A model for twinning and plasticity in tantalum under shock conditions

During deformation of tantalum at high strain rates, a competition takes place between plastic slip and twinning. The higher the strain rate, the harder it is to activate slip systems and twinning becomes predominant. In view of investigating this transition, a phase field model for phase changes including crystal plasticity, written in the finite strain format, is derived. It relies on the Reaction Pathway formalism, which is tailored for dynamic loadings since elastic and inelastic effects are properly split. First, all accessible variants are identified using point group symmetries of the parent phase. In the case of tantalum, two levels of transformation are considered (with 1+36 accessible variants in total), which makes it possible to reproduce secondary twinning. Then, the variants are taken as local minima of an energy landscape from which transformational energy and kinetic relations are obtained. A crystal plasticity model is further embedded at each variant to run polyphase plastic computations. Shock simulations on tantalum bars are performed using a 3D total Lagrangian code with an element-free Galerkin least-squares formulation. Results in terms of both variants repartition and plastic activity are consistent with past experimental works.