13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- other bright regions are equiaxed α phase. In STQ material (Fig. 2 (b)), isolated regions are α phase and the regions surrounding α phase are prior β phase. As seen in Fig. 2 (b), the volume fraction of prior β phase increased with STQ treatment [5]. At the same time, a part of prior β phase was transformed to acicular α' martensite (Fig. 3(a)); however, the result of the electron diffraction showed the existence of metastable β phase. This point is important to think about the ductility of STQ material because strain-induced transformation can arise in metastable β phase. Although the volume fraction of prior β phase was unchanged with the subsequent short-time aging, the inside of prior β phase became dark with increasing the treatment temperature (Fig. 2 (c)). This change in etching degree meant that metastable β phase was decomposed into fine α phase and stable β phase. The change in microstructure with the duplex treatment was summarized as follows (Fig. 4): the untreated material was composed of α phase and stable β phase. Since the volume fraction of prior β phase increased with the short-time solution treatment, concentration of vanadium which is a β stabilizer decreased in prior β phase. As a result, acicular α' phase was generated in prior β phase through quenching. The short-time aging decomposed a part of remained metastable β phase into fine α phase and stable β phase during. The decomposition was accelerated by increasing the aging temperature. 3.2. Hardness and mechanical properties Figure 5 shows the changes in the average hardness, hardness of α phase and prior β phase with the duplex heat treatment. Figure 6 shows the changes in the mechanical properties. As shown in Fig. 5, the hardness of prior β phase was significantly increased by STQ treatment due to the formation of α' phase, although the hardness of α phase was almost unchanged. As a result, the average hardness also increased. The subsequent short-time aging generated fine α phase, so that the hardness of prior β phase further increased and it reached the maximum value at 803 K. Figure 2 Microstructures optically observed after etching.
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