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 $3to150nm$⁠. Whereas quasistatic tests show a strength plateau below about $15nm$⁠, high-rate tests (indentation strain rate of $∼103s−1$⁠) 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 $10–20nm$⁠. High strain rates are also shown to promote shear banding at nanocrystalline grain sizes.