/Mo₅Si₃ eutectic composites. Our previous studies have revealed that their high-temperature mechanical properties and fracture toughness values can be improved through refining the lamellar structure and controlling interfacial properties such as interfacial segregation of ternary elements and lattice misfits. We also have found that the DS ternary alloys with a small addition of C possess a homogeneous three-phase script lamellar structure composed of MoSi₂, Mo₅Si₃, and Mo₅Si₃C and exhibit higher yield strength than the binary two-phase counterpart. In the present study, effects of Ta and Nb addition on the microstructure and mechanical properties of MoSi₂/Mo₅Si₃/Mo₅Si₃C eutectic composites were investigated in order to establish a way to further improve the mechanical properties of MoSi₂/Mo₅Si₃–based eutectic composites. When part of Mo was substituted with Ta or Nb, a relatively homogeneous three-phase eutectic lamellar structure was obtained only at growth rates lower than 10mm/h during the DS process using an optical floating zone furnace, while a heterogeneous eutectic microstructure with a cellular morphology was developed at higher growth rates. Nb atoms were partitioned mostly into Mo₅Si₃C, while Ta atoms were partitioned into both Mo₅Si₃ and Mo₅Si₃C. Both volume fraction and average thickness of MoSi₂ lamellae decreased in the Nb alloyed eutectic composites compared to those in the non-alloyed counterpart, which resulted in higher yield strength and better creep property of the Nb alloyed eutectic composites.

MoSiBTiC alloys are expected to be used for next generation high-pressure turbine blades. However, the phase diagrams of the alloys are still deficient. Our group has developed an ultra-high-temperature thermal analysis using blackbody radiation, which can be used above 2000 °C. In this study, MoSiBTiC alloys with several different compositions were thermally analyzed with this equipment to evaluate their phase transformation temperatures. In addition, the alloys were levitated and melted by electromagnetic levitation (EML) technique in a static magnetic field to observe solidification phenomena, and to obtain rapidly-solidified sample. The solidification paths of the alloys were studied from the thermal analysis and microstructures, and the partial phase diagrams of the alloys were constructed.
Each alloy powder was filled in a CaO-stabilized ZrO₂ crucible having a blackbody cavity, and then heated and cooled in a radio-frequency furnace under Ar atmosphere. The heating and cooling rates were fixed at 10 °C / min. The sample was kept above 2000 °C for over 30 min to ensure homogeneous melt, and then cooled. Temperature change during cooling was measured by a pyrometer, through the radiance from the blackbody cavity. The phase transformation temperatures were obtained from the cooling curves. On the other hand, each alloy was levitated and melted by EML technique in a static magnetic field of 10 T. The static magnetic field was applied to suppress convection in the melts. The temperature of the melts were measured by a pyrometer and controlled by adjusting the power of a heating laser. Multi-recalescence was observed at the surface of the alloys during cooling. The levitated alloys were held at the temperature just after each recalescence for about 10 min and rapidly cooled by blowing He gas to freeze the high-temperature structure. The cross-sectional microstructures of rapidly-solidified alloys were observed by a scanning electron microscope.
As an example, results of Mo-5.0Si-10.0B-8.8Ti-8.8C (mol%) alloy were described as follows; five inflection points appeared on the cooling curve during thermal analysis of this alloy. From the microstructure analysis, the following solidification path was proposed. The first point corresponds to the liquidus temperature, at which the Mo solid solution (Mo_ss) phase begins to precipitate as a primary phase. Subsequently, the Mo₂B phase precipitates from the melt, and the (Mo_ss-TiC) eutectic reaction takes place, which is followed by the (Mo_ss-T₂-TiC) eutectic reaction. Finally, the (Mo_ss-T₂-Mo₂C) eutectic reaction occurred, and the solidification completes below 1800 °C. Thus, utilizing combination of the ultra-high-temperature thermal analysis technique with the EML technique revealed the complicated solidification path of MoSiBTiC alloys. Other compositions of the alloy were similarly studied, and the partial phase diagrams of the alloys were constructed. The details will be presented in the conference.

For improving energy efficiency a new class of high temperature materials based on refractory elements have been investigated. It was already reported that the alloys based on Nb and Mo are composed of BCC solid solution (Nb-Mo) and T₂-silicide (Nb,Mo)₅(Si,B)₃. Further investigation on alloy phase equilibrium are needed for improved mechanical properties and oxidation resistance.
Cr is one of the candidate to modify the properties of the alloy because Cr is expected to stabilize T₂ compound phase with B. In the present study the phase equilibrium among BCC solid solution and T₂ compound are widely investigated in Cr-Mo-Nb-Si-B system. Alloys are prepared using an Ar-arc melting machine and heat-treated at 1400 ^oC for 168 h. Microstructures are investigated using FE-SEM (JEOL, JXA-8530F). Constituent phases are identified using XRD (PHILIPS, X'Pert Pro), FE-EPMA (JEOL, JXA-8530F) and AES (Auger Electron Spectroscopy, JEOL, JAMP-9500F).
In the Cr-Mo-Nb-Si-B system BCC-T₂ two-phase microstructure are found in Mo-rich alloys. B/Si ratio in T₂ phase increases with increasing Cr, while almost no B solubility was found in BCC solid solution. With increasing Si in alloys, A15 silicide phase ((Cr, Mo, Nb)₃Si) and/or Laves phase are stabilized.
This work was supported by the Advanced Low Carbon Technology R&D (ALCA) program of the Japan Science and Technology Agency (JST).

The 1st-generation MoSiBTiC alloy (65Mo-5Si-10B-10TiC, mol.%) has great potential in high temperature structure application because of its comparable density with Ni-based superalloys (~8.8 g/cm³) and excellent high-temperature strength. However, its poor oxidation resistance, especially at intermediate temperatures (700-900°C) prevents them from practical uses. A recent work by Zhao et al. showed that the introduction of Ti₅Si₃ into MoSiBTiC alloy was able to improve the oxidation resistance, but without detailed oxidation mechanisms provided. In this study, the oxidation behavior of a Ti₅Si₃-containing MoSiBTiC alloy is systematically investigated at various temperatures, aiming to clarify the effect of Ti₅Si₃. The alloy was prepared by arc-melting and then annealed at 1700°C for 24 hours. Oxidation tests were carried out in a thermo-gravimetric analyzer (TGA) for different time periods. Microstructures before and after oxidation tests were characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The heat-treated 38Mo-20Ti-17Si-5B-10TiC alloy was composed of Mo_SS, Mo₃Si, Mo₅SiB₂, Ti₅Si₃ and TiC phases. During oxidation test at 700°C, the alloy exhibited an initial transient stage of weight gain followed by a steady state of weight loss. An initial transient stage, a steady stage and an acceleration stage leading to catastrophic failure were observed at 800°C. The oxidation kinetic curves obtained at 900-1200°C showed continuous weight loss, including an initial transient stage and a steady state.

Mo-Si-B-TiC alloys are expected as a candidate for ultrahigh-temperature materials to replace Ni-base superalloys, because they have a low density, a superior high-temperature strength and a high fracture toughness. The alloys have complicated microstructures consisting of molybdenum solid solution (Mo_ss), Mo₅SiB₂ (T₂), (Ti, Mo)C_x and (Mo, Ti)₂C phases and their eutectic phases. However, it has been still unclear how the microstructure affects mechanical properties of these alloys. It was reported so far that the topology like fractal analysis and percolation analysis were useful to correlate the microstructure with mechanical properties such as intergranular fracture. Accordingly, we applied the topological approach to evaluate microstructure in Mo-Si-B-TiC alloys and examined the correlation between topological parameters and the fracture toughness.
Four distinct Mo-Si-B-TiC alloy ingots with the same composition (65Mo-5Si-10B-10TiC (at%)) were produced via conventional arc-melting (20g and 90g), drop-casting and plasma arc-melting techniques in an Ar atmosphere, followed by heat-treatment at 2073 K for 24 hours. Microstructure in those samples was observed by SEM, and then percolation and fractal analyses were performed on the binarized SEM-BSE images. The EPMA was used to evaluate the chemical composition of constituent phases. The fracture toughness, K_Q, was evaluated using a three-point bending test based on the Irwin’s similarity relationship [1]. Three-points bending tests were conducted at a displacement rate of 5 µm/s. The specimens with the Chevron-notch had final dimensions of 2.5 mm (w)×2.5 mm (h)×12.5 mm (l). The Chevron-notches were machined by electro-discharge machining with a brass wire of 0.1 mm in diameter.
The SEM observation and EPMA analysis revealed that the area fraction of constituent phases and their compositions in those Mo-Si-B-TiC alloys were almost the same, but microstructures were different. The percolation probability of hard phases: Mo₅SiB₂, (Ti, Mo)C_x and (Mo, Ti)₂C, was found to be more than 85%, whereas that of Mo_ss was less than 20%. Accordingly, the microstructural feature of those alloys that the Mo_ss clusters were distributed around the continuous hard phases, and then the fractal dimension of dispersion [2] of Mo_ss phase was determined. The fracture-toughness of those alloys were measured to be in the range of 14.6~15.6 MPa (m) ^1/2. There appeared no clear relation between the fractal dimension of dispersion [2] for Mo_ss clusters and the fracture-toughness of those alloys. However, of particular importance is finding that the fracture-toughness monotonously increased with increasing a new parameter that was multiplied the fractal dimension of dispersion by the average perimeter of Mo_ss clusters.
[1]T. Moriyama, K. Yoshimi, M. Zhao, T. Masnou, T. Yokoyama, J. Nakamura, H. Katsui, T. Goto: Intermetallics 84 (2017) 92-102.
[2]Y. Mizuno, K. Terashita and K. Miyanami: Kagaku Kogaku Ronbunshu 19-1 (1993) 21-29.

1st generation MoSiBTiC alloy, which mainly consists of Mo_ss, Mo₅SiB₂(T₂) and TiC phase, is expected to be a possible candidate for novel ultra-high temperature material because of its outstanding mechanical properties. However, its insufficient oxidation resistance at elevated temperature has caused barriers in practical application.
In this study, we attempted to improve the oxidation resistance by two steps. First, by increasing Si with Ti content, a Mo_ss-T₂-Ti₅Si₃-TiC four-phase alloy was developed. The introduction of Ti₅Si₃ phase is aimed to increase a Si resource in the microstructure. Second, Cr was added on the four-phase alloy to reinforce its oxidation resistance especially at intermediate temperature around 800°C. The Cr addition maintains the constituent phases of the alloy in a compositional range, and thus a Cr-added four-phase MoSiBTiC alloy was successfully developed at a composition of Mo-10Cr-28Ti-14Si-6C-6B (mol%).
As expected, newly developed MoSiBTiC alloy exhibited a much better oxidation resistance at 800°C than 1st generation one. The alloy composition modification led to the formation of protective Cr oxide below an outermost Ti oxide layer. In addition, this alloy has a much smaller density of about 7.1 g/cm³, resulting in about 20% improvement in the high-temperature specific strength at 1400°C compared with that of 1st generation alloy.

The influence of composition in Co₃(Al, W)-Co₃Ti pseudo binary system on the positive temperature dependence of strength of the L1₂ intermetallic compound has been examined. Some L1₂ ordered pseudo binary alloys were prepared and those 0.2% flow stresses were determined by a compression test from room temperature to 1273 K. The pseudo binary alloys show the intermediate onset temperature of the anomalous yield stress between the component alloys of Co₃(Al, W) and Co₃Ti. Since the onset temperature is closely related to the thermally activated cross-slip from {111} glide plane to the {010} non-glide plane known as the Kear-Wilsdorf (KW) locking mechanism. The values of the activation energy and of the pre-exponential factor were derived from an Arrhenius-type plot of the increase in the 0.2% flow stress. On the simple assumption where the values of the activation energy and of the pre-exponential term in a pseudo binary alloy have a linear relationship with its composition, the experimentally determined temperature dependence of 0.2% flow stress are roughly represented, though the calculated values of the activation energy and of the pre-exponential term are slightly different to those experimentally determined.

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). Another study indicates that complex stacking fault (CSF) energy as well as γ’ solvus temperature might be the key of improving the high-temperature strength of Co₃(Al,W).
In the present study, we investigate the effect of Ni, Ta alloying on the anomalous temperature range 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 thus determined from yield strength-temperature curves. Dislocations in specimen deformed at high temperatures were investigated with TEM. While, γ’ solvus temperature was determined by differential scanning calorimeter (DSC). Slip trace formed at high temperatures was investigated by scanning electron microscopy with electro back-scatter diffraction.
The result indicates that, with Ni, Ta alloying, reported to be effective to increase the CSF energy, the onset temperature of YSA shifts to the low temperature side. TEM observation reveals that dislocations introduced above onset temperature exhibit strong tendency to along in its screw orientation while that below onset temperature are curved and no special orientation preferred, indicating that YSA in alloyed Co₃(Al,W) is also corresponding to the K-W lock. Thus, it is evident that alloying increasing CSF energy is effective in widening the YSA region by shifting onset temperature to the low temperature side. On the other hand, alloyed Co₃(Al,W), possessing higher γ’ solvus temperature, exhibits also higher peak temperature, compared to ternary Co₃(Al,W). However, the correspondence between peak temperature and γ’ solvus temperature is not so strong among the alloyed Co₃(Al,W), indicating that other effects, such as diffusing, may also influence on the mechanical properties at high temperature. Nevertheless, the slip trace analyses indicates that (111) slip takes place exclusively above the temperature range and no slip plane transition occurs in the vicinity of peak temperature, unlike Ni₃Al, in which the peak temperature corresponds to the (111)=(001) transition.
We confirmed that CSF energy is a dominant parameter of determining the onset temperature of YSA in both ternary and alloyed Co₃(Al,W). On the other hand, γ’ solvus temperature is a very important parameter, although may not be the only one, on peak temperature.

It is well known that various elements substitute for a certain sub-lattice of intermetallic compounds. There are various experimental investigations on the effect of substitution elements on the mechanical properties, however, there are few reports on the effect of multi-elements substitution. In the present study, L1₂ type compound Ni₃Al is selected as a model compound because its substitution behavior is established. It was reported that various elements such as Co, Cu, Pd and Pt substitute for Ni-site, while Si, Ga, Ge, Ti, V, Nb, Ta, Mo, W substitute for Al-site. These elements are expected to introduce local lattice distortion which may have effects on the motion of dislocations not only at room temperature but also at lower temperature region or higher temperature region. Several alloys composed of 5 or more elements including Ni, Co, Al, Mo, W are prepared using Ar-arc melting machine and heat-treated at 900 ^oC for 168 h. Microstructures are investigated using FE-SEM (JEOL, JXA-8530F). Constituent phases are identified using XRD (PHILIPS, X’Pert Pro) and FE-EPMA (JEOL, JXA-8530F). Mechanical properties of phases are investigated using nano-indenter (Hysitron, TI-950 Triboindenter) with a load of 10000 micro-N. Several alloys are found to include (Ni, Co)₃(Al, Mo, W, …) - L1₂compound as one of the constituent phases. The hardness of these L1₂phase investigated using nano-indenter are almost the same or higher than that of high strength Co₃(Al,W) -L1₂ compound, and it is confirmed that multi-elements substitution is an effective way to improve mechanical properties of Intermetallic compound.
This work was partly supported by the Advanced Low Carbon Technology R&D (ALCA) program of the Japan Science and Technology Agency (JST).

A dual two-phase intermetallic alloy exhibits microstructure composed of primary Ni₃Al (L1₂) phase surrounded by a eutectoid microstructure comprised of Ni₃Al and Ni₃V (D0₂₂) phases which is formed by a eutectoid reaction from A1 (fcc) phase. The intermetallic alloy with the dual two-phase microstructure shows attractive mechanical properties as high-temperature structural materials [1]. However, further increasing in mechanical properties is required to be used as advanced high-temperature structural materials. It has been reported that Ti and Nb not only stabilize the two constituent phases [2] but also play a role in effective solid solution strengtheners [3]. In this study, the effect of Ta addition which is other potential solid solution strengthener in the dual two-phase intermetallic alloy is investigated. Ta was added to a base alloy with a composition 75Ni10Al15V (in at.%)+50 wt.ppm B by three substitution methods for Ni, Al and V. Alloy button ingots prepared by arc-melting were solution-treated at 1553K for 5 h in a vacuum, followed by furnace cooling at a rate of 10 K/min. Microstructure observation was carried out by scanning electron microscopy (SEM) and electron probe micro analyzer (EPMA). Lattice parameters of the constituent phases were calculated from X-ray diffraction (XRD). Mechanical properties were evaluated by Vickers hardness test at room temperature.
The dual two-phase microstructure was especially fine in the 2Ta(Al) alloy among three substitution methods where the alphabet between the parentheses indicates the constituent element substituted by Ta. The hardness was observed to be ranked in order, the Base < 2Ta(V) < 2Ta(Ni) < 2Ta(Al) alloys. The solid solution hardening predicted from the unit cell volume increment rate |dV/dC|, where V is unit cell volume and C is Ta content dissolving in the constituent phases, is ranked in order, Ta(Al) ~ Ta(V) < Ta(Ni). Based on the observed results of microstructure and hardness, and the calculation on lattice expansion in the constituent phases, it is suggested that the additional hardening operating on the 2Ta(V) and 2Ta(Ni) alloys is dominated by the solid solution hardening while that operating on the 2Ta(Al) alloy is dominated by the hardening due to fine microstructure in addition to the solid solution hardening, consequently resulting in the largest hardness.

[1] Y. Nunomura, Y. Kaneno, H. Tsuda and T. Takasugi, Acta Materialia, 54 (2006) 851-860.
[2] S. Kobayashi, K. Sato, E. Hayashi, T. Osaka, T.J. Konno, Y. Kaneno, T. Takasugi, Intermetallics, 23 (2012) 68-75.
[3] K. Kawahara, T. Moronaga, Y. Kaneno, A. Kakitsuji and T. Takasugi, Materials Transaction, 51 (2010) 1395-1403.

Use of engineering plastics has been steadily increasing in advanced industry hardware; automobiles, trains, aerospace, electronic parts, and electricity production/storage, where always demand materials with lighter, stronger, and more heat resistant properties, which has been persistent to attain their better performances, as called “high performance plastics”. A recent example is a group of fluorocarbon polymers (PFA) gradually replacing currently popular engineering thermoplastics or metals. PFA, however, stays elastic at low temperatures and shows its plasticity only above a transition temperature (>400 °C ). Engineering plastics have been made into many product shapes of complex geometries through injection molding machines with critical parts made of many conventional tool (high strength, high carbon) steels. However, for advanced engineering plastics, like PFA, we need wrought metals with more heat resistant strength. Another inevitable property required for those parts is resistance against hot corrosion, as those parts' surfaces are continuously exposed to high temperature corrosive gas during the PFA plasticization process. In this study we developed a new Ni-based wrought superalloy, (hereinafter called “F-alloy”) to be used for critical parts of advanced engineering plastics injection molding machines.
F-alloy has a characteristic as follows; the composition of this alloy was determined according to the L18 table of the design of experiment(DOE)，which commonly practiced by a quality engineering method and enabled us to not only reduce a number of experiments for new alloy developments but grasp quantitatively influence of each elements to the target properties; hardness and corrosion-resistance to PFA.
Experiment procedure is as follows; 18 melts of ingots, 10kg each, with 18 different composition were made by a laboratory induction melting furnace. The major alloy elements and composition were selected according to the L18 table of ”DOE” . The mechanical and metallurgical test samples were machined out from bars and plates made through press forging the 18 ingots, followed by heat treatments (solution and aging) at the temperatures predetermined by thermal and metallurgical analyses.
Hardness of F-alloy was measured at room temperature up to at 900 °C and the results were compared to the data of a conventional corrosion-resistant alloy (UNS N10276), a tool steel (UNS T30402), which has been commonly used for the parts of present injection molding machines. Hot corrosion tests were performed for F-alloy and two reference materials, the said corrosion-resistant alloy, and the said tool steel, being exposed to an environment composed with PFA at 400 °C .
The results of those experiments show that this alloy has both hardness (Hv600 at 400 °C ) and good hot corrosion-resistance to PFA at 400 °C , which is an outstanding characteristic compared to conventional materials for the parts of PFA injection molding machines.

Hot dip aluminized steels are practically used as automotive exhaust system parts because of their good heat resistance, corrosion resistance and designability. The coating layers have been believed to mainly consist of a thick η-Fe₂Al₅ phase and a thin θ-Fe₄Al₁₃ phase. The orthorhombic η-Fe₂Al₅ (space group Cmcm) comprises chains of atoms with partial occupancies along the c-axis. Recent studies report that these sites are ordered with different manner in relation to compositions, and that there are variations of ordered phases such as the η’ and η’’ phases, which possess the framework structure of the η phase. Furthermore, another derivative of the η phase (the η''' phase) is suggested to exist by means of powder X-ray diffraction (XRD) analysis. However, the reported structure does not seem to be consistent in terms of the hierarchical ordering of the η phase. In this study, crystal structure of the η’’’ phase is assessed by means of transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and XRD analysis.
Ingot with Al-rich composition Fe-73.7 at.%Al was prepared by arc-melting method. After solution treatment at 1073 K for 8 hours, the ingot was encapsulated in vacuum-sealed quartz ampoules and heat-treated at 523 K for 60 days followed by water quenching. Thin foils for TEM and STEM observations were prepared by electro-polishing.
In the selected-area electron diffraction (SAED) patterns obtained from various incidence azimuth, there are superlattice diffraction spots in addition to fundamental ones of the η structure. For example, the superlattice diffraction spots are located at the positions that divide the distance between the 000 and 1-11 or 3-11 fundamental spots by two. From STEM observations, it is clarified that the η’’’ phase accompanies orderings of the atom sites with partial occupancies as those of the η’ and η’’ phases. Furthermore, the η’’’ phase contains periodic anti-phase boundaries (APBs) normal to the c axis. According to the analysis of XRD pattern, APBs are presumably introduced to compensate for the c-axis lattice incompatibility among adjacent η, η’ and/or η’’ phases.

The so-called sigma phase has long been considered to cause detrimental influences on the mechanical properties of stainless and heat-resistant steels mainly because of its high hardness and brittleness at room temperature that stem from its complex crystal structure (D8_b structure, tP30, space group: P4₂/mnm, c/a ~0.52). Recently, it has been reported that the creep strength, ductility, yield strength and tensile strength of stainless steels can be improved if the distribution and morphology of the sigma phase are properly controlled. These results have caused a growing demand for fundamental understanding of plastic deformation behavior of the sigma phase itself. However, the deformation behavior of the sigma phase is largely unknown because of its brittleness at room temperature. Recently, micropillar compression tests of single crystals have been proved to be useful for studying fundamental deformation behavior of hard and brittle materials such as high temperature intermetallics, semiconductors and ceramics. In the present study, we have prepared single crystalline micropillars of the Fe-Cr sigma phase and compression-tested at room temperature as a function of loading axis orientation and specimen size. Plastic flow was observed at room temperature for all tested micropillars mostly exhibiting a smooth transition from elastic to plastic deformation similar to those observed for ductile materials. Four different slip systems were identified to be operative depending on the loading axis orientations. The values of critical resolved shear stress (CRSS) for the four slip systems identified are extremely high about 1.2 ~ 2.5 GPa. However, it should be noted that very good ductility more than 5 % compressive strain was achieved for most loading axis orientations tested, which clearly indicates that the Fe-Cr sigma phase is not inherently brittle at least under compressive loading.

Porous metals have unique properties like ultra-low density, energy absorption, thermal insulation, sound absorption, fluid permeability, and so on. For example, the thermal conductivity of the porous metals with 80% porosity is about one tenth of the bulk metals. Transition metal aluminides like TiAl, NiAl, and Ni₃Al have high melting point, high strength, high oxidation and corrosion resistance, and low thermal conductivity. Porous transition metal aluminides, which have all of these properties, are expected as a high-strength energy absorber and a heat-resistant thermal insulator, and so on. Our group have developed a powder metallurgical process to synthesize porous titanium aluminides such as TiAl, TiAl₃, and Ti₃Al by combining reactive synthesis and a spacer method, and revealed the high strength of porous TiAl. In the present study, we attempted to synthesize porous nickel aluminides like NiAl and Ni₃Al through the reactive process and control their porous structure. Al powder with 2 μm in size and 99.99% in purity, Ni powder with 1 μm in size and 99% in purity, and sodium chloride (NaCl) particles with 30-50 μm in size and 99.99% in purity were used as starting powders. These powders were blended with various molar ratio of Al/Ni (3, 1, and 1/3). The volume fraction of NaCl was varied within the range of 0-80%. The blended powder was cold compacted at 25 MPa to obtain cylindrical precursors with 10 mm in diameter and height. An electric current sintering was performed under applying a pressure of 5 MPa. Temperature was raised in a rate of 0.5 K/s and held at 923 K for 10.8 ks. The sintered samples were soaked in water for 86.4 ks to leach out NaCl completely. For comparison, porous Al specimens were also fabricated by powder sintering and spacer method. X-ray diffraction measurements were performed operating at 40 kV and 20 mA with a Cu-Kα radiation in order to confirm the constituent phase. The porous structures of fabricated samples were observed with a scanning electron microscopy. Synthesized porous nickel aluminides had bi-modal pore size distribution. Large pore replicated the shape of NaCl particles. Small pore (0.5~4.0 μm in diameter) derived from reactive synthesis are formed where Al particles existed before the reaction. As the size of Al particles was larger, the size of small pores increased. Porosity of small pores increased with increasing the Al/Ni molar ratio. The porous structure can be controlled by the size and the composition of the starting powders.

Pearlitic steels with a lamellar structure composed of alternating ferrite and cementite (Fe₃C) layers are widely used as high-strength steel wires and rails because of their high strength. The strength of pearlitic steel wires has been known to be improved further by drawing, which makes their lamellar structure much finer. These attractive mechanical properties related to their microstructure evolution during drawing have been considered to be strongly influenced by the mechanical properties of cementite. Cementite has the orthorhombic D0₁₁ structure (oP16, space group: Pnma, a = 5.08, b = 6.73, c = 4.52Å), which is composed of corner- and edge-sharing carbon-centered trigonal prisms. Cementite has been considered to be brittle mainly because of its complex crystal structure with low symmetry. However, inherent deformation mechanisms of cementite Fe₃C are still largely unknown, except for some implications of plastic deformation observed as slip band propagation across very thin Fe₃C lamella in pearlite grains and the thickness reduction of Fe₃C lamella during the drawing process of pearlitic steel wires. This is mostly because of the lack of systematical studies using single crystals. Recently, micropillar compression tests have been proved to be useful in studying deformation behavior of various crystalline materials including those with serious difficulty in preparing bulk single crystals. In this study, single crystalline micropillars of cementite were fabricated by focused ion beam technique, and tested in compression at room temperature as a function of loading axis orientation and specimen size. Plastic deformation was observed at room temperature for all tested specimens. Five different slip systems were identified experimentally for the first time. The values of critical resolved shear stress for the five slip systems were confirmed to be very high about 1.0 – 2.0 GPa. Characteristics of identified slip systems such as actual glide plane and dislocation dissociation scheme were investigated by both experimentally through TEM analysis and theoretically by first-principles DFT calculations of generalized stacking fault energy.

Al alloys are widely used as key structural materials especially in aerospace and automobile industries owing to their light weight, high strength, and good workability. In Al alloys, nanosized clusters of solute atoms called Guinier-Preston (G.P.) zones play a significant role in the precipitation hardening effect, in which the Cu nanoclusters impede the movement of dislocations in the Al matrix while maintaining balanced strength and ductility. Thus, it is important to control the size, orientation, and shape of the nanoclusters for the adequate design of Al alloys. The observations with high-resolution transmission electron microscopy revealed that the structures of G.P. zones consist of disk-shaped, monoatomic layered precipitates of Cu atoms along the {100} planes. However, the detailed mechanism of the formation of the zones has not been clarified yet. Understanding of the atomistic mechanism may serve as relevant information for the design and exploitation of advanced Al alloys with excellent mechanical performance.
In this study, in order to elucidate the atomistic behavior of solute atoms and vacancies in the process of the formation of G.P. zones, a framework of atomistic kinetic Monte Carlo modeling for Al-Cu alloys was developed based on density functional theory (DFT). An on-lattice potential model for a dilute Al-Cu-vacancy system was constructed using the DFT-calculated binding energies for the pairs and triplets of Cu atoms and a vacancy in the Al matrix, and then applied to atomistic kinetic Monte Carlo calculations. The DFT results revealed that the Cu-Cu pairs in the first-nearest neighbor (1NN) position and the Cu triplets with a 1NN bond angle of approximately 90 degree exhibit significant attractive interactions whereas a vacancy does not prefer to occupy sites neighboring to these Cu pairs and triplets. This suggested that a vacancy can diffuse rather randomly without strong bindings to Cu atoms and that Cu atoms gradually form clusters via a vacancy-assisted diffusion mechanism with a lowering of the energy of the system. Indeed, the results of atomistic kinetic Monte Carlo calculations supported this view and reproduced the planar segregation of Cu atoms along the {100} planes in a manner consistent with experimental measurements. Also, the effects of temperature and vacancy concentration on the formation of G.P. zone were investigated. The nucleation behavior of G.P. zone obtained from the kinetic Monte Carlo analysis was compared with the counterpart based on classical nucleation theory. The critical nucleus size for the formation of G.P. zone was estimated from the formation free energy of Cu clusters with considering a competition between the enthalpic and entropic contributions to the free energy at finite temperatures.

Long-period stacking ordered (LPSO) phase is an intermetallic compound in high-strength Mg alloys such as Mg₉₇Zn₁Y₂. Basal slip acts as the primary deformation mode in the LPSO phase because of its lower critical resolved shear stress than the others, leading to the formation of slip and kink bands, which are macroscopically parallel and perpendicular to the basal plane, respectively. Although the formation of these deformation bands may be tr