Abstract
A nonlinear continuum phase field theory is developed to describe amorphization of crystalline elastic solids under shear and/or pressure loading. An order parameter describes the local degree of crystallinity. Elastic coefficients can depend on the order parameter, inelastic volume change may accompany the transition from crystal to amorphous phase, and transitional regions parallel to bands of amorphous material are penalized by interfacial surface energy. Analytical and simple numerical solutions are obtained for an idealized isotropic version of the general theory, for an element of material subjected to compressive and/or shear loading. Solutions compare favorably with experimental evidence and atomic simulations of amorphization in boron carbide, demonstrating the tendency for structural collapse and strength loss with increasing shear deformation and superposed pressure.
Highlights
Many crystalline ceramics and minerals undergo structural changes when subjected to stresses of high magnitude
The present paper focuses on boron carbide, a strong ceramic of high stiffness, high hardness, low mass density, and low ductility
One model for amorphization based on results of Density Functional Theory (DFT) calculations suggests that structural collapse and localization follow cross-linking of the C-B-C chain with icosahedral atoms [4]; other calculations suggest segregation is most favorable for polytypes with C-C-C chains [9]
Summary
Many crystalline ceramics and minerals undergo structural changes (e.g., phase transformations, twinning, or fracture) when subjected to stresses of high magnitude. Amorphization of boron carbide has been observed in Diamond Anvil Cell (DAC) compression-decompression [4], indentation [13], and ballistic impact [14]. Regarding the latter, performance of boron carbide ceramic in protection systems such as armor for vehicles and personnel is thought to be severely impeded by its tendency to localize, with cleavage fracture accompanying or closely following amorphization [14]. Regions of glassy phase are often reported to be in the form of planar bands of small thickness, on the order of several nanometers, and may be preferentially located parallel to certain crystallographic planes [4, 14]
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