In Situ Observation of Fracture along Twin Boundaries in Boron Carbide.

The observation of fracture behaviors in perfect and twinned B 4 C crystals via in situ transmission electron microscopy (TEM) mechanical testing is reported. The crystal structure of the synthesized B 4 C, composed of B 11 C icosahedra connected by boron-deficient C-▫-C chains in a chemical formula of B 11 C 3 , 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 B 4 C under both indentation and tension loading. The cracks then propagate along the TBs, thus resulting in the fracture of B 4 C. These results are consistent with the theoretical calculations that show that TBs have a softening effect on B 4 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 B 4 C. Furthermore, they can guide the design of advanced superhard materials via planar defect control.