13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 5 Changes in the average hardness, hardness of α phase and prior β phase with the duplex heat treatment. Figure 6 Changes in the mechanical properties with the duplex heat treatment. 3.3. Fatigue strength Figure 7 shows the S-N curves of all materials, and Fig. 8 shows the change in the fatigue strength by STQ and STA treatments. Figure 8 includes the change in the ratio of the fatigue strength and the tensile strength, σW/σTS. Figure 9 shows the fatigue fracture surfaces of STQ material and STA material (aging temperature: 803 K). The fatigue strength of Ti-6Al-4V alloy greatly increased with STQ treatment (Fig. 8). The maximum improvement in the fatigue strength was achieved by STA treatment (aging temperature: 803 K). As shown in Fig. 9, facets were observed at crack initiations sites of both materials, and their sizes were almost the same to α grain size. Moreover, no non-propagating crack was found on the test sections of the specimens which did not fracture until fatigue cycles of 107. The above results meant that the fatigue strength was the maximum stress amplitude at which fatigue cracks did not initiate from α grain. The difference in the fatigue strength between STQ and STA materials resulted from the difference in the characteristics of prior β phase surrounding α phase. In the case of STA material, since the hardness of prior β phase was markedly higher than α phase, this region suppressed extension of slippages induced in α grain. In consequence, initiation of fatigue cracks was strongly restricted and the fatigue strength was improved. On the other hand, STQ material also showed high fatigue strength although the hardness of prior β phase was lower than that in STA material. This will result from high ductility of prior β phase. That is, prior β phase in STQ material included much metastable β phase. Even if slippages in α phase generated a strain field in prior β phase, such strain field can be effectively cancelled through its strain-induced transformation. Consequently, the fatigue strength was improved by STQ treatment. As mentioned above, the improvement of the fatigue strength by the short-time treatments will be closely related to the two factors: the hardness of prior β phase and its ability for strain-induced transformation. Finally, Fig. 10 summarized the tensile and fatigue strengths of the materials examined in this study. In addition, the figure includes the reference data [9]. As understood in this figure, STQ treatment effectively increased the fatigue strength although the improvement rate of the tensile
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