放電プラズマ焼結法によって作製したTi-Alメカニカルミリング粉末反応焼結鍛造体の硬さに及ぼす熱間等方圧加圧加工の影響

抄録

Mechanically milled powders of mixed powders of Ti and Al (Ti-48 mol%Al) was reactively sintered under relatively low temperatures (773 or 1073 K) by a spark plasma sintering (SPS) machine equipped with a servo press, and then the reactively sintered compacts were subjected to die forging by the SPS machine at a constant strain rate (2×10^-3 s^-1) and a temperature of 1073 K. Thereafter, the reactively sintered and forged compacts were subjected to hot isostatic pressing (HIP) at a holding temperature of 1523 K, a holding pressure of 150 MPa and a holding time of 1 h, and the influence of HIP treatment on properties of the compacts was investigated and compared with the case of high strain rate condition (1×10^-2 s^-1). In both cases, the synthesis reaction was almost ended at the stage of die forging, the hardness before HIP treatment was very hard at 800-900 HV but after HIP treatment it was greatly decreased to around 400-500 HV, and the density (3.77-3.85 Mg/m³) was close to the true density of TiAl (3.81 Mg/m³), so that the density was nearly true one. Reactively sintered and forged compacts before HIP treatment were better deformed than in the case of high strain rate conditions and macroscopic defects such as cracks were hardly found except for the case of pre-sintering temperature of 773 K. This material processing is considered to brings about a dense, high hardness sintered compact in which synthesis reaction is almost achieved at the stage of completion of forging and the forgeability becomes extremely good if the strain rate condition is optimized.