Nanotwinning-induced pseudoplastic deformation in boron carbide under low temperature

Abstract

Nanotwinned metals exhibit enhanced strength due to the twin-boundary-dislocation interaction, but nanotwins may play a different role in strong, yet brittle ceramics due to the absence of mobile dislocations under low temperatures. Here, we carry out molecular dynamics (MD) simulations using a machine-learning force field to illustrate how nanoscale twins influence the mechanical properties and deformation mechanisms of superhard boron carbide (B4C). Surprisingly, in contrast to brittle failure in single-crystal B4C, we identify an abnormal pseudoplastic deformation behavior in nanotwinned B4C (nt-B4C) which arises from successive breakage of stretched inter-icosahedral B-C bonds. This pseudoplastic behavior shows a strong dependence on temperature and twin density, and it becomes more prominent as temperature decreases and twin density increases. This abnormal anti-correlation between pseudoplasticity and temperature mainly arises from enhanced atomic shear strain induced by thermodynamic vibrations at high temperatures. Moreover, a nanotwinning-induced softening mechanism is observed in nt-B4C, since amorphization preferentially nucleates near twin boundaries due to stress concentration. This work demonstrates that introducing high-density nanotwins into B4C is effective to enhance its low-temperature ductility.