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 (∂H/∂ ln ) in 1 μm-thick Cu/Nb nanolayer composites. For t < 10 nm, we find that H correlates with t according to H = H0 = H1t-1/2, suggestive of a Hall–Petch mechanism with layer interfaces replacing grain boundaries as barriers against dislocation motion. The measured levels of ∂H/∂ ln clearly indicate the operation of bulk-like dislocation mechanisms consistent with a Hall–Petch mechanism. However, based on a Haasen-plot activation analysis, it appears that the Hall–Petch coefficient, H1, is strongly rate-dependent, inconsistent with a conventional Hall–Petch 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.