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

Mechanical properties and transmission electron microscopy studies of singl e crystals of gamma -Ti-54.5 at.% Al of various orientations for which ordi nary 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) fo r ordinary slip on {110) and {111} are comparable, and that above T-p, the CRSS on {110) decreases rapidly with increasing temperature. The glide loop s on (110) are elongated along the [001] direction, indicating the low mobi lity of edge dislocations. Below T-p, slip occurs on {111} and the screw di slocations are much less mobile than the edge dislocations, forming effecti vely long locks. Further slip (unlocking) is thought to occur by the genera tion 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} an d partly on {110), the latter resulting in relatively immobile jogs which a ct as obstacles to slip on {111} and to the motion of the superkinks. Bowin g on {110) has been observed in this work and by others. The YSA is conside red to be due to increasing amounts of bowing on {110) with increasing temp erature, and thus the formation of larger jogs on {110) in dislocations gli ding on {111}. The driving forces for this mechanism are attributed to the screw dislocations having a minimum line tension on {110) and, except for a lloys with less than about 54 at.% Al, to a lower resistance from the effec t of short-range order associated with the Al5Ti3 phase for slip on {110) t han on {111}. The large activation volumes observed below T-p are thought t o be associated with mechanisms controlling the generation and motion of ed ge superkinks, involving thermally activated processes including climb at s mall jogs, and various mechanisms of cross-slip. Above T-p the rate-control ling process is considered to be thermally activated or climb-controlled gl ide of edge dislocations on {110) planes. The model for the YSA is discusse d in the light of results from other studies.