Cross-slip and glide behavior of ordinary dislocations in single crystal γ-Ti-56Al

Single crystals of Ti-56A1 deformed at 873 K in [1 6 3] and [1 12 5] orientations exhibit anomalous hardening. The anomalous hardening increases with increasing the Schmid factor for cross-slip in (1 1 0) plane as well as a (1 1 1) cross-slip plane. The yield stress and CRSS show temperature dependence as well as orientation dependence. The deformation structure of these crystals was observed by TEM. The TEM study shows that ordinary dislocations of screw character were the main deformation structure, but these dislocations were frequently pinned by "superjogs" in (1 1 0) planes. The alloy with a higher flow stress shows a wide spread presence of the (1 1 0) edge dislocation segments. These edge segments appear to have formed through a double cross-slip mechanism in which a screw dislocation cross-slipped onto (1 1 0)plane from the (1 1 1) primary slip plane and subsequently cross-slip again onto a (1 1 1) plane. This double cross-slip leaves a pair of short edge segments "superjogs" in (1 1 0) plane. It appears that these superjogs are immobile in the forward direction, but mobile in the lateral directions. As a result, these (1 1 0) superjogs act as pinning points, but also, provide a Frank-Read source for formation of dislocation loops as well as dislocation dipoles. These loops and dipoles probably contribute to alloy hardening at high temperatures. (C) 1999 Elsevier Science Ltd. All rights reserved.