Papers by Author: Ryota Kobayashi

Abstract: Deformation behavior of Fe-15Al-18Co-3Ti (at.%) single crystals containing the Co₂AlTi precipitates was examined. In the single crystals furnace-cooled (FC) from 1373 K to room temperature, coarse Co₂AlTi phase with the L2₁ structure was precipitated in the bcc matrix. The L2₁ phase showed a cuboidal shape with a misfit strain of 0.59%. It is also noted that large amount of Fe substituted for Co in the Co₂AlTi precipitates. The FC single crystals exhibited high yield stress above 600 MPa up to 823 K while further increase in temperature resulted in a decrease in yield stress. In the FC crystals, 1/2<111> dislocations in the bcc matrix bypassed the coarse L2₁ precipitates due to their large misfit strain, resulting in high strength. In contrast, the fine L2₁ precipitates about 30 nm in diameter were observed in the crystals after solutionization and annealing at 823 K. The crystals with the fine L2₁ precipitates demonstrated high yield stress above 1400 MPa at room temperature. Paired 1/2<111> dislocations cut the fine L2₁ precipitates, which led to high strength. The dependence of the yield stress on the precipitate size was also discussed.

Abstract: AlN–SiC solid solutions with p-type electrical conduction were fabricated with the addition of small amounts of Al and C. Powder mixtures of AlN and SiC with small amounts of Al and C (below 10 mol%) were consolidated by spark plasma sintering (SPS) at 2000°C for 10 min under 1 atm Ar, and then heat-treated at 2200°C for 3 h in an Ar flow to afford 2H AlN–SiC solid solutions. The relative densities of the 50AlN-50SiC-Al4C3 (A50-1AC) and 50AlN-50SiC-3Al4C3 (A50-3AC) samples were about 95%, whereas that of the 75AlN-25SiC-Al4C3 (A75-1AC) sample was about 86%. X-ray diffractometry (XRD) analysis showed that the samples comprised only the 2H phase, and except in the case of the A50-3AC sample, no diffraction peaks of Al and C were observed. Although the samples without the additives (Al and C) were electrical insulators, addition of Al and C introduced p-type semiconduction. The electrical conductivities at 300°C of the A50-1AC and A50-3AC samples were about 30 and 100 S/m, respectively, whereas that of the A75-1AC sample was about 10–1 S/m. It was found that addition of Al and C brought about electrical conduction in AlN–SiC solid solutions.