[SY-G3] Symposium G-3

[SY-G3] Shear relaxation behind the shock front in <1 1 0> molybdenum

^○Roman Kositski¹, Dominik Steinberger², Stefan Sandfeld², Dan Mordehai¹ (1.Department of Mechanical Engineering, Technion - Israel Institute of Technology, 32000 Haifa, Israel, 2.Chair of Micromechanical Materials Modelling (MiMM), Institute of Mechanics and Fluid Dynamics, TU Bergakademie Freiberg, 09599 Freiberg, Germany)

In this work we study shock-induced plasticity in Mo single crystals, using a method to relate information of dislocations from the atomic to the continuum scale [1]. We use molecular dynamics (MD) simulations to simulate the shock propagation along the <110> crystal orientation. Shock waves with compressive axial stresses of about 120 GPa are simulated. These stresses induce homogeneous nucleation of dislocations but barely lead to a phase transition. Using the Dislocation Extraction Algorithm (DXA) and the newly developed Discrete to Continuum (D2C) technique, the atomistic information is turned into continuous dislocation fields. In conjunction, we extract from the MD simulations thermo-dynamical macroscopic quantities such as macroscopic strain, density, temperature and stress tensor during the propagation of the shock wave. Correlating the stress evolution and the evolution of the continuous dislocation fields, we analyze the attenuation of the sharp elastic jump at the front of the shock wave and the plastic deformation behind it. Analyzing the MD simulations, we show that the initially an elastic precursor wave overshoots the dislocation nucleation stress, after which dislocations on a specific group of slip planes (which we denote as out-of-plane) are nucleated, slightly lagging behind the elastic front. As dislocations are nucleated in the out-of-plane direction, the resolved shear stress on these planes is relaxed, but the principal lateral stresses increase. This leads to an increase in the shear stresses on a plane parallel to the shock wave (denoted as in-plane), 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 close to isotropic stress state. Interestingly, the DXA-D2C analysis shows that non-zero contributions in the Kroener-Nye tensor appear only in the first stage of plasticity, right behind the shock front. 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.

[1] Kositski, et al. Computational Materials Science 149 (2018): 125-133.