酸素固溶強化Ti-6Al-4V焼結合金の高延性発現機構の解明

抄録

The previous study indicated powder metallurgy (PM) Ti-64 alloys with oxygen showed the increment of not only their tensile strength but elongation. This study investigated the elongation improvement mechanism of Ti-64 alloys with oxygen atoms. The elemental mixture of Ti-64 alloy powder and TiO₂ particles (0~0.4 mass%) was used as starting materials, and consolidated by spark plasma sintering (SPS). The following heat treatment in vacuum was applied to sintered materials. β transus temperature increased by oxygen addition because it was one of α-phase stabilizer elements. Prior-β grains size and aspect ratio of α-Ti grains were changed by heat treatment conditions. For example, Ti-64 + O alloys after heat-treated at β-phase temperature range showed needle-like α-Ti grains with a large aspect ratio (6.1~7.0) although those with heat treatment at α + β-phase temperature had α-grains with a small aspect ratio of 3.3~4.0. These grain morphology changes strongly depended on the heat temperature, not oxygen contents. In addition, the latter materials indicated high elongation (16~17%) compared to the former with 9~10%. When Ti-64 + O alloy specimens after tensile test were analyzed by SEM-EBSD, Kernel average misorientation (KAM) maps showed many plastic strains induced in small aspect ratio α-Ti grains.