Atomistic modeling of the effect of twins on plastic deformation in nanocrystalline cobalt

It has been observed by experiments and computer simulations that deformation twinning and grown-in twins play important roles during the plastic deformation in nanocrystalline (nc) metals. In this study, molecular dynamics simulation is employed to investigate the role of grown-in twins in the tensile deformation of hexagonal close-packed nc cobalt. Plastic deformation processes in three types of nc samples, namely nc-cobalt without twins, with (0002) twin faults and with (1121) twins, are compared. The results show that the pre-existing twins significantly lower the mechanical strength and do not enhance the ductility of nc cobalt. Instead deformation induced hcp-to-fcc allotropie transformation is found to result in remarkable strain hardening in the modeled nc cobalt.