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Representative data from the initial impact event on a grain size Ni–W specimen. The upper panel demonstrates the output displacement-time data, from which the velocity, shown in the lower panel, is calculated.
Dynamic hardness as a function of grain size on double logarithmic scales. Hall–Petch scaling is observed down to a grain size of , followed by a breakdown manifested as a strengthening peak and region of inverse Hall–Petch weakening. The quasistatic hardness measurements from our previous study (Ref. 15) are included for reference.
Hardness as a function of strain rate for specimens with grain sizes in the range . The dynamic measurements from the present work align well with the extrapolation from quasistatic nanoindentation results reproduced from Ref. 15. Corresponding SEM images of the residual impressions reveal discernible shear offsets around the impact sites for grain sizes as large as . (Some of the impressions have a scar across them, made upon withdrawal of the tip; these scars are formed after the conclusion of the test and are not relevant to the discussion).
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