Super-elastic response of metals to laser-induced shock waves

Abstract

The structure and evolution of ultrashort shock waves generated by femtosecond laser pulses were explored in single-crystal nickel films via molecular dynamics and two-temperature hydrodynamics simulations. Ultrafast laser heating induces pressure build-up in a 100-nm-thick layer below the surface of the film. For low-intensity laser pulses, the stress-confined subsurface layer breaks into a single elastic shock wave with an amplitude that may exceed the conventional Hugoniot elastic limit. Comparative analysis with available experimental data confirms the existence of super-elastic states attainable through ultrashort shock compression. At high laser intensities, the two shock waves, elastic and plastic, form independently from the initial pressure profile. Because the laser heating was isochoric, the pressure and temperature at the melting front was fixed independent of absorbed fluence and results in a fluence-independent amplitude of the elastic wave generated at the liquid-solid interface. Elastic amplitude does not attenuate during propagation due to support from acoustic pulses emitted by the plastic front; whereas the unsupported plastic front undergoes significant attenuation and may fully decay within the metal film.