3D atomic density tomography: A novel approach to quantify microstructural heterogeneity and shear transformation zones of metallic glasses

Characterizing the complex microstructural heterogeneity of metallic glasses (MGs) has long been challenging, as experimental and computational tools for resolving atomic-scale localized structures are lacking. In this study, we introduce and validate a structural analysis technique: three-dimensional atomic density tomography (ACT-3D). This approach allows for the direct quantification of local atomic packing density and precise identification of potential shear transformation zone (STZ) regions in molecular dynamics (MD) simulations. By leveraging ACT values, regions prone to STZ formation were identified in a MD simulation constructed Al₉₀Sm₁₀ MG systems derived under varying cooling rates, allowing for accurate quantification of key STZ properties such as quantity, cluster size, size distribution, and spatial uniformity. An integrative evaluation index (I) was developed, which aggregates five crucial STZ parameters to quantitatively assess the overall STZ network structure. The weights of these parameters were determined from their coefficients of determination (R²) with a plasticity index β obtained from tensile simulations, and then normalized to construct a single composite score. The simulation results suggest that, within the range of cooling rates examined, a moderate cooling rate of 1 K/ps is most favorable for enhancing the macroscopic plasticity and toughness of the material. This improvement is attributed to the promotion of a uniform distribution of STZs, moderate network connectivity, and a balanced cluster size distribution. This study establishes 3D-ACT as a reliable tool for visualizing and quantifying STZ networks in MGs, while revealing how processing parameters tune microstructural heterogeneity to optimize mechanical performance.