Yield stress anomalies in single crystals of Ti–54.5 at.% Al III. Ordinary slip

Abstract

Mechanical properties and transmission electron microscopy studies of single crystals of γ–Ti–54.5 at.% Al of various orientations for which ordinary slip occurs show that at the temperature T _p of the peak stress of the yield stress anomaly (YSA) the critical resolved shear stresses (CRSSs) for ordinary slip on {110) and {111} are comparable, and that above T _p, the CRSS on {110) decreases rapidly with increasing temperature. The glide loops on (110) are elongated along the [001] direction, indicating the low mobility of edge dislocations. Below T _p, slip occurs on {111} and the screw dislocations are much less mobile than the edge dislocations, forming effectively long locks. Further slip (unlocking) is thought to occur by the generation and glide of edge superkinks along the screw direction. In the region of the YSA, before unlocking occurs, the screws bow out, mainly on {111} and partly on {110), the latter resulting in relatively immobile jogs which act as obstacles to slip on {111} and to the motion of the superkinks. Bowing on {110) has been observed in this work and by others. The YSA is considered to be due to increasing amounts of bowing on {110) with increasing temperature, and thus the formation of larger jogs on {110) in dislocations gliding on {111}. The driving forces for this mechanism are attributed to the screw dislocations having a minimum line tension on {110) and, except for alloys with less than about 54 at.% Al, to a lower resistance from the effect of short-range order associated with the Al₅Ti₃ phase for slip on {110) than on {111}. The large activation volumes observed below T _p are thought to be associated with mechanisms controlling the generation and motion of edge superkinks. involving thermally activated processes including climb at small jogs, and various mechanisms of cross-slip. Above T _p the rate-controlling process is considered to be thermally activated or climb-controlled glide of edge dislocations on {110) planes. The model for the YSA is discussed in the light of results from other studies.