Cluster Line Criterion And The Formation Of Cu-Zr(Hf)-based Ternary Bulk Metallic Glasses

Bulk metallic glasses (BMGs) are a special kind of metastable complex metallic alloys with important potential applications. Though many BMGs have been developed, this research is mainly empirical one, relying on extensive trial-and-error experiments. In the present study, the formation and composition optimization of BMGs in Cu-Zr-Al, Cu-Zr-Ti, Cu-Hf-Al and Cu-Hf-Ti ternary systems are comprehensively investigated by using our composition criteria, among which the cluster line criterion is particularly focused.The cluster line criterion, previously named the e/a-variant criterion, refers to a specific composition line linking a specific cluster composition to the third element, which reflects the structure relationship between the optimized ternary amorphous alloy and the basic binary cluster. It can be regarded as the growth pathway from the binary cluster to a ternary phase. The selection of the binary cluster is thus the priority before the criterion can be applied. The Cu-Zr cluster structures are then studied for such a purpose, and the cluster selection rules relevant to glass formation are proposed which include topological packing rule (atomic size rule), chemical short range order rule (optimized dissimilar atom coordinations) and kinetic rule (cluster composition close to deep eutectic point and far away from crystalline phases). Then, three special clusters centered by Cu atom are selected by using these three rules: icosahedron Cu₈Zr₅ (Cu₈Hf₅), capped Archimedean antiprisms Cu₆Zr₅ (Cu₆Hf₅) and CU₅Zr₆ (Cu₅Hf₆). The cluster lines are constructed by linking the special clusters to the third alloying element. For comparison, other special binary composition points, such as deep eutectic points and the optimum binary glass-forming composition are also used to construct composition lines.A series of alloy compositions are designed along these specific composition lines in the Cu-Zr-Al, Cu-Zr-Ti, Cu-Hf-Al and Cu-Hf-Ti ternary systems. The formation ranges of BMGs with a diameter of 3 mm by using copper mould suction casting method are determined. Furthermore, XRD, TEM and thermal analyses are carried out on the bulk amorphous samples.The effective electron contribution e/a of Zr (Ti, Hf) in different BMG compositions is calculated with experimental diffraction method. The e/a value of the alloying Zr is about 2.0 in Cu-based amorphous alloys, and that of the basic Zr is about 1.5 in Zr-based amorphous alloys. Thermal analysis indicates that the thermal parameters of Cu-Zr-Al and Cu-Hf-Al serial BMGs, which characterize the thermal stabilities and glass forming abilities (GFAs), increasemonotonically with increasing Al content (electron concentration e/a). While the thermal parameters of Cu-Zr-Ti and Cu-Hf-Ti serial BMGs decrease with increasing Ti content (average atomic size Ra). In these four systems, the BMGs located on the cluster lines linking icosahedron CugZrs to the third element have relatively high thermal stabilities (high Tg) and large reduced glass transition temperature (Trg).The relationship between the BMG compositions with the highest Trg and the largest GFA is further studied. It is pointed out that the BMG composition with the highest Trg corresponds to the most stable ternary cluster composition (the optimum cluster composition), but not the finally optimized BMG composition. The largest GFA composition is located on the intersecting point of two composition lines, one being e/a-constant line with e/a value of the ternary optimum cluster composition, the other being i?a-constant line with Ra value of binary basic cluster. The experimental results verify the validity of the relationship. In Zr-Al-Ni system, the ternary optimum cluster composition Zr53Ni23.sAl23.5 is located on the icosahedron ZrgNLt-Al line, and the largest GFA composition is Zrsg.elNfoojAko.?. The Cu-Zr-Al BMGs can be divided into Cu-based and Zr-based BMGs, and the Cu and Zr-based optimum cluster composition Cu58.1Zr35.9Al6 and Cu39.7Zr47.iAli3.2 are respectively located on the dense packing CugZrs-Al and CusZre-Al cluster lines, which coincide with the largest GFA compositions. In Cu-Zr-Ti system, the optimum cluster composition Cu57.2Zr35.3Ti7.5 is located on the CusZrs-Ti cluster line, and the largest GFA composition is 0157^32.9^9.9. Therefore, the BMG compositions with the highest Trg values will not always have the largest GFAs.Finally, Cu-based BMG Cu58.1Zr35.9Al6 is annealed at different temperatures, and the devitrification phases are CuioZr7 and CugZ^. These two crystalline phases contain the local cluster structures relevant to glass formation.