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

Abstract

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.

Velocity and mass density of the ejecta produced from sinusoidal grooves in laser shock-loaded tin

“…One of the processes governing such ejection is the interaction of a shock wave with a free surface presenting geometrical defects such as pits, scratches, grooves, or machining-inherited roughness. This process, usually referred to as "material ejection" or "microjetting" (because ejecta often consist of "jets" breaking up into micrometer-scale particles), has been the subject of extensive research work for about six decades [5], and it still motivates both theoretical and experimental investigations worldwide [6][7][8][9][10][11][12][13][14]. For a few years, we have explored how laser-driven shocks can provide some valuable, complementary insights into ejecta physics under pulsed pressure loads of high amplitude (some tens of GPa) and very short duration (a few ns).…”

“…Figure12. Comparison of the jet tip velocities inferred from the shadowgraph (vjt, same as Figure5), those resulting from the 99% mass criterion (v 99% , see text), and those predicted by the Richtmyer-Meshkov Instability theory and models.…”