Molecular dynamics study of anisotropic shock responses in oriented alpha-quartz single crystal

HD Zhang and MK Shukla and S Larson and AM Rajendran and S Jiang, JOURNAL OF MATERIALS SCIENCE, 57, 6688-6705 (2022).

DOI: 10.1007/s10853-022-07076-0

This paper presents an investigation aimed at understanding the shock wave propagation response of oriented alpha-quartz single crystals by using molecular dynamics (MD) simulations. Several orthorhombic unit cells with different crystal orientations converted from an original monoclinic alpha-quartz crystal were used to construct the supercells with the crystallographic orientations of 100, 120, and 001 aligned with the shock direction. The shock wave propagation responses were analyzed via position-time (x-t) diagrams of several thermal and mechanical properties. Atomic shear strain and radial distribution function (RDF) were used to investigate the shock-induced material deformation and phase change from crystal to disordered fluid-like flow. The MD simulations enabled the construct of the shock Hugoniot, in terms of the shock velocity Us versus the impact/particle speed Up (i.e., U-s- U-p plane), and the Hugoniot elastic limit or HEL+ response with reference to precursor decay. It was found that the single crystal alpha-quartz sample exhibits noticeable anisotropic behaviors in terms of kinetic temperature distribution, stress distribution, and Hugoniot shock velocity response. Among the three studied crystal directions at a relatively low Up, the 120 sample showed a non-uniform shock-induced deformation pattern, and the 001 crystal showed the most prominent energy absorption capacity. At a given high impact speed (e.g., U-p = 2.5 km/s), the 001 sample showed a relatively longer amorphous shocked region followed by a shorter deformed crystal region, which was very different from the compressed regions behind the shock front in the other two samples. For all oriented crystals, the RDF results predicted an amorphous structure of silica emerging in the compressed region at the higher speed impact, in addition to a few "shearbands'' or crystal sliding in the 120 sample. The shock Hugoniot Us-Up also indicated a noticeable anisotropic behavior of the alpha-quartz. At a given value of Up above 1.5 km/s, the 001 crystal yielded the largest Us while the 120 crystal yielded the smallest. A "two-wave'' structure was evidently found in the 001 sample at U-p = 2.5 km/s, while such a structure was not clearly seen for the other two orientations. The precursor decay phenomenon was observed in 001 direction, indicating a strong strain rate effect on rHEL; however, the rHEL decay was not easy to identify in 100 and 120 directions due to the instantaneous microstructural sliding/collapse or fast transition to an extensive amorphous structure behind the shock front. In summary, the MD simulationbased studies reported in the present work demonstrate strong orientationdependent shock responses of the monoclinic single crystal alpha-quartz. GRAPHICS .

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