Substantial change in medium range ordering and its influence on glass forming ability and mechanical properties of ZrCu and ZrCuAl metallic glasses

This paper presents the substantial changes in structural heterogeneity in ZrCu and ZrCuAl metallic glasses (MGs) as a function of composition and elucidates its correlation with their glass-forming ability (GFA) and mechanical properties. Utilizing the angular correlation function and direct mapping of MRO based on four-dimensional scanning transmission electron microscopy, we conducted a detailed analysis of nanoscale heterogeneity in binary Zr₅₀Cu₅₀, ternary eutectic Zr₅₀Cu₄₀Al₁₀, and hypoeutectic Zr₆₅Cu₂₅Al₁₀ MGs. Our findings reveal significant differences in medium-range ordering (MRO), including MRO type, size, and spatial distribution among these MGs, which correspond to substantial variations in their nanoscale heterogeneity. Especially, the results show the formation of nanoscale Cu-rich MRO in the binary MG and their further change when Al is added in the eutectic composition. The mechanism of such change in crystal-like MRO and its correlation with GFA is discussed. We also show that Zr₅₀Cu₄₀Al₁₀, with the largest and most sparsely distributed crystal-like MRO, exhibits the least ductile behavior, while Zr₆₅Cu₂₅Al₁₀, with a much more homogeneous structure with finely distributed MRO, demonstrates the highest ductility. Based on experimental results and theoretical insights, we explain how the homogeneous distribution of MRO interferes with the formation and propagation of shear bands, ultimately leading to substantially different ductility behavior as the composition is altered.