Thermostastitical modelling of deformation twinning in HCP metals

Abstract

Deformation twinning in HCP metals is described by a novel thermostatistical approach. Thermodynamic descriptions for the critical conditions for twin nucleation and growth are derived. These are obtained by accounting for the competition between the strain energy in the material from local stress concentrations and dislocation slip. Central to this theory is the introduction of a statistical entropy term that accounts for the energetically favourable dislocation migration paths, which determine the dynamic recovery, twin nucleation and growth rates. Deformation by dislocation slip, at strains before twinning occurs, is described by theory previously derived for FCC metals and now applied to HCP materials without additional considerations. A dislocation generation term accounting for twin propagation is added to the evolution equation. Such term becomes active once a critical strain for twin nucleation is reached. Only physical parameters are employed as input. The new theory is successful in describing work hardening and twin volume fraction evolution of Ti, Zr, Mg and Mg-based alloys for various temperature and orientation conditions.