Size-induced weakening and grain boundary-assisted deformation in 60 nm grained Ni nanopillars

Nanocrystalline metals generally exhibit high strengths and good fatigue resistance. Their strengthening scales with the inverse of grain size through square root dependence down to grain sizes of ∼20 nm, representing the well-known Hall–Petch relation. Here we show that in surface-dominated structures with sub-micron dimensions, i.e. nanopillars, 60 nm grained Ni–W alloys exhibit lower tensile strengths with decreasing pillar diameter, form shear bands and undergo mechanical twinning. Moreover, there appears to be a transition in the deformation mechanism – from dislocation-driven deformation in pillars with diameters larger than 100 nm to grain-boundary-mediated deformation in pillars of 100 nm and below, including grain rotation and grain-boundary migration, processes previously observed only in grain sizes below 20 nm in materials of the same composition. We postulate that the presence of free surfaces activates these grain-boundary-mediated deformation processes at much larger grain sizes than observed before and results in lower attained strengths.