Shock-compression and release behavior near melt states in aluminum

Abstract

The response of porous aluminum (ρ0=1.6 g/cm3) when shock-compressed to states of partial melting and then released has been investigated. There are no detectable changes in any of the Hugoniot properties in the region where melting should occur. However, release wave speeds in the completely compacted state of the porous aluminum specimens were found to depend critically upon the magnitude of the initial shock loading. For impact stresses below about 7 GPa, the measured release-wave velocity is well described by the assumption that the initial release is elastic. For initial impact stresses above 7 GPa, the speed of the release wave is observed to be about 20% lower than the elastic wave velocity and approaches the bulk sound speed predicted by equilibrium thermodynamics. All calculations of wave speeds were made with a complete equation of state which was obtained by constructing semiempirical free-energy functions for both solid and liquid pure phases. These were developed by first assuming self-consistent forms for the second derivatives of the Helmholtz free energy and then adjusting coefficients in the second derivatives using known quasistatic thermodynamic properties for each phase until agreement with all available data was achieved. Calculation of the intersection of the Hugoniot with the melt boundary (7.5 GPa, 1340 K) using this equation of state when compared with the observed change in sound speed gives strong support to the assumption that melting occurred in these experiments. The release-wave technique shows promise for measuring phase boundaries in regions of pressure and temperature not accessible by ordinary quasistatic techniques.