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Shock-Induced Amorphization in Covalently Bonded Solids

  • Author(s): Zhao, Shiteng
  • Advisor(s): Meyers, Marc A
  • et al.
Abstract

Pulsed lasers with a power of the order of terawatts, once deposited on a target surface, will launch a stress pulse that propagates into material. Owing to the ultrashort duration of the laser pulses, unprecedented experimental conditions which combine high pressures (and/or shear stresses), strain rates and temperatures can be generated in materials, yielding a yet unexplored regime of study: materials science at extremes.

High-power, short-duration, laser-driven, shock compression and recovery experiments were carried out on four covalently bonded materials, namely, silicon (Si), germanium (Ge), boron carbide (B4C) and silicon carbide (SiC). These materials were chosen because of their high Peierls-Nabarro stress and negative Clapeyron slope. The profile of the shock waves was measured by a velocity interferometer system for any reflectors (VISAR). The shock deformation microstructure has been revealed by high resolution transmission electron microscopy and all the materials exhibit shock-induced amorphization. For Si and Ge with [001] orientation, two distinct amorphous regions were identified: (i) a bulk amorphous layer close to the surface and (ii) amorphous bands initially aligned with {111} slip planes. The VISAR measurements show that the estimated thresholds for such a crystalline-to-amorphous transition is estimated to be ~10 GPa (for silicon) and ~4 GPa (for germanium). Further increase of the shock stress leads to the crystallization of amorphous domain into nanocrystals with high density of nano-twins. For polycrystalline boron carbide, only amorphous bands inclined to the direction of shock wave propagation have been observed at a shock stress above ~45 GPa. At lower shock stress, planar faults have been seen below the shocked surface. For [0001] oriented monocrystalline silicon carbide, in addition to the amorphous bands inclined to the shock direction, some amorphous bands perpendicular to the direction of shock wave propagation were observed.

We propose that the amorphization is produced by the combined effect of high magnitude hydrostatic and shear stresses under dynamic shock compression. This study reveals that amorphization is a general inelastic deformation mechanisms in covalently bonded elements and compounds subjected to shock compression. Their formation yields a decrease in the overall hydrostatic and deviatoric elastic energy. Shock-induced defects play a very important role in the onset of amorphization. Calculations of the free energy changes with pressure and shear, using the Patel-Cohen methodology, agree with the experimental results. Molecular dynamics simulation corroborates the amorphization, showing that it is initiated by the nucleation and propagation of partial dislocations. The nucleation of amorphization is analyzed by classical nucleation theory.

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