| Summary: | 碩士 === 元智大學 === 機械工程學系 === 99 === This research is aimed to study the grain refinement and mechanical properties of Ti-6Al-4V (Ti64) alloy, which is α (HCP) + β (BCC) type titanium alloy, with isothermal hydrogenation treatment. Two kinds of initial microstructural Ti64 specimens, lamellar and equiaxed, will be studied. The specimens are treated with different phase transformation process such as α + β ↔ αH + βH (0.1 H/M), α + β ↔ βH (0.3, 0.5, 0.6 H/M) and α + β ↔ βH + δ (FCC)(0.7, 0.9 H/M) at the temperature 600 ℃. X-ray diffraction analysis showed that there were no residual metal hydrides in Ti64 after hydrogenation and dehydrogenation treatment. Grain refinement occurs on the lamellar and equiaxed Ti64, which caused the full width at half maximum (FWHM) of α phase peak broadening. Metallographic observations showed that the major microstructural feature of lamellar and equiaxed is preserved, after hydrogenation treatment. These results are helpful to maintain their original mechanical properties. According to scanning electron microscopy (SEM) observations, the refined plate-like nanostructures within the α matrix is seen in both structures. The thickness of the refined α grains is approximately 50-100 nm. However, lamellar and equiaxed Ti64 specimens, which are treated with α + β ↔ βH + δ (0.7,0.9 H/M) phase transformation, becomes embrittlemen due to the occurrence of the heavy strain associated with titanium hydrides (δ) during hydrogenation. The width of the crack size is nanometer and micrometer for 0.7 and 0.9 H/M, respectively. According to the Hall-Petch equation, formation of the nanostructures helps to increase the hardness of the Ti64 alloys. In the optimum hydrogen loading around 0.6 H/M, the hardness of the lamellar and equiaxed specimens were enhanced significantly about 28 %. However, over hydrogen loading of 0.7 H/M, the hardness of specimens were decreased (embrittlemen) because the δ hydride has been once formed.
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