In Situ Observation of Fracture along Twin Boundaries in Boron Carbide

The observation of fracture behaviors in perfect and twinned B₄ C crystals via in situ transmission electron microscopy (TEM) mechanical testing is reported. The crystal structure of the synthesized B₄ C, composed of B₁₁ C icosahedra connected by boron-deficient C-▫-C chains in a chemical formula of B₁₁ C₃ , is determined by state-of-the-art aberration-corrected scanning TEM. The in situ TEM observations reveal that cracking is preferentially initiated at the twin boundaries (TBs) in B₄ C under both indentation and tension loading. The cracks then propagate along the TBs, thus resulting in the fracture of B₄ C. These results are consistent with the theoretical calculations that show that TBs have a softening effect on B₄ C with amorphous bands preferentially nucleated at the TBs. These findings elucidate the atomic arrangement and the role of planar defects in the failure of B₄ C. Furthermore, they can guide the design of advanced superhard materials via planar defect control.