High strain rate deformation mechanisms in fcc alloys as a function of load triaxiality

While dislocation-based plasticity is often the dominant deformation mechanism for fcc alloys, twinning and dynamic recrystallization play important roles under certain loading conditions. Reducing temperature or increasing rates favors twinning in fcc metals with low stacking fault energies. Here, large-scale molecular dynamics simulations reveal that loading triaxiality can also induce twinning and twinning-mediated grain refinement. At strain rates between ${10}^{8}\phantom{\rule{4pt}{0ex}}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ and ${10}^{9}\phantom{\rule{4pt}{0ex}}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, we observe a transition from a dislocation-dominated regime for uniaxial tension to one dominated by twinning and, eventually, grain refinement as the deformation triaxiality is continuously changed toward biaxial tension. The simulations reveal how strain rate and stacking fault energy affect the transitions between regimes and underlying mechanisms.