Cluster line criterion and Cu–Zr–Al bulk metallic glass formation

The cluster line approach is presented and applied for searching the optimum glass-forming composition in the Cu–Zr–Al system. Three cluster selection rules are proposed: topologically dense packing (atomic size-related), chemical short-range order as evaluated by dissimilar atomic pairs, and composition distance to deep eutectics. Three Cu–Zr clusters Cu8Zr5, Cu6Zr5 and Cu5Zr6 satisfy all the three rules and are selected to construct cluster lines. The bulk metallic glass-forming range in the Cu–Zr–Al ternary system is thus determined. It is found that the thermal stabilities and glass-forming abilities of the bulk metallic glasses manifest monotonously increased tendencies with the increased Al content. The optimum Cu-rich and Zr-rich bulk metallic glass compositions are Cu58.1Zr35.9Al6 and Cu39.7Zr47.1Al13.2 (number indicate at.%) and they are, respectively, located along the Cu61.8Zr38.2–Al and Cu45.7Zr54.3–Al cluster lines, where the deep eutectics exactly correspond to the dense packing clusters Cu8Zr5 and Cu5Zr6. The characteristic parameters (glass transition temperature Tg and melting temperature Tl) of Cu58.1Zr35.9Al6 BMG are Tg = 760 K and Tg/Tl = 0.648, and those of Cu39.7Zr47.1Al13.2 BMG are Tg = 731 K and Tg/Tl = 0.638.