A thermodynamical model for the nucleation of mechanical twins in TiAl

Mechanical twinning has long been recognized as an important deformation mechanism in many intermetallics including γ(TiAl) based alloys. The generation of a twin can be triggered by dislocations at grain boundaries. The reduction of the stored strain energy in relation to a configuration without any twins can be shown by applying an established transformation criterion, considering twinning as a transformation shearing process. In addition, the application of a thermodynamical extremal principle allows one to predict a distinct twin nucleus in good agreement with experimental observations. The concept is rather general and can be adapted to any initial eigenstress field produced by dislocations present at misfitting interfaces.