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 Cu-58.1,Zr35.9Al6 and Cu39.7Zr47.1Al13.2 (number indicate at.%) and they are, respectively, located along the Cu61.8Zr32.2Al 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 T-g and melting temperature T) of Cu58.1Zr35.9Al6 BMG are T-g =760 K and T-g/T-l =0.648, and those of Cu39.7Zr47.1Al13.2 BMG are T-g =731 K and Tg/T-l =0.638. (c) 2006 Elsevier B.V. All rights reserved.