The Hall-Petch breakdown at high strain rates: Optimizing nanocrystalline grain size for impact applications

The breakdown of Hall-Petch strength scaling is investigated in nanocrystalline Ni-W alloys at high strain rates, using dynamic hardness testing at grain sizes ranging from 3 to 150 nm. Whereas quasistatic tests show a strength plateau below about 15 nm, high-rate tests (indentation strain rate of ∼ 10³ s^-1) exhibit a pronounced strength peak and a regime of "inverse Hall-Petch" weakening. This effect is shown to be the result of a grain size-dependent rate sensitivity that exhibits a maximum at a grain size near 10-20 nm. High strain rates are also shown to promote shear banding at nanocrystalline grain sizes.