Kyosuke Kishida

Gamma Titanium Aluminide Alloys

Extensive progress and improvements have been made in the science and technology of gamma titanium aluminide alloys within the last decade. In particular, the understanding of their microstructural characteristics and property/microstructure relationships has been substantially deepened. Based on these achievements, various engineering two-phase gamma alloys have been developed and their mechanical and chemical properties have been assessed. Aircraft and automotive industries arc pursuing their introduction for various structural components. At the same time, recent basic studies on the mechanical properties of two-phase gamma alloys, in particular with a controlled lamellar structure have provided a considerable amount of fundamental information on the deformation and fracture mechanisms of the two-phase gamma alloys. The results of such basic studies are incorporated in the recent alloy and microstructure design of two-phase gamma alloys. In this paper, such recent advances in the research and development of the two-phase gamma alloys and industrial involvement are summarized.

Temperature dependence of yield stress, tensile elongation and deformation structures in polysynthetically twinned crystals of Ti-Al

Abstract The plastic deformation behaviour of polysynthetically twinned (PST) Ti-Al with three different orientations has been studied in tension and compression as a function of temperature in a range from −196 to 1100°C. With increase in temperature, the yield stress decreases rather rapidly at low temperatures and then decreases gradually at intermediate temperatures for all orientations studied although the temperature dependence at low temperatures is less significant for an orientation where shear deformation occurs parallel to the lamellar boundaries. When the loading axis is perpendicular to the lamellar boundaries, the yield stress again rapidly decreases with increasing temperature at high temperatures. This is also the case for PST Ti-Al whose lamellar boundaries are parallel to the loading axis, although a small and broad anomalous yield stress peak is observed at 800°C for this orientation. When the lamellar boundaries are inclined at an intermediate angle to the loading axis, the yield stres...

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.”

Deformation and fracture of PST crystals and directionally solidified ingots of TiAl-based alloys

Abstract The results of recent studies on the deformation and fracture properties of PST crystals, the directional solidification (DS) and microstructural control and the mechanical properties of DS ingots of TiAl-based alloys are summarized.

Plastic deformation of polycrystals of Co

The plastic behaviour of Co3(Al,W) polycrystals with the L12 structure has been investigated in compression from 77 to 1273 K. The yield stress exhibits a rapid decrease at low temperatures (up to room temperature) followed by a plateau (up to 950 K), then it increases anomalously with temperature in a narrow temperature range between 950 and 1100 K, followed again by a rapid decrease at high temperatures. Slip is observed to occur exclusively on {111} planes at all temperatures investigated. The rapid decrease in yield stress observed at low temperatures is ascribed to a thermal component of solid-solution hardening that occurs during the motion of APB-coupled dislocations whose core adopts a planar, glissile structure. The anomalous increase in yield stress is consistent with the thermally activated cross-slip of APB-coupled dislocations from (111) to (010), as for many other L12 compounds. Similarities and differences in the deformation behaviour and operating mechanisms among Co3(Al,W) and other L12 compounds, such as Ni3Al and Co3Ti, are discussed.

Fabrication of Ni3Al thin foil by cold-rolling

Abstract Thin foils of stoichiometric Ni 3 Al with thicknesses ranging from 57 to 315 μm were successfully fabricated by heavily cold-rolling without intermediate annealing. Starting materials were produced by directional solidification using the floating zone method. The total reduction in thickness obtained was as much as 95.5%. This high rolling ductility is considered to be due to the monocrystalline or near monocrystalline form of the starting materials. X-ray pole figures showed the formation of {110} rolling texture. This {110} texture is considered to develop mainly as a result of compressive deformation normal to the rolling plane. The foils recrystallized at temperatures over 1273 K had some tensile ductility (3.0–14.6%) at room temperature in air, in contrast to the usual brittleness of polycrystalline Ni 3 Al. Electron back scatter diffraction measurements revealed that low angle and Σ3 coincidence site lattice boundaries, which are considered to be crack-resistant, comprised 41–84% of the total grain boundary area in the recrystallized foils. This large fraction is probably a chief cause of the observed ductility. These results demonstrate that it may be possible to utilize Ni 3 Al thin foils as lightweight, high-temperature structural materials, e.g. honeycomb structures.

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.

Improvement of Grain-Boundary Conductivity of Trivalent Cation-Doped Barium Zirconate Sintered at 1600°C by Co-doping Scandium and Yttrium

Scandium and yttrium co-doped barium zirconate [BaZr 0.85 Sc x Y 0.15-x O 3-δ (x = 0, 0.05, 0.075, 0.10, 0.15)] have been investigated in terms of phase relationship, microstructures, and electrical conductivity. The bulk conductivity of the scandium and yttrium co-doped barium zirconate increased with the dopant ratio of yttria. BaZr 0.855 Sc 0.05 Y 0.10 O 3-δ had the highest grain-boundary conductivity among the scandium and yttrium co-doped barium zirconates in this study. But, BaZr 0.85 Sc 0.15 O 3-δ BaZr 0.85 Sc 0.10 Y 0.05 Ο 3-δ , BaZr 0.85 S 0.75 Y 0.075 O 3-δ , and BaZr 0.85 Sc 0.05 Y 0.10 Ο 3-δ consisted of a single cubic perovskite phase at 1600°C and their densities of grain-boundary were smaller than that of BaZr 0.85 Y 0.15 Ο 3-δ . From the observation of microstructure and results of grain boundary-conductivity measurement, we can say that yttrium is a dopant that increases specific grain-boundary conductivity and bulk conductivity, and scandium is a dopant that increases the grain size. Thus, there is a trade-off relation between the grain size and specific grain-boundary conductivity based on the mixing ratio of scandia to yttria. The total conductivity of BaZr 0.85 Sc 0.05 Y 0.10 O 3-δ at 600°C was estimated to be 1.6 X 10 -2 S cm -1 , which is the highest-class conductivity among reported trivalent cation-doped barium zirconates.

Recent progress in our understanding of deformation and fracture of two-phase and single-phase TiAl alloys

Extensive work has been carried out on the microstructural characteristics and property/microstructure relationships of two-phase TiAl alloys in the last decade. Based on these achievements, various engineering two-phase TiAl alloys have been developed and industries are seeking to accomplish their introduction for various structural components. This article reviews recent progress in understanding the deformation and fracture not only of such two-phase TiAl alloys but also single-phase TiAl. The single-phase TiAl is very brittle and exhibits mechanical properties which are considerably different from those of the TiAl phase in the two-phase alloys. However, clarifying the difference in mechanical properties between two-phase and single-phase TiAl alloys and its reasons is indispensable to deepen our understanding of the deformation mechanisms of the TiAl phase.