Pressure-composition isotherms and reversible hydrogen-induced phase transformations in Ti-6Al-4V

Reversible stepwise phase transformations induced by the addition and removal of hydrogen in pure Ti and Ti-6Al-4V (Ti64) were examined by means of pressure-composition (P-C) isotherm measurement from 550 to 700 °C. Both Ti-H and Ti64-H systems exhibit two pressure plateaus in the P-C isotherms which indicate phase transformations. Phase transformations take place at lower hydrogen content and higher hydrogen pressure in Ti64 than in Ti due to the effect of substitutional elements Al and V. Upon hydrogenation, the Ti-H system exhibits the sequence of phase transformation α ↔ αH ↔ αH + βH ↔ βH ↔ βH + δ ↔ δ, whereas the Ti64-H system shows a sequence of α + β ↔ αH + βH ↔ βH ↔ βH + δ ↔ δ because of the presence of the original β phase. Partial phase diagrams for Ti-H and Ti64-H were established based on their P-C isotherms. Modification of microstructure of Ti64 occurs after cyclic hydrogenation. This can be ascribed to hydrogen-induced atomic migration during the sequential phase transformations.