Cluster-based Bulk Metallic Glass Formation In Cu-Zr-based Ternary System

Metallic glasses have been obtaining more and more interests for distinctive properties being unusual for crystalline materials and it is always an important topic for researchers to find bulk Metallic glasses. In this paper, a series of new bulk metallic glasses have been developed in Cu-Zr-based ternary system according to the e/a-variant criterion based on cluster. The minor alloying mechanism is discussed in the light of atomic size and electron concentration factors.The atomic cluster Cu8Zr5, derived from the Cu8Zr3 phase structure, corresponds to an eutectic composition Cu0.618Zr0.382. Cu0.64Zr0.36 is the best glass-forming composition in the Cu-Zr system. Two e/a-variant lines (Cu0.618Zr0.382)1-xMx and (Cu0.64Zr0.36)1-xMx are constructed by Unking these two specific compositions with the third constitute M, where M represents Nb, Sn, Mo, Si, V or Ag. Ingots of the alloys are prepared by arc melting under argon atmosphere and alloy rods with a diameter of 3mm are prepared by copper mould suction casting. The glass-forming ranges and abilities of the alloys are determined by using X-ray diffraction, thermal analysis and optical morphology. The results indicate that minor Nb, Sn, Mo and Ag additions improve the glass forming abilities of the Cu-Zr alloys. (Cu0.64Zr0.36)0.99Nb0.01, (Cu0.64Zr0.36)0.99Sn0.01 and (Cu0.618Zr0.382)0.92Ag0.08 have the best glass-forming abilities and the lengthes of the fully amorphous rods are respectively 20,12 and 25mm.Cu8Zr5 cluster is an decisive role on the glass formation. The R*(the topological packing density) values are increased after minor alloying with Nb and Sn additions, but not with Si and V, so the glass-forming abilities of Cu-Zr alloys are increased by the Nb and Sn additions and V and Si adversely affect the glass-forming abilities. Cu0.64Zr0.36 composition is close to (Cu8/13Zr5/13)13/14Cu1/14, that is, the amorphous structure is composed of the icosahedral Cu8Zr5 cluster plus the glue atom Cu. Accordingly, the minor alloyed amorphous structures might maintain similar structures, i.e. [Cu8Zr5-based icosahedral cluster]13/14(glue atoms)1/14. Thereof the best ternary amorphous composition can be described as (Cu0.643Zr0.357)1-uMu. Electron concentration is another factor to affect the glass formation. Except for the alloys with Ag addition, the glass forming abilities of other series of bulk metallic glasses are increased with increasing e/a ratios. Though Mo addition decreases the R* value of Cu8Zr5, but it still improves the glass forming abilities of the Cu-Zr alloys due to its electron concentration factor.Ag additions not only improve the R* value of Cu8Zr5, but also act as the glue atom Unking Cu8Zr5. So the optimum composition is located along the e/a-variant line (Cu0.618Zr0.382)1-xAgx with equal to or larger than 8 at.% Ag addition. The e/a ratios are decreased with increasing Ag additions, but its varieties are smaller than 0.002, therefore electron concentration is comparatively a secondary factor.