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Nanocrystalline metals contain a large fraction of high-energy grainboundaries, which may be considered as glassy phases. Consequently, withdecreasing grain size, a crossover in the deformation behaviour of nanocrystalsto that of metallic glasses has been proposed. Here, we study this crossoverusing molecular dynamics simulations on bulk glasses, glass-crystalnanocomposites, and nanocrystals of Cu64Zr36 with varying crystalline volumefractions induced by long-time thermal annealing. We find that the grainboundary phase behaves like a metallic glass under constraint from the abuttingcrystallites. The transition from glass-like to grain-boundary-mediatedplasticity can be classified into three regimes: (1) For low crystalline volumefractions, the system resembles a glass-crystal composite and plastic flow islocalised in the amorphous phase; (2) with increasing crystalline volumefraction, clusters of crystallites become jammed and the mechanical responsedepends critically on the relaxation state of the glassy grain boundaries; (3)at grain sizes $\geq$ 10 nm, the system is jammed completely, prohibiting puregrain-boundary plasticity and instead leading to co-deformation. We observe aninverse Hall-Petch effect only in the second regime when the grain boundary isnot deeply relaxed. Experimental results with different grain boundary statesare therefore not directly comparable in this regime.

acta materialia

From metallic glasses to nanocrystals: Molecular dynamics simulations on  the crossover from glass-like to grain-boundary-mediated deformation  behaviour