Cold-rolling of Ti based alloys: How does plastic deformation improve the hydrogen sorption?

Different amounts of cold rolling were applied to a commercial pure hexagonal Ti – taken as a “model material” - to gain new insights into the effect of plastic deformation on hydrogen absorption properties (activation and kinetics). Reproducible specific H 2 charging procedures were set to discriminate between different material parameters such as structural defects (dislocations, vacancies), grain boundaries, texture and native oxide stability. The positive effect of increasing the amount of plastic deformation on improving the absorption kinetics was maintained despite the occurrence of a recrystallisation process - that removes most of the structural defects induced by rolling - before any H 2 loading. Decisive factors responsible for improved absorption kinetics such as higher proportion of high-angle grain boundaries, texture strengthening, as well as surface oxide layer modification induced by increasing cold rolling are analysed and thoroughly discussed. The H-absorption curves in Ti are characterized by three different stages, having possibly very different durations. It is essentially the formation of the hydrogen (hcp) α-Ti solid solution (stage I) and its transformation into the (bcc) β-Ti phase (stages II) that are affected by these decisive parameters. In particular, cold rolling has a major effect on destabilizing the surface oxide which, in turn, shows faster reduction towards its metal form. The subsequent kinetics, towards the formation of the δ-TiH 2 hydride (stage III), being related to the volume expansion cracking and the formation of fresh surfaces, is always rather fast even at low (400 – 450°C) charging temperatures.