Strength and strain rate sensitivity for hcp and fcc nanopolycrystal metals

[摘要] While there is overwhelming evidence that strengthening from grain size refinement persists into the nanocrystalline grain size regime consistent with extrapolation of classical Hallâ€�?�Petch (Hâ€�?�P) behaviour, there are indications of a transition to an inverse Hâ€�?�P dependence, i.e. grain boundary weakening behaviour, occurring below a grain size of ∼ 10â€�??20 nm. When Hallâ€�?�Petch strengthening predominates, and the stress intensity, i.e. Hâ€�?�P slope value, ð‘�?_ðœ�?, is thermally-activated (as is the case for pure fcc and the easy basal slip hcp metals), the strain rate sensitivity, defined as [𝜕ðœ�?/ðœ�? ln(dð›�?/d𝑡]_ð‘�??, also is predicted to follow an Hâ€�?�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 b³ in conventional grain size fcc Cu and Ni materials to 10â€�??100 b³, for nanomaterials. At the smallest grain sizes, the transition to an inverse Hâ€�?�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 Hâ€�?�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.

[学科分类] 材料工程

[关键词] Activation volume;grain boundary strengthening/weakening;Hallâ€�?�Petch equation;inverse Hâ€�?�P effect;nanocrystalline grain size;strain rate sensitivity;thermal activation.