Communication: Non-monotonic evolution of dynamical heterogeneity in unfreezing process of metallic glasses

Flow units as dynamic defects in metallic glassy materials

In a crystalline material, structural defects such as dislocations or twins are well defined and largely determine the mechanical and other properties of the material. For metallic glass (MG) with unique properties in the absence of a long-range lattice, intensive efforts have focused on the search for similar 'defects'. The primary objective has been the elucidation of the flow mechanism of MGs. However, their atomistic mechanism of mechanical deformation and atomic flow response to stress, temperature, and failure, have proven to be challenging. In this paper, we briefly review the state-of-the-art studies on the dynamic defects in metallic glasses from the perspective of flow units. The characteristics, activation and evolution processes of flow units as well as their correlation with mechanical properties, including plasticity, strength, fracture, and dynamic relaxation, are introduced. We show that flow units that are similar to structural defects such as dislocations are crucial in the optimization and design of metallic glassy materials via the thermal, mechanical and high-pressure tailoring of these units. In this report, the relevant issues and open questions with regard to the flow unit model are also introduced and discussed.

Effects of atomic interaction stiffness on low-temperature relaxation of amorphous solids

While low-temperature relaxations show significant differences among metallic glasses with different compositions, the underlying mechanism remains mysterious. Using molecular dynamics simulation, low-temperature relaxation of amorphous solids is investigated in model systems with different atomic interaction stiffness. It was found that as the interaction stiffness increases, the low-temperature relaxation is enhanced. The fraction of mobile atoms increases with increasing interaction stiffness, while the length scale of dynamical heterogeneity does not change. The enhanced relaxation may be due to increased dynamical heterogeneity. These findings provide a physical picture for better understanding the origin of low-temperature relaxation dynamics in amorphous solids, and the experimentally observed different β-relaxation behaviors in various metallic glasses.