Perfect dislocation pole models for twinning in the f.c.c. and b.c.c lattices

Abstract

As a prerequisite for the continued operation of a twinning pole mechanism it is proposed that the pole dislocation in the twin must be perfect. The pole models of Cottrell and Bilby (1951) and of Venables (1961), for twinning in b.c.c, and f.c.c, respectively, are shown to produce imperfect pole dislocations in the twin. As a result, the Cottrell-Bilby mechanism very probably comes to a halt when the twin has grown to a thickness of less than 50 atomic layers. The topology of Venables' model is shown to be in error. When this is corrected, the mechanism results in a stacking fault in the twin attached to the imperfect pole dislocation, which impedes further growth.

A perfect pole dislocation in the twin can be obtained for twinning in f.c.c, if the mechanism is nucleated from a triple node of perfect dislocations. Under the influence of the applied stress the two Shockley partials of which one of these dislocations is composed may rotate in mutually opposite sense around the other two perfect dislocations, producing a single twin lamella. For twinning in b.c.c, a perfect pole dislocation in the twin results from a mechanism nucleated from a jogged 〈001〉 dislocation. The dislocation decomposes in the jog and produces a two-layer fault on 〈112〉, After having produced one two-layer twin, the two ⅙〈111〉 twinning dislocations are interconnected by a local annihilation reaction, and the twin lamella can grow further on each side by the motion of a single twin partial.