Haasen plot analysis of the Hall-Petch effect in Cu/Nb nanolayer composites

We investigate the effects of layer thickness (t) on hardness (H) and rate sensitivity of the hardness (partial derivative H / partial derivative In is an element of over dot) in 1 mu m-thick Cu/Nb nanolayer composites. For t > 10 nm, we find that H correlates with t according to H = H-0 + H(1)t(-1/2), suggestive of a Hall-Fetch mechanism with layer interfaces replacing grain boundaries as barriers against dislocation motion. The measured levels of partial derivative H / partial derivative In (is an element of) over dot clearly indicate the operation of bulk-like dislocation mechanisms consistent with a Hall-Fetch mechanism. However, based on a Haasen-plot activation analysis, it appears that the Hall-Fetch coefficient, H-1, is strongly rate-dependent, inconsistent with a conventional Hall-Fetch mechanism. For specimens with t < 10 nm there is a saturation in hardness, but the rate sensitivity data indicate no clear evidence of a corresponding change in mechanism. Simple models are proposed.