13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- Hereafter, this heat-treated material is called "STQ material". In the short-time aging, STQ material was kept at 753, 803, 853 and 903 K for 40 s and air-cooled. They are called "STA materials". For comparison, the untreated material was also prepared. The test sections of the button specimens (Fig.1 (a)) were polished to mirror surfaces with emery papers and alumina powder. The button specimens were used in the observation of the microstructure and hardness measurement. The test sections of the tensile and fatigue specimens (Fig. 1 (b), (c)) were polished with emery papers and electro-polished to mirror surfaces. For TEM observation, thin small disks (diameter: 3 mm, thickness: 50 µm) were prepared from STQ and STA materials. The observation regions were polished with alumina powder and finally thinned by ion milling. Electron diffraction patterns of STQ material were obtained at the same positions where the microstructure was observed. In the hardness measurement, a super micro-Vickers hardness tester with a CCD camera was used. The average hardness was measured under the test force of 2.94 N, and the hardness of α phase and prior β phase was measured under the test force of 19.6 mN. The tensile test was conducted in air at room temperature. The plane-bending fatigue test was performed under the conditions of frequency of 33 Hz and stress ratio R=-1 in air at room temperature. After the fatigue test, crack initiation sites were observed on the fracture surfaces by SEM (scanning electron microscopy). Table 1 Chemical composition of Ti-6Al-4V alloy used in this study (mass %) Al V C N O Fe H Ti 6.18 4.20 0.001 0.01 0.17 0.17 0.0004 Bal. Figure 1 Specimen shapes (mm): (a) button specimen; (b) tensile specimen; (c) fatigue specimen. 3. Results and Discussion 3.1. Microstructure Figure 2 shows the microstructures of the untreated, STQ and STA materials, and Fig. 3 shows the microstructures of prior β phase observed by TEM. Figure 3 includes the electron diffraction profiles of STQ material. Figure 4 shows a schematic illustration to explain the change in microstructure with the duplex heat treatment. In the untreated material (Fig. 2(a)), a lot of small dark points correspond to β phase, and the
RkJQdWJsaXNoZXIy MjM0NDE=