Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
1500799 | Scripta Materialia | 2011 | 4 Pages |
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.