A three-dimensional model was proposed for twin nucleation in hexagonal close-packed materials. This was based upon the non-planar dissociation of the leading dislocation in a pile-up of <a> slip dislocations. Continuum linear elastic dislocation theory was used to calculate the change in free energy with extension of the dissociated configuration; consisting of a stair-rod and glissile twinning dislocation loops. The model was applied to Mg, which deformed mainly by basal slip, and to Zr which deformed mainly by prismatic slip. It was found that dissociation from an isolated <a> slip dislocation was energetically unable to produce a stable twin fault loop; at least one larger than the core-width of the initial <a> slip dislocation. However, for some reactions, dissociation of the lead dislocation in a basal or prismatic dislocation pile-up could lead to a stable and sizable twin loop. In this case, the loop size was found to increase with decreasing twin-boundary energy and increasing number of dislocations in the pile-up.

Nucleation and Stability of Twins in HCP Metals. L.Capolungo, I.J.Beyerlein: Physical Review B, 2008, 78[2], 024117