Size Effect in the Uniaxial Compression of Polycrystalline Ni Nanopillars with Small Number of Grains

Molecular dynamics (MD) simulations are used to investigate the compression of nickel nanopillars in the ranges from 3 to 18 nm grain size (d) and from 12 to 30 nm specimen size (D). The results reveal that grain size play a more significant role than specimen size. There is a strong grain size effect—smaller is weaker—on the yield as well as the flow stress. The deformation is mainly governed by grain boundary (GB) motion for d < 12 nm, while for d≥12nm, it is governed by dislocation activity. When the grain size is small enough, the deformation of nanopillar is specimen size independent. For larger grain sizes, an irregular fluctuation in flow stress is observed resulting from sensitivity to grain orientation, fracture, and coalescence. Extended dislocations and twins play important roles in grain-shape evolution during compression. Due to the high fraction of surface grains, the “theory based on GB-shear and surface layer” is found inapplicable to explain the size effect in the plasticity of specimens with small number of grains across their diameter. Consequently, an expression of flow stress is proposed that includes effects of grain and specimen sizes, and the specimen-to-grain size ratios.