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

TL;DR

This study uses molecular dynamics simulations to explore how nanocrystalline metals transition from glass-like to grain-boundary-mediated deformation as grain size decreases, revealing three distinct regimes of plasticity behavior.

Contribution

It provides a detailed classification of the crossover from glass-like to grain-boundary-mediated deformation in nanocrystals based on crystalline volume fraction and grain size.

Findings

Grain boundary phases behave like metallic glasses under constraints.

Plasticity regimes depend on crystalline volume fraction and grain boundary relaxation.

Inverse Hall-Petch effect occurs only in a specific intermediate regime.

Abstract

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.