NEW MODEL FOR THE SHOCK-INDUCED ALPHA-QUARTZ -]STISHOVITE PHASE-TRANSITION IN SILICA

A new model has been developed to describe the alpha-quartz to stishovite phase transition in silica under shock-loading conditions. During hydrodynamic simulations, individual global equations of state for the alpha-quartz and stishovite phases of silica are mixed in accordance with process-dependent constraints on the Gibbs free energy difference between the phases (DELTAG). For the shock-induced transition, DELTAG is required to equal a simple two parameter function of the mass fraction of stishovite. Unlike previous models, the new constraint equation assumes that the shock induced phase transition begins at the equilibrium phase boundary, but is not completed unless peak stresses on the order of 40 GPa are achieved.On release, the reverse transition is required to satisfy the usual thermodynamic equilibrium condition (DELTAG=0). It is shown that hydrodynamic simulations combining this hysteretic phase transition model with a strength model that assumes partial softening after yielding are capable of reproducing experimental Hugoniot and release data for crystalline silica in the mixed-phase region.