Cluster Dynamics and Anomalous Transport in Metallic Glasses

Quenching a metallic liquid sufficiently fast can give rise to an amorphous solid, typically referred to as a metallic glass. This out-of-equilibrium material has a long suite of remarkable mechanical and physical properties but suffers from property deterioration via structural relaxation, also known as physical aging. In search for a structural understanding of aging of metallic glasses, we exploit here the ability to track atomic-scale dynamics via x-ray photon correlation spectroscopy (XPCS). Conducted across temperatures and under the application of stress, the results reveal strong signatures of intermittent aging and structural dynamics (Nature Communications 10 (2019) 5006). Non-monotonically evolving and fluctuating relaxation times persist throughout isothermal conditions over several hundred thousands of seconds, demonstrating heterogeneous dynamics at the atomic scale. In concert with microsecond molecular dynamic simulations, we identify possible mechanisms of correlated atomic-scale dynamics that can underly the temporal fluctuations and structural decorrelations. Through simulated XPCS, we find strong evidence of atomic-scale cluster dynamics that underly the intermittent structural decorrelations seen in experiments (Acta Materialia 267 (2024) 119730). Furthermore, a transition from classical stretched exponential to power-law decorrelations emerges at sufficiently long waiting times, which we interpret as a signature of anomalous transport (Nature Communications (2024) in press). We discuss these findings in the context of an emerging microstructure in metallic glasses.