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

Abstract

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 Cu 8Zr 5, Cu 6Zr 5 and Cu 5Zr 6 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 Cu 58.1Zr 35.9Al 6 and Cu 39.7Zr 47.1Al 13.2 (number indicate at.%) and they are, respectively, located along the Cu 61.8Zr 38.2–Al and Cu 45.7Zr 54.3–Al cluster lines, where the deep eutectics exactly correspond to the dense packing clusters Cu 8Zr 5 and Cu 5Zr 6. The characteristic parameters (glass transition temperature T g and melting temperature T l) of Cu 58.1Zr 35.9Al 6 BMG are T g = 760 K and T g/ T l = 0.648, and those of Cu 39.7Zr 47.1Al 13.2 BMG are T g = 731 K and T g/ T l = 0.638.