MD simulations of plasticity response in gamma-tin under compression

Experiments suggest that under shock compression body-centered tetragonal gamma-tin presents near zero mechanical strength and continuously varying c/a ratio. To gain insights into these peculiar behaviors we developed a SNAP-style interatomic potential fitted to a DFT dataset containing configurations pertaining to beta and gamma crystalline phases of tin. The resulting SNAP potential correctly reproduces geometry of BCT gamma-tin and gamma-to-delta transition pressure. Using the newly developed potential we performed full-scale MD simulations of single crystal gamma-tin under compressive deformation. Simulations reveal that under compression along its c-axis gamma-tin possesses high mechanical strength, however when compressed along one of it's a-axes its plastic strength is indeed near zero. Careful analysis unambiguously related the latter dynamics to a displacive "flip" transformation by which the compression a-axis and the c-axis of BCT tin are flipped. The resulting flip transformation has a gross effect on plasticity response irrespective of presence (or not) of pre-existing dislocations thus accounting for near zero plastic strength. To gain additional insights into competition between the newly discovered flip transition and a previously hypothesized BCT BCC transformation, we map relevant transformation pathways onto a 2D transformation energy landscape devised specifically for the purpose. We then compare the transformation landscape computed with the SNAP potential with the same landscape computed using fully converged DFT calculations. Except for non-essential details the two landscapes agree with each other both suggesting that, although BCT BCC transition cannot be ruled out, the BCT BCT flip is rather more likely to be responsible for the peculiar zero-strength response of gamma-tin observed experimentally.