Molecular dynamics simulation of BCC Ta with coherent twin boundaries under nanoindentation

We performed molecular dynamics (MD) simulations of nanoindentation on monocrystalline and nanotwinned Ta (nt-Ta) films, aimed to uncover the effects of twin boundary (TB) and twin thickness on hardness and the corresponding deformation mechanism. We found that the inelastic deformation of monocrystalline Ta under indentation could be attributed mainly to the propagation of dislocation loops and emergence of deformation twin. Inverse Hall-Petch-like relationship was found in the material with various twin thicknesses. It was also found that dislocation absorption, desorption and direct transmission are three kinds of dislocation-TB interaction in nt-Ta, resulting in the migration of TBs, the formation of steps along TBs, and the formation of new sites for dislocations nucleation, which contribute to the softening. On the other hand, TBs can block the propagation and migration of dislocations and confine dislocations in the twin lamellae, which may contribute to hardening. Overall, softening mechanisms dominate the deformation of nt-Ta with small twin thickness during nanoindentation process.