Avalanches mediate crystallization in a hard-sphere glass

Significance Glasses are formed from the supercooled liquid state when motion is arrested on the scale of the particle diameter. Such states are thermodynamically unstable: An apparently deeply arrested amorphous material can transform into a crystal (devitrification) without large-scale particle motion. The prediction and avoidance of devitrification represent major formulation issues in materials science, hence the importance of understanding its mechanism. Using molecular-dynamics simulations, we study the crystallization of the simplest glass-forming system, a hard-sphere glass. We find that crystallization is caused by a subset of particles cooperatively undergoing large rearrangements in an intermittent stochastic fashion (avalanches). Particles involved in an avalanche are not themselves crystallizing, but they induce crystallization in nearby regions that already show incipient local order. By molecular-dynamics simulations, we have studied the devitrification (or crystallization) of aged hard-sphere glasses. First, we find that the dynamics of the particles are intermittent: Quiescent periods, when the particles simply “rattle” in their nearest-neighbor cages, are interrupted by abrupt “avalanches,” where a subset of particles undergo large rearrangements. Second, we find that crystallization is associated with these avalanches but that the connection is not straightforward. The amount of crystal in the system increases during an avalanche, but most of the particles that become crystalline are different from those involved in the avalanche. Third, the occurrence of the avalanches is a largely stochastic process. Randomizing the velocities of the particles at any time during the simulation leads to a different subsequent series of avalanches. The spatial distribution of avalanching particles appears random, although correlations are found among avalanche initiation events. By contrast, we find that crystallization tends to take place in regions that already show incipient local order.