13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- strength was relatively low; however, the subsequent short-time aging conducted at 803 K showed the great improvement of both strengths, and the reached level was markedly higher than the reference data. Figure 7 S-N curves. Figure 8 Change in the fatigue strength and the change in the ratio of the fatigue strength and tensile strength, σW/σTS. Figure 9 Fatigue fracture surfaces of STQ material and STA material (aging temperature: 803 K). Figure 10 Changes in the tensile strength and the fatigue strength of Ti-6Al-4V alloy, including reference data [9]. 4. Conclusions (1) The short-time solution treatment transformed a part of prior β phase to acicular α' martensite phase. The subsequent short-time aging precipitated fine α phase in metastable β phase. These changes in microstructure increased the hardness of prior β phase. (2) The short-time duplex heat treatment greatly increased the tensile strength and fatigue strength
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