Unified Hall-Petch description of nano-grain nickel hardness, flow stress and strain rate sensitivity measurements

It is shown that: (i) nano-grain nickel flow stress and hardness data at ambient temperature follow a Hall-Petch (H-P) relation over a wide range of grain size; and (ii) accompanying flow stress and strain rate sensitivity measurements follow an analogous H-P relationship for the reciprocal “activation volume”, (1/v*) = (1/A*b) where A* is activation area. Higher temperature flow stress measurements show a greater than expected reduction both in the H-P kε and in v*. The results are connected with smaller nano-grain size (< ∼20 nm) measurements exhibiting grain size weakening behavior that extends to larger grain size when tested at very low imposed strain rates.