On the mechanistic origins of maximum strength in nanocrystalline metals

The maximum strength of polycrystalline metals/alloys has been suggested to occur at nanoscale grain sizes where the governing deformation mechanism transitions from dislocation plasticity to grain boundary mediated deformation. Despite tremendous progress recently uncovering links between transitions in nanoscale mechanisms and peak strength, the scientific literature is mostly devoid of any quantitative support, owing to the difficulty in measuring the resolved contribution of individual mechanisms to microstructural strain accommodation. In this study, the contribution of individual nanoscale mechanisms to the overall deformation of nanocrystalline Ni is calculated from atomistic simulations leveraging continuum-based kinematic metrics to compute mechanistic contributions to microstructural strain. By employing such a quantitative approach to analyze deformation behavior, it is shown that the realization of maximum strength in nanocrystalline metals corresponds to a grain size regime where the operative nanoscale mechanisms transition, and are thus equally competing to accommodate strain. However, the transition occurs between intergranular and intragranular mediated mechanisms, as it is found that dislocation plasticity alone is not the governing mechanism at all grain sizes above the peak strength regime.