Effects of grain structure inhomogeneity on dynamic deformation mechanisms and spallation of medium entropy alloy CoCrNi under ramp wave loading

By leveraging large-scale molecular dynamics simulations, the effects of grain structure inhomogeneity on the ramp wave response and spall failure behavior of CoCrNi medium entropy alloy are thoroughly investigated, with loading velocities varying between 800 m/s and 1400 m/s. In the uniform nanocrystalline (UNC) models with homogeneous grain size distribution, the Hugoniot elastic limit (HEL) initially increases as the grain size grows from 3 nm to 9 nm but decreases as the grain size further increases to 12 nm. This behavior suggests a transition in the dominant deformation mechanism from intergranular grain boundary (GB) deformation to intragranular activities. For comparison, gradient nanocrystalline (GNC) models, featuring a linear grain size variation along the loading direction from 3 nm to 12 nm, and the heterogeneous nanocrystalline (HNC) models, exhibiting an abrupt grain size change from 3 nm to 12 nm, are also examined. Both GNC and HNC models exhibit a combination of intragranular defect multiplications (involving dislocation slip, stacking fault (SF), and twin boundary (TB) expansion) and intergranular GB movement during the plastic deformation, resulting in shear strain homogenization, especially at the GBs. Consequently, the pronounced strain delocalization caused by grain structure inhomogeneity promotes cooperative deformation between GBs and grain interiors, effectively hindering void nucleation and expansion. This leads to enhanced spall strength in both the GNC and HNC models. This study provides a strategy of grain structure engineering for designing polycrystalline MEA under dynamic loading conditions.