Dynamic behavior of single-crystal iron from shock compression to spall fracture

We present molecular dynamics simulations of the dynamic response of single crystal iron from shock compression to spall fracture. Behind the shock front, accompanied by twin-mediated plasticity and bcc to hcp phase transformation as already reported extensively in the literature, the unloading wave is found to evolve into a rarefaction shock during its propagation and a pressure hysteresis between the direct and reverse phase transformations is observed. When this incident release wave interacts with the rarefaction wave reflected from the sample free surface a tensile pulse is induced within the crystal, which is found to drive a bcc to fcc phase transition whose mechanism is consistent with a Bain transformation path. Depending on the tensile wave magnitude, the spall fracture process is observed to occur through voids nucleation and growth either at favorable sites (twin boundaries, bcc-fcc grain boundaries) or within the fcc phase. All these results are consistent with experimental observations including very recent ones obtained under sub-nanosecond laser shock compression, while our simulations provide new insights into the governing mechanisms at the atomic scale and their kinetics.