Nonisentropic Release of a Shocked Solid

Shock release is the fundamental process that takes place when a material at high pressure undergoes rapid decompression. It is commonly accepted that rarefaction of this sort takes place isentropically, and is thus attended by substantial cooling due to the thermoelastic effect. However, this treatment fails to account for the fact that rapidly releasing material within the first few microns of the free surface typically exhibits material strength of order gigapascals, and therefore suffers copious plastic-work heating. Moreover, an isentropic treatment of release neglects the energy that can be recovered via the annihilation of crystal defects that ensues during rarefaction. Here, we present molecular dynamics simulations of shock and release in micron-scale tantalum crystals that exhibit release temperatures far exceeding those expected under the standard assumption of isentropic release. We show via an energy-budget analysis that this is due primarily to heating from material strength that largely counters thermoelastic cooling. The simulations are corroborated by experiments where the release temperatures of laser-shocked tantalum foils are deduced from their thermal strains via femtosecond x-ray diffraction, and are found to be close to those behind the shock itself.