Local and bulk melting of shocked columnar nanocrystalline Cu: Dynamics, anisotropy, premelting, superheating, supercooling, and re-crystallization.

We perform large-scale molecular dynamics simulations to study shock-induced melting transition of idealized hexagonal columnar nanocrystalline Cu. The as-constructed nanocrystalline Cu consists of unrotated (reference) and rotated columnar crystals, relative to the columnar axis. Shock loading is applied along three principal directions of the columnar Cu: two transverse (zigzag and armchair) and one longitudinal directions. Dynamic local melting processes are highly anisotropic with respect to the shock directions. For the transverse directions, hotspot effect and disparate dynamic responses of grains with different orientations may lead to partial or complete premelting of the initially rotated grains, which in turn leads to transient supercooling and heterogeneous recrystallization, and thus, the formation of nanocrystalline solids with modified grain structures or solid-liquid mixtures, depending on the extent of supercooling. With increasing shock strengths, the reference grains melt heterogeneously at interfaces and homogeneously inside. Conversely, "bulk" premelting of the rotated grains is absent for the longitudinal direction, except for grain boundary melting. The progression of recrystallization or heterogenous melting diminishes and eventually eliminates the transient premelting or superheating of the system via latent heat and thermal diffusion. Premelting or superheating appears unlikely for bulk melting or well-defined Hugoniot states, if the thermal and mechanical equilibria are achieved, and the thermodynamic melting curve coincides with the partial melting Hugoniot states of a polycrystalline solid.