Double twinning mechanisms in magnesium alloys via dissociation of lattice dislocations

In this work, we propose dislocation mechanisms for the formation of {10 $(1) over bar $1}-{10 $(1) over bar $2} double twin structures in hexagonal close packed (HCP) crystals through the nucleation of secondary twins within primary twin domains. The model considers that secondary twins associated with the most commonly observed double twin variants (i.e. type 1 and type 2) nucleate and thicken by a sequence of three distinct dissociation reactions of mixed basal dislocations. Provided that the less frequently observed double twin variants (i.e. type 3 and type 4) also form by a dislocation-based mechanism, we show that their development must proceed by a separate set of dissociation reactions involving pyramidal < c + a > slip dislocations. Mechanistic, crystallographic and energetic considerations indicate that the type 1 variant should be the most prevalent. The mechanisms proposed here would also apply to the analysis of {10 $(1) over bar $3}-{10 $(1) over bar $2} compound twins and HCP metals other than Mg that exhibit double twinning, such as titanium.