Yield stress anomalies in single crystals of Ti-54.5at.%Al I.: Overview and ⟨011] superdislocation slip

Single crystals of gamma-Ti-54.5 at.% Al with two different orientations have been deformed between room temperature and 1223 K. Both exhibit a yield stress anomaly. The operating slip systems and dislocation structure have been determined by optical and transmission electron microscopy, and the dissociation of the [011] superlattice dislocations has been studied by the 'weak-beam' technique. For both crystals the major part of the slip occurs via the [011] superdislocations in the temperature range below the yield stress peak. The anomaly therefore reflects mainly an anomaly for [011](1 (1) over bar 1) slip. At room temperature these dislocations are dissociated in the (1 (1) over bar 1) glide plane, but at higher temperatures the screw dislocations form Kear-Wilsdorf-type locks which are the cause of the anomaly, as suggested by previous workers. Above the peak stress temperature, glide occurs by ordinary slip on {110) planes for one crystal and by [001]((1) over bar 10) slip and ordinary slip and climb for the other. Evidence for a yield stress anomaly for 1/2[(1) over bar 12)(1 (1) over bar 1) slip has been found. The results are discussed in the light of previous work.