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

• Published in: Scripta materialia 2011, Vol.64 (1), p.77-80
• Main Authors: Jang, Dongchan, Greer, Julia R.
• Format: Article
• Language: English
• Subjects: Compression test and tension test microscopy and microanalysis techniques; Deformation; Dislocations; Grain size; Grains; Mechanical properties testing; Nanocomposites; Nanocrystalline microstructure; Nanomaterials; Nanostructure; Pillars; TEM nanocrystalline materials mechanical properties; Twinning microstructure
• ISSN: 1359-6462; 1872-8456
• DOI: 10.1016/j.scriptamat.2010.09.010

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.