Engineering nanoscale heterogeneities in thin film metallic glasses via alloying with immiscible elements

Controlling nanoscale heterogeneity in metallic glasses offers a promising route to tailor their mechanical behavior. In this work, we investigate (Zr 34 Cu 66 ) 100-x Fe x thin film metallic glasses (TFMGs) across a wide compositional range (x = 0 – 76 at.%) to reveal how immiscibility between Cu and Fe can lead to heterogeneity, thereby influencing their structure and mechanical properties. Guided by thermodynamic modeling, we show that (Zr 34 Cu 66 ) 100-x Fe x retains an amorphous structure up to 77.5 at.% Fe, enabling systematic investigation of a wide range of compositions within ZrCu-rich and Fe-rich regions. Ab initio molecular dynamics simulations reveal that Fe addition in the range of 17-29 at.% maximizes the population of highly stable icosahedral <0,0,12,0> motifs, thereby increasing hardness ( H ) from ∼ 7.9 up to ∼ 8.5 GPa, while further Fe increase leads to a reduction in H as a result of the weakening of atomic bonds. At 76 at.% Fe, atom probe tomography reveals the presence of Cu-rich amorphous clusters, accompanied by H decrement down to ∼ 7.6 GPa. Moreover, we show that nanoscale phase separation in the form of Cu-rich clusters controls the deformation mode, affecting the amplitude of strain rate serrations during nanoindentation, decreasing from ∼ 16 down to ∼ 5 for high Fe content, indicating a transition from localized to more homogeneous plastic flow. Overall, we demonstrate a simple approach to tailor local heterogeneities of TFMGs by introducing immiscible elemental constituents, successfully managing to control the mechanical behavior, which can be beneficial for advanced material applications.