Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
1451244 | Acta Materialia | 2005 | 13 Pages |
Molecular simulations of nanocrystalline nickel are used to investigate the effect of loading state on mechanical response. Simulations at grain sizes near the amorphous limit (2–4 nm) show a clear strength asymmetry, with specimens stronger under uniaxial compression than under uniaxial tension. A full biaxial yield surface is obtained for a grain size of 2 nm, and its shape reflects the asymmetry seen in the uniaxial simulations: the compressive lobe is proportionally larger than the tensile lobe. This biaxial yield surface cannot be well described using traditional yield criteria based on the maximum shear stress, but a good fit can be attained if a pressure or normal stress dependence is included. The simulations also show a monotonic trend towards larger strength asymmetry at larger grain sizes, suggesting the existence of a maximum asymmetry at finite grain sizes. This trend is validated by other mechanistic considerations in the ultrafine range of grain sizes, and discussed relative to the experimental literature.