Katsushi Tanaka

Temperature dependence of thermal expansion and elastic constants of single crystals of ZrB2 and the suitability of ZrB2 as a substrate for GaN film

Coefficients of thermal expansion (CTE) and elastic constants of single crystals of ZrB2 have been determined in the temperature ranges from room temperature to 1073 K and from room temperature to 1373 K, respectively. The elastic constants of ZrB2 are best characterized by the large value of the Young modulus (as high as 500 GPa) and the small values of the Poisson ratio (0.13–0.15), indicating the high stiffness and hardness and the brittleness, respectively. The values of CTE along the a- and c-axis directions are 6.66×10−6 and 6.93×10−6 K−1, respectively, when averaged over the temperature range from room temperature to 1073 K. The CTE value along the a-axis direction of ZrB2 is only moderately larger than the corresponding value for GaN. This together with the small lattice mismatch along the a-axis direction between ZrB2 and GaN in the heteroepitaxial orientation relationship of (0001)GaN//(0001)ZrB2 and 〈1120〉GaN//〈1120〉ZrB2 indicate that only a small compressive stress develops in the GaN thin-f...

Thermoelectric properties and crystallographic shear structures in titanium oxides of the Magnèli phases

The thermoelectric properties of Magneli phase titanium oxides TinO2n−1 (n=2,3,…) have been investigated, paying special attention to how the thermoelectric performance can be altered by changing the microstructure. Dense polycrystalline specimens with nominal composition of TiO2−x (x=0.05, 0.10, 0.15, and 0.20) prepared by conventional hot-pressing are all identified to be one of the Magneli phases, in which crystallographic shear planes are regularly introduced according to the oxygen deficiency. Electrical conduction is n-type for all specimens and the carrier concentration increases with the increase in the oxygen deficiency. The values of lattice thermal conductivity, on the other hand, decrease with the increase in the oxygen deficiency, which can be attributed to phonon scattering at the crystallographic shear plane. The largest value of thermoelectric figure of merit Z, 1.6×10−4 K−1 was obtained at 773 K for the hot-pressed specimen of TiO1.90.

Elastic constants of Al-based icosahedral quasicrystals

The elastic constants of several Al-based icosahedral alloys were measured over a temperature range from 4K to 1073 K by the rectangular parallelepiped resonance method. The values of the bulk moduli of these quasicrystals are similar to those of conventional cubic crystals having similar melting temperatures, while the values of Youngs and shear moduli are larger in comparison with conventional aluminium alloys. The latter feature is considered to be due to the strong directional bonding in the quasicrystals.

Single-crystal elastic constants of Co3(Al,W) with the L12 structure

Single-crystal elastic constants of Co3(Al,W) with cubic L12 structure have been experimentally measured by resonance ultrasound spectroscopy at liquid helium temperature. The values of all three independent single-crystal elastic constants and polycrystalline elastic constants of Co3(Al,W) experimentally determined are 15%–25% larger than those of Ni3(Al,Ta) but are considerably smaller than previously calculated values of the constants. When judged from the values of Poisson’s ratio, Cauchy pressure, and Gh∕Bh, the ductility of Co3(Al,W) is expected to be sufficiently high so that Co3(Al,W) can be practically used as the constituent phase of “Co-base superalloys.”

Rafting mechanism for Ni-base superalloy under external stress: elastic or elastic–plastic phenomena?

Abstract Rafting mechanism in Ni-base single-crystal superalloys has been discussed with the total mechanical energy calculated for typical microstructures. We found that the actual rafting phenomena cannot be explained within the coherent elastic regime. The present calculations reveal that (i) only the transverse rafted structure with laminates normal to the stress direction can be realized, regardless of tensile or compressive stresses, and (ii) the lattice misfit is not relevant to the choice of the rafted structures. However, when the eigenstrain of the spherical (dilatational) symmetry changes into that of the tetragonal symmetry with misfit dislocations on the γ/γ′ interfaces, the signs of lattice misfit and external stress govern the choice of the transverse or longitudinal rafts. It is concluded that the rafting belongs to an elastic–plastic phenomenon.

Creep deformation of single crystals of new Co–Al–W-based alloys with fcc/L12two-phase microstructures

The creep deformation behaviour of single crystals of Co–Al–W-based alloys with γ + γ′ two-phase microstructures has been investigated in tension under a constant stress of 137 MPa in air at 1000°C as a function of the γ′ solvus temperature and the volume fraction of the γ′ phase. When described by the creep strain rate versus time curve, the creep deformation of Co–Al–W-based alloys consists of transition and accelerating regions without a steady-state region, as observed in many modern nickel-based alloys. However, the creep strength of the present Co–Al–W-based alloys is comparable with nickel-based superalloys of the first generation but is much weaker than those of the second and higher generations. Unlike in nickel-based superalloys, the so-called p (parallel)-type raft structure, in which the γ′ phase is elongated along the tensile axis direction, is formed during creep in Co–Al–W-based alloys, being consistent with what is expected from the positive values of lattice misfit between the γ and γ′ phases. As a result, of the alloys investigated, the best creep properties are obtained with the alloy possessing the highest volume fraction (85%) of the γ′ phase, which is far larger than usual for nickel-based superalloys (55–60%).

Elastic and anelastic behavior of Zr55Al10Ni5Cu30 bulk metallic glass around the glass transition temperature under ultrasonic excitation

Elastic and anelastic properties of Zr55Al10Ni5Cu30 bulk glass have been investigated at high temperatures with the electromagnetic acoustic resonance technique within the frequency range of 300–1500 kHz. The elastic constants of ascast sample decrease monotonically up to the glass transition temperature Tg and jump up just above Tg, but such jumps disappear in the cooling and subsequent heating processes. This indicates that an irreversible structural stabilization occurs at Tg under ultrasound vibration. As for ultrasonic attenuation, prominent peaks appear at Tg in the first heating process. These attenuation peaks can be attributed to the atom movements in the initial glassy state. 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Size effect, critical resolved shear stress, stacking fault energy, and solid solution strengthening in the CrMnFeCoNi high-entropy alloy

High-entropy alloys (HEAs) comprise a novel class of scientifically and technologically interesting materials. Among these, equatomic CrMnFeCoNi with the face-centered cubic (FCC) structure is noteworthy because its ductility and strength increase with decreasing temperature while maintaining outstanding fracture toughness at cryogenic temperatures. Here we report for the first time by single-crystal micropillar compression that its bulk room temperature critical resolved shear stress (CRSS) is ~33–43 MPa, ~10 times higher than that of pure nickel. CRSS depends on pillar size with an inverse power-law scaling exponent of –0.63 independent of orientation. Planar ½ < 110 > {111} dislocations dissociate into Shockley partials whose separations range from ~3.5–4.5 nm near the screw orientation to ~5–8 nm near the edge, yielding a stacking fault energy of 30 ± 5 mJ/m2. Dislocations are smoothly curved without any preferred line orientation indicating no significant anisotropy in mobilities of edge and screw segments. The shear-modulus-normalized CRSS of the HEA is not exceptionally high compared to those of certain concentrated binary FCC solid solutions. Its rough magnitude calculated using the Fleischer/Labusch models corresponds to that of a hypothetical binary with the elastic constants of our HEA, solute concentrations of 20–50 at.%, and atomic size misfit of ~4%.

Crystal structure and thermoelectric properties of type-I clathrate compounds in the Ba-Ga-Ge system

The crystal structure and thermoelectric properties of type-I clathrate compounds in the Ba–Ga–Ge system have been investigated as a function of Ga content. The solid solubility of Ga in the type-I clathrate compounds is determined to be X=16 when expressed with the formula of Ba8GaXGe46−X. As the Ga content increases, the crystal structure changes from a superlattice structure to the normal type-I clathrate structure with the transition occurring at X=3.5–5. The density of Ge vacancies in the type-I clathrate phase decreases as the Ga content increases. The absolute values of electrical resistivity and Seebeck coefficient increase, while that of lattice thermal conductivity decreases with the increase in the Ga content. The changes in electrical resistivity and Seebeck coefficient are explained in terms of the number of excess electrons, while the change in lattice thermal conductivity is explained in terms of the extent of the rattling motion of Ba atoms encapsulated in the cage structure.

Effect of In additions on the thermoelectric properties of the type-I clathrate compound Ba8Ga16Ge30

The thermoelectric properties of quaternary type-I clathrate compounds, Ba8Ga16−xInxGe30 (x=0–9), have been investigated as a function of In content and temperature. The substitution of In atoms for Ga atoms leads to a decrease in electrical resistivity, as well as a decrease in thermal conductivity. The decrease in electrical resisitivity is explained in terms of the In occupancy behavior in the 6c sites, whereas the decrease in thermal conductivity in terms of the increased extent of the rattling motion of Ba atoms due to the increased lattice constant. As a result, the value of thermoelectric dimensionless figure of merit (ZT) of Ba8Ga16Ge30 is improved by In substitutions from 0.49 to 1.03 at 670°C when x=6.