AN IN-SITU TRANSMISSION ELECTRON-MICROSCOPY STUDY OF PYRAMIDAL SLIP IN TI3AL .1. GEOMETRY AND KINETICS OF GLIDE

Ti3Al single crystals have been strained along the c axis in situ in a transmission electron microscope, in order to investigate the mechani sms of glide in pyramidal planes. Between -150 and 400 degrees C, supe rdislocations are shown to dissociate into two superpartials with c a/2 Burgers vectors. They glide in the type I pyramidal (pi(1)) planes , in contrast with type II pyramidal (pi(2)) planes observed in macros copic compression along the c axis. This difference is proposed to res ult from the reversal of the applied stress. Glide softening takes pla ce, probably associated with intensive disordering in the slip plane. The most mobile dislocations are 30 degrees in character lying paralle l to the second c + a/2 direction in the slip plane other than the Bur gers vectors. The least mobile are, on average, heavily cusped 60 degr ees dislocations which nucleate a high density of rows of loops. It is concluded that the critical resolved shear stress of pi(1) slip is de termined by the density of pinning points on the less mobile dislocati ons.