A model for the formation of annealing twins in F.C.C. metals and alloys

Abstract

A model for the formation of annealing twins in f.c.c. metals and alloys is proposed. According to it, annealing twin formation proceeds in two stages: initiation and propagation. Initiation takes place at grain boundary ledges. After 'popping out' of the grain boundary, the twin grows into the grain by the migration of the noncoherent twin boundary, that can be represented by an array of partial Shockley dislocations with total Burgers vector equal to zero. Since the twin 'pops out' of the bondary and grows into the grain, the model does not require associated migration of the existing grain boundaries. The twinned region is separated from the grain by means of two parallel coherent twin boundaries. There are two relative orientations of adjacent grains for which the model can operate: 1. (a) when they are at twin orientation (but the boundary is not a coherent twin boundary) and 2. (b) when they are amenable to forming a 'special' boundary. Experimental evidence supporting the model is presented. © 1978.