Nanoindentation hardness and elastic modulus of nano-grained titanium produced by asymmetric and symmetric rolling

To understand the nanomechanical properties of nano-grained (NG) Ti produced by combination of asymmetric and symmetric rolling, nanoindentation hardness (H(n)) and elastic modulus (E(n)) of different planes within the NG Ti specimens were measured using continuous stiffness measurement mode at room temperature. For comparison, the nanomechanical properties of the as-received hot-rolled coarse-grained (CG) Ti and ultrafine-grained (UFG) Ti with only asymmetric rolling process were also investigated. It was found that H(n) of the Ti samples increased significantly with the decrease of grain sizes, while E(n) exhibited a slight decrease as the grain sizes decreased from CG to NG regime. The increase of H(n) was expected to be caused by higher density of dislocations and finer grains attained by severer plastic deformation, while the slight decrease of E(n) was considered as a result of the increased density of lattice defects and volume fraction of the grain boundary atoms. Furthermore, the nanomechanical properties of different planes of the Ti specimen exhibited a little difference which can be expressed as H(n(RD-TD)) > H(n(N-RD)) > H(n(TD-ND)) and E(n(RD-TD)) > E(n(ND-RD)) > E(n(TD-ND)). These differences were ascribed to crystallographic textures formed by rolling deformation.