AN IN-SITU TRANSMISSION ELECTRON-MICROSCOPY STUDY OF PYRAMIDAL SLIP IN TI3AL .2. FINE-STRUCTURE OF DISLOCATIONS AND DISLOCATION LOOPS

An in-situ transmission electron microscopy investigation has been con ducted of the dynamic behaviour of dislocations in the type I pyramida l (pi(1)) slip plane of Ti3Al single crystals which operates when spec imens are strained in tension along the c-axis. Superpartial dislocati ons are split asymmetrically into two unlike partials. The high-energy complex stacking fault involved is thought to relax by short-range di ffusion and this can account for the very strong disordering, the glid e softening, and the nucleation of loops reported in part I. Three loo p families have been identified, all vacancy type. Mechanisms are prop osed to explain the formation of loops, their interactions with mobile dislocations, and their alignment along a c + a/2 direction which is different from the Burgers vector direction of the gliding dislocation s. It is concluded that the critical resolved shear stress of pyramida l slip is determined by the nucleation process of loops.