An in-situ transmission electron microscopy study of pyramidal slip in Ti₃Al: I. Geometry and kinetics of glide

Abstract

Ti₃Al single crystals have been strained along the c axis in situ in a transmission electron microscope, in order to investigate the mechanisms of glide in pyramidal planes. Between -150 and 400°C, superdislocations are shown to dissociate into two superpartials with c + a/2 Burgers vectors. They glide in the type I pyramidal (π₁) planes, in contrast with type II pyramidal (π₂) planes observed in macroscopic compression along the c axis. This difference is proposed to result from the reversal of the applied stress. Glide softening takes place, probably associated with intensive disordering in the slip plane. The most mobile dislocations are 30° in character lying parallel to the second c + a/2 direction in the slip plane other than the Burgers vectors. The least mobile are, on average, heavily cusped 60° dislocations which nucleate a high density of rows of loops. It is concluded that the critical resolved shear stress of π₁ slip is determined by the density of pinning points on the less mobile dislocations.