To quantitatively understand the thin foil effect in in situ observations of the B2 to B19′ transformation in Ti–Ni alloy, the microstructure of the B19′ martensite in thin foil and bulk specimens was compared. The transformation temperatures decreased with decreasing specimen thickness. There were large habit plane variants more than several tens of micrometers in size in the area of the specimen less than 10 µm thick. The critical thicknesses for reproducing the transformation behavior in the bulk material was about 20 µm based on the self-accommodation morphology and 4 µm based on the twin width ratio of the 〈011〉 type II twin.

A new reduction process for producing titanium (Ti) with an ultra-low oxygen concentration directly from TiO₂, employing yttrium (Y) as the reductant, was developed in this study. Several methods for the direct reduction of TiO₂ have been proposed to lower the cost of production of Ti. However, none of them have yet been applied industrially. In addition, Y has never been used as a reductant for reducing TiO₂ although its reducing ability is the highest among the reductants capable of reducing TiO₂ to Ti with low oxygen concentration. In this study, the reduction reactions of TiO₂ using Y/Y₂O₃ equilibrium and Y/YOCl/YCl₃ equilibrium, employing various molten salts as solvents, were investigated. TiO₂ pellets and metallic Ti pieces were placed in several types of solvents along with sufficient Y and heated at 1300 K for 86 ks. When YCl₃ or CaCl₂ was used as the solvent, the TiO₂ pellets were reduced to metallic Ti. The oxygen concentrations in the Ti pieces after heating in YCl₃ and CaCl₂ were 90 ± 40 mass ppm O and 350 ± 60 mass ppm O, respectively. However, when NaCl or KCl was used as the solvent, a small amount of metallic Ti and a large amount of complex oxides were obtained. It is considered that the reduction reaction did not proceed sufficiently owing to the low solubility of the oxide ions in molten salts. It was experimentally demonstrated that Ti with an ultra-low oxygen concentration (100 mass ppm O or less) can be directly produced from TiO₂ by using Y as the reductant in an appropriate solvent. This method is expected to lead to the development of a new industrial process for the production of Ti with an ultra-low oxygen concentration directly from its ore.

Co–Cr– and Co–Cr–Mo-based alloys are commercially used in the industry especially for high wear resistance and superior chemical and corrosion performance in hostile environments. These alloys were widely recognized as the important metallic biomaterials. Here, the first development of Co–Cr–Mo–Fe–Mn–W and Co–Cr–Mo–Fe–Mn–W–Ag high-entropy alloys (HEAs) based on Co–Cr–Mo metallic biomaterials is reported. Ingots of six-component Co_2.6Cr_1.2Mo_0.2FeMnW_0.27 (Co_41.5Cr_19.1Mo_3.2Fe₁₆Mn₁₆W_4.3, at%) HEAs with a minor σ phase and of seven-component Co_4.225Cr_1.95Mo_0.2FeMnW_0.2Ag_0.5 (Co_46.6Cr_21.5Mo_2.2Fe₁₁Mn₁₁W_2.2Ag_5.5, at%) and Co_2.6Cr_1.2Mo_0.1FeMnW_0.1Ag_0.18 (Co_42.1Cr_19.4Mo_1.6Fe_16.2Mn_16.2W_1.6Ag_2.9, at%) HEAs without an σ phase were fabricated. The alloy was designed by a taxonomy of HEAs based on the periodic table, a treelike diagram, predicted phase diagrams constructed by Materials Project, and empirical alloy parameters for HEAs. The σ phase formation prevented the formation of solid solutions in Co–Cr–Mo-based HEAs without a Ni element. The σ phase formation in as-cast ingots was discussed based on the composition dependence and valence electron concentration theory.

The present work reports the effect of elemental combination on microstructure and mechanical properties of quaternary refractory medium entropy alloys (RMEAs) having equi-atomic compositions. As-cast RMEAs ((1) HfNbTaTi, (2) HfNbTaZr, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr) were fabricated by vacuum arc-melting of pure elements under Ar atmosphere, homogenization was then performed at 1150°C for 24 hours with Ar atmosphere. Firstly, microstructures of both as-cast and homogenized RMEAs were observed by SEM-BSE. Three kinds of microstructures consisting of annealed grains (AG), granular morphology (GM) and dendritic morphology (DM) were found to be distributing along solidification direction in the as-cast RMEAs. Inter-dendritic segregation in the as-cast RMEAs was characterized by SEM-EDX. At the same time, grain boundary precipitates were found in the as-cast (2) HfNbTaZr and (4) HfTaTiZr alloys. After homogenization at 1150°C, a fraction of AG greatly increased while that of DM largely decreased. It was also found that the degree of segregation was largely reduced after homogenization. In addition, grain boundary precipitates having equiaxed morphology and HCP structure were observed in the homogenized (2) HfNbTaZr alloy. Subsequently, tensile tests of both as-cast and homogenized RMEAs were performed at room temperature (RT) to characterize mechanical properties of the RMEAs. After homogenization, ductility of (1) HfNbTaTi, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr alloys was highly improved while (2) HfNbTaZr alloy still showed early brittle fracture. Better ductility of the homogenized (1) HfNbTaTi, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr alloys could be attributed to the elimination of inter-dendritic segregation as well as grain boundary precipitates through homogenization.

Non-equiatomic high entropy alloys (HEAs) and medium entropy alloys (MEAs) are expected to have the potential to exhibit good mechanical properties due to abundant composition designs compared to equiatomic alloys. It has been reported that an equiatomic CoCrNi MEA shows better strength-ductility balance than CoCrFeMnNi HEA, and there is a possibility that the mechanical properties can be further improved by changing chemical composition. Among the constituent elements, cobalt (Co) has the effect of decreasing stacking fault energy (SFE). In this study, we clarified the effect of Co-content on mechanical properties of non-equiatomic Co–Cr–Ni MEAs with different amounts of Co through investigating deformation behaviors and deformation microstructures. Co_x(CrNi)_(100−x) (x = 20 (Co20), 40 (Co40), 60 (Co60) at%) MEAs were processed to very high plastic strains by high-pressure torsion (HPT) and subsequently annealed under proper conditions to obtain FCC single-phase and uniform fine grain sizes. Mechanical properties of the specimens with fully recrystallized microstructures were characterized by tensile tests at room temperature. Their deformed microstructures at different tensile strain levels were observed by electron microscopy. The result of the tensile tests showed that the work-hardening rate was enhanced with increasing the Co-content although early fracture before reaching plastic instability condition occurred in Co60. Planar slip of dislocations and deformation twinning were observed in Co20 (SFE = 30 mJ/m²), while, in addition to them, deformation-induced martensitic transformation to HCP ε-martensite was observed in Co40 having lower SFE (SFE = 10 mJ/m²), leading to higher work-hardening rate. By increasing Co-content (decreasing SFE) further, phase fraction of ε-martensite greatly increased in Co60 (SFE = 0 mJ/m²) compared with Co40, and early fracture occurred due to stress concentration at intersects between martensite and grain boundaries. The present results suggested that the mechanical properties of the present materials could be effectively designed by controlling the SFE.