Strengthening-softening transition and maximum strength in Schwarz nanocrystals

Recently, a Schwarz crystal structure with curved grain boundaries (GBs) constrained by twin-boundary (TB) networks was discovered in nanocrystalline Cu through experiments and atomistic simulations. Nanocrystalline Cu with nanosized Schwarz crystals exhibited high strength and excellent thermal stability. However, the grain-size effect and associated deformation mechanisms of Schwarz nanocrystals remain unknown. Here, we performed large-scale atomistic simulations to investigate the deformation behaviors and grain-size effect of nanocrystalline Cu with Schwarz crystals. Our simulations showed that similar to regular nanocrystals, Schwarz nanocrystals exhibit a strengthening-softening transition with decreasing grain size. The critical grain size in Schwarz nanocrystals is smaller than that in regular nanocrystals, leading to a maximum strength higher than that of regular nanocrystals. Our simulations revealed that the softening in Schwarz nanocrystals mainly originates from TB migration (or detwinning) and annihilation of GBs, rather than GB-mediated processes (including GB migration, sliding and diffusion) dominating the softening in regular nanocrystals. Quantitative analyses of simulation data further showed that compared with those in regular nanocrystals, the GB-mediated processes in Schwarz nanocrystals are suppressed, which is related to the low volume fraction of amorphous-like GBs and constraints of TB networks. The smaller critical grain size arises from the suppression of GB-mediated processes.