Papers by Author: Katsushi Tanaka

Abstract: The effect of elastic driving force on the microstructural change of superalloys in the secondary creep stage is evaluated by elastic energy calculations with the concept of effective eigen strain where both lattice mismatch and creep strain are taken into account The elastic energy calculations indicates that the elastic state in the secondary creep stage is totally different to that in the initial one where the lattice misfit between γ and γ' phases is over accommodated along the [100] and [010] directions by creep deformation in the γ phase. The excess creep dislocations for the over accommodation are required so as to develop an internal stress field to prevent further creep deformations. The planer raft structure with the plane normal oriented to the [001] direction is unstable in the over accommodated state. The γ/γ' lamellar interfaces will be inclined to make a wavy raft structure of which elastic energy is lower than the ideal 001 planer raft structure.

Abstract: The mechanical properties of Co3(Al,W) with the L12 structure have been investigated both in single and polycrystalline forms. The values of all the three independent single-crystal elastic constants and polycrystalline elastic constants of Co3(Al,W) experimentally determined by resonance ultrasound spectroscopy at liquid helium temperature are 15~25% larger than those of Ni3(Al,Ta) but are considerably smaller than those previously calculated. When judged from the values of Poisson’s ratio, Cauchy pressure and ratio of shear modulus to bulk modulus (Gh/Bh), the ductility of Co3(Al,W) is expected to be sufficiently high. In the yield stress-temperature curve, a rapid decrease and an anomalous increase in yield stress is observed in the low and intermediate (1000-1100 K) temperature ranges, respectively. The former is concluded to be due to the solid-solution hardening effect while the latter is attributed to thermally activated cross-slip of APB-coupled a/2<110> superpartial dislocations from octahedral to cube slip planes.

Abstract: The physical and mechanical properties of Co3(Al,W) with the L12 structure have been investigated both in single and polycrystalline forms. The values of all the three independent single-crystal elastic constants and polycrystalline elastic constants of Co3(Al,W) experimentally determined by resonance ultrasound spectroscopy at liquid helium temperature are 15~25% larger than those of Ni3(Al,Ta) but are considerably smaller than those previously calculated. When judged from the values of Poisson’s ratio, Cauchy pressure and Gh (shear modulus)/Bh (bulk modulus), the ductility of Co3(Al,W) is expected to be sufficiently high. Indeed, the value of tensile elongation obtained in air is as large as 28 %, which is far larger than that obtained in Ni3Al polycrystals under similar conditions.

Abstract: The values of all the three independent single-crystal elastic constants and polycrystalline elastic constants of Co3(Al,W) experimentally determined by resonance ultrasound spectroscopy at liquid helium temperature are 15~25% larger than those of Ni3(Al,Ta) but are considerably smaller than those previously calculated. Because of the large value of E111/E100 and cij of Co3(Al,W), two-phase microstructures with cuboidal L12 precipitates well aligned parallel to <100> and well faceted parallel to {100} are expected to form very easily in Co-base alloys, as confirmed indeed by experiment. Values of yield stress obtained for [001]-oriented L12/fcc two-phase single crystals moderately decrease with the increase in temperature up to 800°C and then decrease rapidly with temperature above 800°C without any anomaly in yield stress.

Abstract: Manganese-alloyed Ru2Si3-based alloys with various alloy compositions have been prepared and the phase relationships of these alloys have been investigated as a function of the Mn concentration using X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. A series of Ru1-xMnxSiy chimney-ladder phases is confirmed to be formed over a wide compositional range (x ≥0.12). These chimney-ladder phases are considered to be formed to stabilize the HT-Ru2Si3 chimney-ladder phase through the substitution of Ru with Mn. The compositions of the chimney-ladder phases are, however, significantly deviated from the idealized composition satisfying the valence electron concentration rule: VEC=14.

Abstract: Electrochemical properties and microstructure evolution during lithiation and delithiation of Ni3Sn2 single crystals with the hexagonal B82-type related ordered structure have been studied as a function of the crystal orientation by charge/discharge tests, powder X-ray diffraction and transmission electron microscopy. Charge and discharge capacities are strongly dependent on the crystal orientation. When the surface of the thin plate anode is perpendicular to the basal plane of the parent B82 unit cell, the capacities are approximately twice as high as those for the sample whose surface is parallel to the basal plane, which is considered to be related to the difference in the diffusion behavior of lithium with respect to the crystal orientation.

Abstract: The temperature dependence of single-crystal elastic constants of L10-ordered single-crystals of FePd . A complete set of elastic constants has been determined with the resonance ultrasound spectroscopy technique. The compounds clearly show a tetragonal elastic anisotropy, c11 < c33 and c44 < c66. The temperature dependencies of the anisotropies are not simply explained by the variation of axial ratio (c/a) of the crystal.

Abstract: The effect of hydrostatic pressure mechanically applied to hydride on the equilibrium hydrogen gas pressure and temperature have been examined theoretically. From free energy calculations where elastic energy is taken into account, equilibrium gas pressure and temperature increases and decreases with increasing applied compressive stress, respectively. In the case of magnesium hydride, equilibrium temperature decreases to 63 °C at the hydrogen gas pressure being 1 atom when volume expansion at hydriding is perfectly suppressed by an external compressive stress. The temperature is remarkably lower than that obtained by alloying various elements.

Abstract: The crystallographic orientation distribution, and its change as a function of creep deformation in Ni-based single crystal superalloys have been investigated by X-ray diffractometry. The distribution of the crystallographic orientation has significantly broadened by creep deformations. Directional broadening of the distribution agrees with creep dislocations having the burgers vector of 1/2<101>. High temperature creep strain of superalloys can be estimated by a non-destractive test where the width of rocking curve of a diffraction peak is measured.

Abstract: Effect of the ternary element of Mo on the crystal structure and thermoelectric properties of ReSi1.75 has been investigated. The crystal structure of Mo-containing ReSi1.75 has shear structure up to 1.5 at%, the structure changes into adaptive structure at larger Mo contents. The concentration of Si vacancies estimated from the crystal structures determined decreases with increasing Mo content. Thermoelectric properties of Mo-containing ReSi1.75 indicate that the character changes from p- to n-type semiconductor upon alloying with Mo, which is explained by the decrease in Si vacancy concentration.