High pressure and shock induced amorphization in boron carbide

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By combining Raman microscopy, nanoindentaion and high-pressure diamond anvil cell, the pressure-induced amorphization in single-crystal B_4C have been systemically investigated. The localized amorphization was found to start with the applied pressure at about 25GPa and extensive amorphization was observed with the applied pressure up to 50GPa. Interestingly, the amorphization only occurs during unloading. Phase transitions have not been seen during loading or still at high pressures. Low- and high-temperature Raman spectroscopy study reveals that amorphous B_4C is composed of carbon aromatic rings and B11C icosahedra. The amorphization of B_4C is associated with the destruction of the C-B-C chains and the formation of sp2 carbon clusters. This disordered structure with relatively weak carbon sp2 bonds is believed to be responsible for the loss of B_4C shear strength at high pressures. The possible polytypes of the chain structure in B_4C was investigated by ab initio calculations. The … More B-C-B chain structure was found to have the lowest energy than other polytypes. The structure was experimentally confirmed by Cs-corrected high-resolution scanning transmission electron microscopy that owns the point-to-point resolution better than 1.0 angstrom. In addition to the quasi-static study, amorphization of B_4C was also observed in the shock-loaded fragments and scratched debris. A large amount of amorphous phase mixed with nanocrystalline particles was characterized in the shock fragments with the impact pressure of about 40GPa. The compositions of the both amorphous and nanocrystalline phases are the same as the crystalline boron carbide, suggesting a polymorphic transition occurs during shocking loading. The observations of amorphization of B_4C under both quasi-static and dynamic loading indicate that the high pressures play the dominant role in the elastic instability of B_4C and irreversible high elastic deformation caused by high pressures is responsible for the amorphization during unloading. Less