| The transition temperature-hydrogen content relationships and microstructural refinement of Ti-6Al-4V alloy have been studied by thermochemical hydrogenation processing. As the hydrogenation temperature increased, the total hydrogen in-diffusion decreased. Also, hydrogen content was independent of the hydrogenation time. In specimens hydrogenated at various temperatures, as the hydrogen content increased, β_T, transition temperature decreased linearly. In the hydrogenation at 700 and 800℃, microstructures showed the fine equiaxed α phase, lamellar of α and β phase and hydrides. However, the microstructure after hydrogenation at 900℃ showed the formation of orthorhombic martensite α″_H and the heterogeneous precipitation of the black plates at prior β grain boundaries. The black plates were a mixture of tetragonal hydride, β and α phase. The broadening of α peaks could be attributed to the build-up of a high stress field, resulting from the formation of fine hydride particles. In the hydrogenation at 700℃ for 8hr, the finest microstructures were obtained. Accordingly, the significant variations in a unique class of microstrucuures of Ti-6Al-4V alloy could be achieved by charging with hydrogen as a temporary alloying element and subsequently dehydrogenating. |
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