Using time-resolved XRD to understand the behavior of the elastic properties along the Hugoniot and shock melting

Traditional shock experiments have measured the velocity of the leading edge of a rarefaction wave following shock to determine the melting point on the Hugoniot. For a typical metal, the velocity of the rarefaction front increases with pressure before thermal softening occurs along with a significant reduction in velocity before again increasing with pressure. This reduction in velocity is associated with the melt transition, as the leading edge shifts from propagating at the longitudinal sound speed in the solid (C_L) to the bulk sound speed of the liquid phase (C_B). Typically, incipient melt (melt onset) is determined by an empirical fit to the transition from C_L to C_B or determining the point where C_L is equal to the bulk sound speed in the solid phase using an equation of state. We have examined shock melting in both Mg and Ce by combining sound speed measurements taken at LANL with time-resolved diffraction experiments performed at the Dynamic Compressions Sector (DCS). The results, presented here, have yielded insight into the behavior of C_L leading into the melt boundary and the melt transition, leading to a greater understanding of shock melting and methods for determining incipient melt and melt completion from sound speed data.