Nucleation and growth of deformation twins:: a perspective based on the double-cross-slip mechanism of deformation twinning

The nucleation and growth of deformation twins are discussed, assuming that twinning occurs via the double-cross-slip mechanism first postulated by Pirouz for twinning in silicon. The dislocation energetics in this model are described in detail. In all cases, dislocation dissociation occurs and gives rise to a stationary partial and a twinning partial; twin growth involves the twinning partial undergoing double cross-slip. We discuss three specific geometries: firstly, the dissociation of a perfect dislocation into three collinear partials of equal Burgers vectors, which describes basal twinning in sapphire and twinning in bcc metals; secondly, the dissociation of a perfect dislocation into two collinear partials with different Burgers vectors, which describes rhombohedral twinning in sapphire; thirdly, the dissociation of a perfect dislocation into two non-collinear Shockley partials, which is used to describe twinning in silicon. Finally, the double-cross-slip mechanism readily explains the formation of emissary dislocations at the twin-matrix interface of deformation twins in bcc metals.