Activating deformation twinning in cubic boron nitride.

Deformation twinning, a phenomenon primarily documented within metallic systems, has remained essentially unexplored in covalent materials due to the formidable challenges posed by their inherent extreme hardness and brittleness. Here, by employing a five-degree-of-freedom nano-manipulation stage inside a transmission electron microscope, we reveal a loading-specific twinning criterion for cubic boron nitride and successfully activate extensive deformation twinning with substantial improvements in mechanical properties in <100>-oriented cubic boron nitride submicrometre pillars at room temperature. Beyond cubic boron nitride, this criterion is also proven widely applicable across a spectrum of covalent materials. Investigations on the twinning dynamics at the atomic level in cubic boron nitride suggest a continuous-transition-mediated pathway. These findings substantially advance our comprehension of twinning mechanisms in covalent face-centred cubic materials, and herald a promising avenue for microstructural engineering aimed at enhancing the strength and toughness of these materials in their applications.