Static recovery in titanium alloys at lower temperatures

A significant and well-documented limitation in the application of titanium alloys containing a large volume fraction of the hexagonal-close-packed a phase is that they suffer time-dependent deformation at lower temperatures. During the course of our recent investigations into the creep behavior of these alloys, several new phenomena, which must be considered to insure the safe implementation of these alloys, have been uncovered. The most important of these phenomena, a dramatic recovery of strain hardening when pre-crept samples are naturally aged for long times, has been observed in both the single-phase α-alloy Ti–6 wt.% Al and the two-phase α/β alloy Ti-6242. This paper summarizes observations of the variation in recovery response with respect to time spent in the unloaded state, exposure temperature, accumulated plastic strain, and microstructure. It is proposed that intense a-type dislocation pile-ups at grain/phase boundaries resulting from the planar nature of slip in these alloys provide the driving force for a dislocation level recovery mechanism that is shown to be thermally activated.