Shear relaxation behind the shock front in $\langle$110$\rangle$ Molybdenum - From the Atomic Scale to Continuous Dislocation Fields

TL;DR

This study uses molecular dynamics simulations to analyze shear relaxation and dislocation behavior behind shock fronts in molybdenum crystals, linking atomic-scale phenomena to continuum dislocation fields.

Contribution

It introduces a multiscale approach combining MD simulations and continuous dislocation fields to understand shock-induced plasticity in Mo along the <110> orientation.

Findings

Dislocation nucleation lags behind elastic shear stress overshoot.

Shear stress relaxation occurs through specific slip plane dislocation activity.

Plastic deformation leads to an isotropic stress state parallel to the shock.

Abstract

In this work we study shock-induced plasticity in Mo single crystals, impacted along the <110> crystal orientation. In particular, the shear relaxation behind the shock front is quantitatively inspected. Molecular dynamics (MD) simulations are employed to simulate the deformation during shock, followed by post-processing to identify and quantify the dislocation lines nucleated behind the shock front. The information on the dislocation lines is ensemble averaged inside slabs of the simulation box and over different realizations of the MD simulations, from which continuous dislocation fields are extracted using the Discrete-to-Continuous method. The continuous dislocation fields are correlated with the stress and strain fields obtained from the MD simulations. Based on this analysis, we show that the elastic precursor overshoots the shear stress, after which dislocations on a specific group of slip planes are nucleated, slightly lagging behind the elastic front. Consequently, the resolved shear stress is relaxed, but the principal lateral stress increases. The latter leads to an increase in the resolved shear stress on a plane parallel to the shock wave, resulting in an additional retarded front of dislocation nucleation on planes parallel to the shock front. Finally, the two-stage process of plasticity results in an isotropic stress state in the plane parallel to the shock wave. The MD simulation results are employed to calculate the dislocation densities on specific slip planes and the plastic deformation behind the shock, bridging the gap between the information on the atomic scale and the continuum level.