Strengthening of α+β titanium alloys by thermomechanical processing

Solution treatment and aging (STA) after hot working in the alpha + beta region has been widely used as an effective strengthening method for alpha + beta titanium alloys. In the first part of the paper, microstructural change during STA process of Ti-5.5Al-1Fe, one of the low cost substitutes for Ti-6Al-4V, is examined and compared to that of Ti-6Al-4V. The main phase transformation contributing to strengthening of Ti-5.5Al-1Fe is the martensite transformation on quenching from the solution treatment temperature and stabilization during aging, which is basically the same as that of Ti-6Al-4V. Some inherent microstructures are also observed in the Ti-5.5Al-1Fe, such as the isolated beta phase in the alpha grains, the traces of the omega phase in this beta phase, the FeTi formation in the materials aged for longer times, etc. In the second half of the paper, a new thermochemical process is proposed, in which less amount of hydrogen is used and extremely brittle hydride is not needed. In this process, hydrogen is used to lower the Ms temperature and increase hardenability, which temporarily converts an alpha + beta alloy to a beta alloy and makes it possible for the fine acicular a phase to directly precipitate in the beta matrix. Mechanical properties of the dehydrogenated material are excellent compared to the materials having so-called beta annealed structures. (C) 1998 Elsevier Science S.A. All rights reserved.