Mesoscale Modeling of Dynamic Compression of Boron Carbide Polycrystals.

摘要

An anisotropic nonlinear elastic model is advanced for crystals belonging to either of two polytypes of boron carbide ceramic. Crystals undergo transformation to an isotropic, amorphous phase upon attainment of a local state-based criterion associated with a loss of intrinsic stability. The model is implemented using the dynamic finite element method, and is demonstrated on a representative volume consisting of fifty polyhedral grains subjected to uniaxial strain at a uniform high strain rate and shock compression at axial pressures ranging from 10 to 50 GPa. Predicted stress-strain behavior is in close agreement with experimental data. For polycrystals consisting of both polytypes, amorphization initiates at stress levels slightly below the experimental Hugoniot elastic limit, and occurs more readily than observed in experiment. For polycrystals consisting only of the CBC (polar) polytype, amorphization initiates at impact pressures similar to those suggested by experiment. In either case, transformation is promoted by dynamic stress interactions and elastic coefficient mismatch among anisotropic crystals. Results support a previous conjecture that amorphization is related to shear instability and cross-linking of the CBC chain. in the polar polytype.