Influence of the phase transitions of shock-loaded tin on microjetting and ejecta production using molecular dynamics simulations

摘要

We perform very large scale molecular dynamics (MD) simulations to investigate the ejection process from shock-loaded tin surfaces in regimes where the metal first undergoes solid to solid phase transitions and then melts on release. In these conditions, a classical two-wave structure propagates within the metal. When it interacts with the surface, our MD simulations reveal very different behaviors. If the surface geometry is perfectly flat or contains almost flat perturbations (sinusoidal type), a solid cap made of crystallites forms at the free surface, over a thickness of a few tens of nanometers. This surface cap melts more slowly than the bulk, and as a result, the ejection process is greatly slowed down. If the surface geometry contains V-shape geometrical perturbations, the oblique interaction of the incident shock wave with the planar interface of the defect leads to a sharp increase of temperature at the defect's bottom. At this place, the metal undergoes a solid to liquid phase change over the entire length of the groove, and this promotes the ejection of matter in the form of sheets of liquid metal. However, this phase change is not spatially uniform, and the sheets keep in memory this process by exhibiting a non-uniform leading edge and large ripples. These ripples grow over time, which ends up causing the fragmentation of the sheets as they develop. In this case, the fragmentation is non-uniform, and it differs from the rather uniform fragmentation process observed when the metal directly melts upon receiving the shock.