Yield stress anomalies in single crystals of Ti-54.5 at.%Al - II. 1/2((1)over-bar-12](1(1)over-bar-1) slip

A yield stress anomaly has been observed for 1/2[(1) over bar 12](11 (1) ov er bar) single crystals of gamma-Ti-54.5 at.% Al. Weak-beam electron micros copy studies have been carried out to elucidate the origin of this anomaly. At room temperature, screw and 60 degrees dislocations are dissociated in the glide plane, and 30 degrees dislocations are dissociated by glide on in tersecting {111} planes, forming weak locks. At high temperatures, 1/2[(1) over bar 12] dislocation loops are elongated along their edge direction, an d these edge dislocations are dissociated by climb and glide into sessile c onfigurations consisting of Frank and Shockley partials on different inters ecting {111} planes, with a stair-rod dislocation along the line of interse ction. There is some evidence for further dissociation of the Frank partial . These strong locks are responsible for the yield stress anomaly. The driv ing force for the anomaly is the reduction in dislocation energy; the mecha nism involves climb, which becomes easier with increasing temperature.