Indentation size effect on hardness in the body-centered cubic coarse grained and nanocrystalline tantalum

Indentation size effect (ISE) on the hardness in the nanocrystalline (NC) (similar to 24 nm) Ta and coarse-grained (CG) counterpart has been investigated by the nanoindentation tests at room temperature. The experimental results show that at shallow indentation depth ranging from 50 nm to 200 nm, the measured hardness is obviously smaller than the values predicted by the Nix-Gao model in the CG Ta, which is in contrast to the tendency in the NC Ta. At indentation depth less than 600 nm, obvious ISE can be observed in the both of CG and NC Ta. When the indentation depth is larger than 600 nm, the observed ISE in the CG Ta would gradually disappear in the NC Ta. Based on the deformation mechanism at different indentation depth, the density of geometrically necessary dislocations (GNDs) (rho(G-NC)) and statistically stored dislocations (SSDs) (rho(S-NC)) constructed by the Nix-Gao model are used to explain this phenomena.