An improved multiphase equation of state for aluminum in hypervelocity impact

Equation of state (EOS) contains information about the relationships between the thermodynamic variables of materials, and is widely applied in the field of hypervelocity impact (HVI) study. In this paper, based on the framework of the Gray EOS, three improvements are introduced to refine its physical descriptions, including the correction of the entropy function of metals in the liquid phase, the more reasonable description for the cold term, and the modification of the Young-Alder EOS, thereby an improved multiphase EOS is developed, accounting for solid, liquid, gas and mixed-phase (melting and vaporization). Further to this, supported by numerous existing experimental data and molecular dynamics simulation results, the complete parameters of the improved EOS for aluminum are obtained, and the thermodynamic behaviors of aluminum during shock compression and isentropic release are studied, simultaneously a systematic comparison is made in the prediction differences of three EOSs, i.e., the improved EOS, the Gray EOS, and the Tillotson EOS. The results show that the improved EOS can best predict the thermodynamic properties of aluminum in a wide range of states, from the high-pressure dense state (<1 TPa) to the low-density expanded fluid state (> 0.1 g/cm³). Then, the improved EOS is embedded into the AUTODYN-SPH hydrocode, and the phase evolution process of aluminum in HVI is displayed.