Mechanical and transmission electron microscopic studies of single crystals of γ-phase Ti-54.5at%Al specimens, having orientations for which ordinary slip occurred, showed that - at the temperature (Tp) of the peak stress in the yield stress anomaly - the critical resolved shear stresses for ordinary slip on {110) and {111} were comparable. Above Tp, the critical resolved shear stress on {110) decreased rapidly with increasing temperature. Glide loops on (110) were elongated along the (001) direction; thus indicating a low mobility of edge dislocations. Below Tp, slip occurred on {111} and screw dislocations were much less mobile than edge dislocations, and formed long locks. Further slip (unlocking) was thought to occur via the generation and glide of edge super-kinks along the screw direction. In the region of the yield stress anomaly, before unlocking occurred, the screws bowed out: mainly on {111}, and partly on {110). The latter resulted in relatively immobile jogs which acted as obstacles to slip on {111}, and to the motion of super-kinks. Bowing on {110) was observed. The yield stress anomaly was considered to be due to increasing amounts of bowing on {110), with increasing temperature, and thus to the formation of larger jogs on {110) in dislocations which glided on {111}. The driving forces for this mechanism were attributed to the facts that screw dislocations had a minimum line tension on {110) and that, except for alloys containing less than some 54at%Al, a lower resistance to the effects of short-range order which were associated with the Al5Ti3 phase for slip on {110) rather than on {111}. Large activation volumes which were observed below Tp were thought to be associated with mechanisms which controlled the generation and motion of edge super-kinks. These involved thermally activated processes which included climb at small jogs, and various cross-slip mechanisms. Above Tp, the rate-controlling process was considered to be the thermally activated or climb-controlled glide of edge dislocations on {110) planes.

Yield Stress Anomalies in Single Crystals of Ti-54.5at%Al - III. Ordinary Slip. S.Jiao, N.Bird, P.B.Hirsch, G.Taylor: Philosophical Magazine A, 2001, 81[1], 213-44