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