Development of a Mechanistic Model for Prediction of Deformation Twinning in BCC Transition Metals and Alloys

Twinning is an important plastic deformation mechanism in crystalline materials. In BCC structures, deformation twinning exhibits strong temperature, loading-rate/direction and composition dependence, and is distinctly different among Fe, Group VB/VIB transition metals (TMs), and W/Mo/Ta-Re alloys. Despite their fundamental importance, the twin nucleation and growth mechanisms remain controversial and the classical -line can not be applied directly to assess twinning behaviour, leaving current theory unable to predict deformation twinning unequivocally. We recently introduced a reduced-constraint (RC) slip method and newrc-lines to replace the classical -lines. The new approach unveils the path to twin nucleation, growth and associated energy barriers in the entire BCC TM family and a mechanism to reverse the ductile-to-brittle transition in BCC TM alloys. Furthermore, the RC path shows that twinning is surprisingly but essentially controlled by the energy difference _Ebetween the HCP and BCC structures in elemental TMs, and can be effectively tuned and quantitatively predicted by first-principles calculations in TM alloys. The new RC method andrc-lines provide a bridge towards developing the long-sought model for deformation twinning in BCC structures. The project here leverages the above insights and proposes a multi-scale approach to develop a unified, mechanistic model for quantitative prediction of deformation twinning in both elemental TMs and alloys. Specifically, the research will establish the theoretical basis and computational framework to determine three primary effects on twinning behaviour: temperatureT, normal stress _nand solute composition ci. Temperature effects will be quantified by replacing _Ewith free energy difference _G. Stress effects will be determined by computing therc-lines, opening displacement and generalized stacking fault enthalpy as functions of slip and normal stresses. Average solute effects will be determined via virtual crystal approximation in DFT and EMTO-CPA for high order alloys. Local solute effects and fluctuations will be treated in statistical models with DFT-determined solute properties. Finally, all effects will be cross-validated in atomistic simulations with explicit solutes modelled using a new generation of interatomic potentials. Success will generate new generalized stacking fault energiesrc(T, _n, ci) and develop therc(T, _n, ci)-based twinning theory applicable to all BCC structures. The computational approach, with fracture mechanics theory andrc(T, _n, ci)-based barriers, will (i) enable predictions of twinning as a function of solute type, concentration, temperature and loading condition, and (ii) provide a practical tool for rapid screening of alloy compositions favouring desired twinning behaviour.