Microstructural features of thermochemical processing in a Ti-6Al-4V alloy

Thermochemical processing (TCP) with hydrogen acting as a temporary alloying element to refine the microstructure of a Ti-6Al-4V alloy has been evaluated in order to examine the microstructural variations. The processing includes a hydrogenation treatment, an elevated-temperature beta solution treatment, a moderate-temperature eutectoid decomposition and finally a vacuum dehydrogenation treatment. After the hydrogenation treatment, an atmosphere containing hydrogen is introduced to stop the hydrogen from escaping from the hydrogenated specimens in the subsequent solution treatment and eutectoid decomposition treatment, which is different from conventional TCP. The results have shown that gamma titanium hydride precipitates in the alpha phase in the hydrogenated specimen. Also, during the eutectoid decomposition treatment a mixture consisting of alpha + gamma hydride nucleates from the beta(H) matrix while being continuously cooled to the decomposition temperature. The mixture continues to nucleate in the same way during the subsequent isothermal aging treatment. Finally, during the dehydrogenation treatment, the gamma hydrides gradually lose their hydrogen content, resulting in the recrystallization of fine alpha/beta phases. In this investigation, the microstructure resulting from each step of TCP is carefully examined, and particular attention is paid to the control of eutectoid decomposition conditions to shorten the time required to achieve a fully transformed microstructure. In addition, the optimal conditions pertaining to eutectoid decomposition treatment are proposed in this study. (C) 1998 Elsevier Science S.A. All rights reserved.