Strength asymmetry in nanocrystalline metals under multiaxial loading

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.