Modeling laser material interactions at the time and length scales of in situ diffraction experiments*

The characterization of microstructure evolution during the interaction of a laser with metallic materials has been an active region of interest in the scientific community. Recent experimental efforts use in situ X-ray Diffraction (XRD) to identify modes of plastic deformation (dislocation slip, twinning) and phase transformation behavior to characterize this response. However, the ability to quantitatively characterize contributions from the various modes of plasticity or phase fractions and complex stress states to the shifts, broadening, or splitting behavior of the peaks in the diffraction patterns is challenging. As a result, current efforts aim to virtually model the interaction of lasers with metallic materials to understand microstructure evolution. This capability is currently available using a hybrid atomistic-continuum method that combines classical molecular dynamics (MD) simulations with a two-temperature method (TTM) to accurately incorporate the mechanisms of laser energy deposition and dissipation in an atomic scale microstructure. Further extension of this capability to the experimental scales is made possible by Quasi-Coarse-Grained (QCGD) method that scales up MD simulations to the larger length and time scales. In addition, simulated diffraction patterns can be created using these mesoscale datasets that allow a comparison with the patterns generated at the time and length scales of in situ diffraction experiments. This talk presents the capability of this virtual framework (QCGD-TTM, virtual in situ XRD) to characterize the microstructure evolution in metallic materials (FCC- Al, Cu) under laser-shock conditions using a laser with a fluence ranging from 10- 100 kJ/m² for a pulse duration of 100 ps. The simulations are carried out for foils with a thickness of 500 nm having an average grain size of 200 nm. The simulations reveal the role of laser ablation and the melting behavior of the sample on the rear surface velocities generated. The framework of the QCGD-TTM method, the temporal and spatial evolution of temperature and pressure in the system, the evolution of the microstructure, and simulated diffraction patterns to understand the contributions from melting for the various laser fluences will be presented.