High-temperature phase transitions. Properties and equilibrium of phases under shock-wave loading

Introducing the temperature as a parameter in the shock-wave experiment can significantly enlarge the scope of phenomena that can be studied. The influence of temperature on the elastoplastic processes accompanying the high-rate deformation and the phase transitions in shock waves is nontrivial and far from complete understanding. The currently existing experimental technique with laser Doppler diagnostics of specimens heated to 1400 K has already been successfully used to study the influence of temperature on the shock-wave behavior and the “dynamic” phase diagrams of both pure metal elements (U, Ti, Fe, Co, Ag, Cu) and ionic and covalent compounds (KCL, KBr, Al₂O₃). These studies showed the typical behavior, which was first discovered by Kanel and his colleagues for pure fcc (Al, As, Co, Cu) and some other (Sn, U) metals and ionic crystals under shock-wave loading, is that their shear strength increases with temperature. At the same time, similar “thermal strengthening” was not discovered in pure metals with bcc lattice. Sharp anomalies of (both shear and spallation) strength were observed near various phase transitions (polymorphic, magnetic, and related to melting). These studies showed that the shear strength of a pure metal increases by 50–100% near the phase boundary (i.e., the line of phase transitions of the first kind). At the same time, the presence of a trace amount (∼0.5%) of any impurity can lead to a fivefold decrease in the strength, as in the case of technically pure nickel near its Curie point. The same experimental technique used to study the shear stress relaxation in shock-wave loaded specimens can be extremely useful when studying mechanisms responsible for these anomalies.