Strength and strain rate sensitivity for hcp and fcc nanopolycrystal metals

While there is overwhelming evidence that strengthening from grain size refinement persists into the nanocrystalline grain size regime consistent with extrapolation of classical Hall-Petch (H-P) behaviour, there are indications of a transition to an inverse H-P dependence, i.e. grain boundary weakening behaviour, occurring below a grain size of similar to 10-20 nm. When Hall-Petch strengthening predominates, and the stress intensity, i.e. H-P slope value, k(epsilon), is thermally-activated (as is the case for pure fcc and the easy basal slip hcp metals), the strain rate sensitivity, defined as [partial derivative sigma/partial derivative In(d gamma/dt](T), also is predicted to follow an H-P type dependence, thus, increasing with decrease in grain size. As a consequence, the activation volume that is inversely proportional to the strain rate sensitivity, is found to decrease by an order of magnitude, from around 1000 b(3) in conven tional grain size fcc Cu and Ni materials to 10-100 b(3), for nanomaterials. At the smallest grain sizes, the transition to an inverse H-P dependence has been proposed to occur because of onset of effective high temperature grain boundary weakening behaviour that is well known in limiting creep property descriptions. If the inverse H-P effect (grain boundary weakening) is genuine, we predict that the strain rate sensitivity and corresponding inverse activation volume dependence on grain size should also have to show a reversal.