We report a unique way of using mechanical milling/in situ consolidation at both liquid-nitrogen and room temperature to produce artifact-free nanocrystalline $\mathrm{Cu}\left(23\mathrm{nm}\right)$ with a narrow grain size distribution. This nanocrystalline Cu exhibits an extraordinarily high yield strength $\left(770\mathrm{MPa}\right)$ , as predicted from a Hall–Petch extrapolation, along with good ductility (comparable with $\sim 30\%$ uniform tensile elongation). Possible factors leading to this excellent optimization of strength and ductility are discussed.

We have recently reported that parallel stacking faults (SFs) can tremendously increase the strength of a magnesium alloy. The strengthening is found to increase linearly with the reciprocal of the mean SF spacing, d. In this study we analyze dislocation interactions with SFs, and then propose a physics-based model to explain the observed relationship between yield strength and SFs spacing. Similar to the empirical Hall-Petch relationship for grain size, it is expected that this strengthening mechanism will hold true for a variety of materials engineered with parallel spaced stacking faults over a wide range of fault spacing.