Molecular dynamics simulation on the deformation mechanism of monocrystalline and nano-twinned TiN under nanoindentation

Abstract

In this research work, MD simulations are performed to investigate the nanoindentation deformation mechanisms of monocrystalline and nano-twinned TiN with various twin thicknesses. It is found that the nanoindentation plastic deformation of monocrystalline TiN with loading on the (111) surface is mainly due to the formation of triangular pyramid stacking faults, embryonic dislocation loop and then the ribbon dislocation loops. As for the nano-twinned TiN films with large twin thickness, the twin boundaries can efficiently block the propagation of dislocations, which could contribute to hardening. As for the nano-twinned TiN films with small twin thickness, the twin boundary could contribute to hardening in the initial plastic deformation due to the twin boundary-induced dislocation blockage, but the twin boundary could cause the softening effect in the middle and later stage of plastic deformation due to the following two aspects: (1) the formation of steps and local damaged regions in the twin boundary; (2) the steps and local damaged regions in the twin boundary serve as new sites for dislocations nucleation. Overall, twin boundary-induced softening mechanism dominates the deformation of nano-twinned TiN with small twin thickness during nanoindentation. Thus, nano-twinned TiN films with various twin thicknesses present the inverse Hall-Petch type relationship.