DEFORMATION IN NIAL BICRYSTALS

The effects of a concentrated local stress arising from elastic and plastic incompatibility at internal NiAl grain boundaries have been investigated in oriented NiAl bicrystals. Observations of surface slip markings and TEM characterization of the dislocations responsible for bicrystal deformation have been made. The glide of a <110> dislocations on {110} planes is the primary mode of deformation in the vicinity of the oriented bicrystal interfaces investigated. The dominant mode of a <110> dislocation generation is the nucleation of loops away from the bicrystal interface. The macroscopic stress required to activate these dislocations, in the presence of a stress concentration such as that produced by a dislocation pile-up at a grain boundary, is the same as that required to nucleate a <100> dislocations in "soft" single crystals of NiAl. Although the statistical relevance of this phenomenon as it pertains to the deformation of NiAl polycrystals has not been established, it is speculated that the glide of non-a <100> dislocations may play an important role in the ductility of polycrystalline NiAl at the ductile-to-brittle transition temperature. The movement of a <100> dislocations provides the primary mode of deformation for several bicrystals investigated far (> 3 mm) from the bicrystal interface. It is suggested that the vacancies required for the extensive climb of a [001] vacancy loops is supplied by the climb of a [100] and a [010] dislocations, and that the gradient in the concentration of vacancies provides the driving force for the motion of the a [100] and a [010] dislocations.