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A new route to achieve high strength and high ductility compositions in the Cr-Co-Ni medium entropy alloys (MEAs) is proposed, by controlling the solid solution hardening parameter (Mean Square Atomic Displacement, MSAD) and twinning propensity parameter (Stacking Fault Energy, SFE), respectively. The MSAD is calculated to increase with the increase in the Cr content and with the increase in the Ni/Co ratio at high Cr concentrations, while the SFE is calculated to decrease with the increase in the Cr content and with the increase in the Co/Ni ratio at high Cr concentrations. In experiment, the strength at 0 K (derived from the temperature dependence of yield stress) increases as the Cr content increases and/or as the Ni content increases for a given high Cr content, so that a linear correlation is found between the yield strength at 0 K and MSAD. The SFE also decreases as the Cr content increases and as the Co content increases for a given high Cr content. However, while the tensile elongation increases with the decrease in SFE down to SFE values of 10–12 mJ/m2, it abruptly decreases once the SFE decreases below this value due to a change in major deformation mode from deformation twinning to deformation-induced ε-martensite transformation. Based on the established connection between the theoretical calculation and experimental measurement, outstanding combinations of strength and ductility are predicted and experimentally confirmed at high Cr compositions and at a bit Ni-rich side of the Co/Ni equi-composition line. The proposed composition (around 45Cr-20Co-35Ni) exhibits a greater 0 K strength and a superior 77 K tensile ductility by 32 % and 13 %, respectively, compared to those of the equiatomic Cr-Co-Ni alloy., 新規高強度高延性合金の開発に成功 --計算と実験の融合による合金設計法の確立--. 京都大学プレスリリース. 2023-05-25.

The effects of Ni and Ta additions on the mechanical properties in the L1$_{2}$ compound Co$_{3}$(Al,W), the strengthening phase of Co-based superalloys, have been investigated by compression tests between room temperature and 1000 °C, in order to elucidate the effects of stability of the L1$_{2}$ phase on the mechanical properties. The additions of Ni and Ta, both of which are L1$_{2}$-stabilizers that increase the L1$_{2}$ solvus temperature, increase the yield strength at intermediate and high temperatures. The strength increase is shown to be more significant as the amount of additions of these elements and thereby the stability of the L1$_{2}$ phase increases. Two factors account for the strength increase at intermediate temperatures: The reduction of the onset temperature of yield stress anomaly (YSA-onset) due to the increased complex stacking fault (CSF) energy and the increase in both the base strength and the intensity of the yield stress anomaly associated with an increased anti-phase boundary (APB) energy on (111) planes. The strength increase at high temperatures, on the other hand, arises from the increase in the peak temperature due to the increased L1$_{2}$ solvus temperatures. The increased strength of the L1$_{2}$ phase due to a higher phase stability thus partly accounts for the improved creep strength of Co-based superalloys upon alloying with Ni and Ta.

Phase relationships of manganese-substituted ruthenium sesquisilicide alloys have been investigated by using X-ray powder diffraction and scanning and transmission electron microscopy. A series of chimney–ladder phases Ru1−xMnxSiy (0.14 less-than-or-equals, slant x less-than-or-equals, slant 0.97, 1.584 less-than-or-equals, slant y less-than-or-equals, slant 1.741) are formed over a wide compositional range between Ru2Si3 and Mn4Si7. The compositions of these chimney–ladder compounds deviate slightly from the composition line connecting Ru2Si3 and Mn4Si7, which corresponds to the ideal composition line satisfying VEC (valence electron counting) = 14 rule. The occurrence of this compositional deviation is discussed in terms of the VEC rule and the atomic packing. The thermoelectric properties of the directionally solidified Ru1−xMnxSiy alloys (0.55 less-than-or-equals, slant x less-than-or-equals, slant 0.90) have also been investigated as a function of the Mn content and temperature. The dimensionless figure of merit (ZT) for those alloys with a high Mn content (x greater-or-equal, slanted 0.75) increased with the increase in Mn content. The ZT value for a crystal with x = 0.90 was as high as 0.76 at 874 K.

Optimum conditions for laser irradiation to achieve fully dense high carbon steel SLS (selective laser sintering) specimens have been investigated as a function of carbon content in steel powders with the use of steel powders with different carbon contents in the range of 0.33–1.05 mass%C (corresponding to S33C, S50C, S75C and S105C in the JIS standard). Full densification is found to be easily achieved by SLS processing for all high carbon steel powders. The energy density during the SLS process required for full densification decreases as the carbon content increases from 400 J/mm3 for 0.33 and 0.50 mass%C to 267 J/mm3 for 0.75 and 1.05 mass%C. This is considered to be due to the increased wettability of molten Fe–C alloys for the higher carbon contents. The values of microhardness and yield stress of fully dense SLS specimens tend to increase as the carbon content in steel powders increases. At a given carbon content, the values of microhardness and yield stress of fully dense SLS specimens tend to be higher for those produced with a lower energy input (with higher laser scan speeds and larger scan spacings).

The evolution of orientation distributions of γ and γ′ phases in crept Ni-base single crystal superalloys have been investigated by theoretical calculations with elastic–plastic models and by experiments. As creep deformation proceeds, the crystallographic orientation distributions for both phases are broadened as a result of the waving of the raft structure, which occurs to reduce the total mechanical energy. The broadening of the orientation distribution occurs in such a way that the 0 0 1 pole broadens isotropically while the h k 0 poles broaden preferentially along the left angle bracket0 0 1right-pointing angle bracket directions. Since the extent of the broadening increases almost linearly with the number of creep deformation, the measurement of the broadening by X-ray diffraction can be utilized in non-destructive methods to predict the lifetime of Ni-base superalloys.

We have grown TiAl crystals of a nearly stoichiometric composition composed of a single lamellar grain as well as those composed of two lamellar grains and studied their deformation behaviour at room temperature. The deformation behaviour of lamellar grains depends strongly on the lamellar orientation. Because of such an anisotropy in the deformation behaviour of lamellar grains with respect to the lamellar orientation, a large strain discontinuity and therefore crack initiation may occur near boundaries between coarse lamellar grains. This is suggested to be a reason for low ductility of TiAl with a structure composed of coarse lamellar grains.

The lamellar structure of Ti-rich TiAl has been investigated on the atomistic scale by high-resolution transmisison electron microscopy (HREM). To analyze the HREM images of the lamellar boundary, computer simulation of HREM images has been fully utilized. The formation mechanism of the lamellar structure is briefly discussed in the light of the results of the present investigation.