Nucleation of deformation twins in zinc bicrystals

Abstract

The effects of stress concentration on the nucleation of deformation twins have been investigated in zinc bicrystals grown with controlled orientations. It is found that the apparent resolved shear stress for twinning varies from 25 to 4900 g/mm² depending upon the orientations of the individual crystal grains of the bicrystals as well as their relative orientations. The maximum stress concentration for the bicrystals twinned at the lowest r.s.s.t. is estimated to be approximately 15 Kg/mm², a value too low for a homogeneous nucleation model, but high enough to form two coherent twin interfaces. A model of twin nucleation based on the movement of pre-existing twin partials in a twin embryo associated with the strain field of a total slip dislocation to form a thin twin lamella which grows to a stable thickness of approximately 400 A is proposed. It appears that the twin fault energy is the controlling factor of the nucleation of twins in zinc.