Ni-Nb-based Bulk Metallic Glasses Issued From Cluster Formula Of Basic Binary Eutectic

Composition design is considered to be one of basic issues in the study of bulk metallic glasses (BMGs). Cluster-plus-glue atom model, as one of the composition design methods, has been successfully applied in several metallic glass systems. In this paper, we illustrate the composition design procedures in metal-metal type Ni-Nb based BMGs using cluster-plus-glue atom model systemically. According to this model, the structure of glassy alloys could be dissociated into a cluster part and a glue atom part, demonstrated as [cluster](glue atom)1,3. Here the cluster refers to a certain coordination polyhedron and the glue atoms lie in the cluster-packing interstitials. The first step in applying cluster-plus-glue atom model is the determination of appropriate cluster and then builds cluster formulae for basic binary eutectic and metallic glass compositions. Next, the alloying element is added in the basic binary composition formula to further enhance the glass forming ability (GFA). In Ni-Nb binary system, M-Ni6Nb6 icosahedron derived from a eutectic phase NiNb (Fe7W6 type) is determined to be principle cluster. It was then pointed out that eutectic composition Ni59.5Nb40.5 and the best binary glass former Ni62Nb38 could be interpreted as [M-Ni6Nb6]Ni3= Ni59.4Nb40.6 and [Ni-Ni6Nb6]Ni3=Ni62.5Nb37.5, respectively. Besides, the BMG-related eutectic compositions Ni59.5Nb40.5 in Ni-Nb system could also be deciphered by the mean combination of two cluster-plus-glue atom formulae derived from the two eutectic phases. Moreover, the two formulae corresponds to the stable liquid compositions in both sides of eutectic, exhibiting the intrinsic relationship between eutectic and BMG structure. Ni-Ta binary system is quite similar with Ni-Nb system. The eutectic and binary BMG compositions in Ni-Ta system could also be deciphered by cluster-plus-glue atom model.Based on the binary BMG composition formula [Ni-Ni6Nb6]Ni3 and similar element substitution principle, the third elements are added to enhance GFA. Ni-Nb-(Zr, Ta, Ag, Cu) ternary BMGs with critical size of 3 mm are obtained using copper mould suction method. The GFA is further enhanced with additions of the fourth elements. The critical sizes of Ni-Nb-Ta-(Zr, Cu) glassy alloys are 4 mm, which are the largest in Ni-Nb based BMGs. The results proved the availability of cluster-plus-glue atom model in designing Ni-Nb based BMG compositions.The thermal, mechanical and corrosion resistance properties of the obtained Ni-Nb based BMG alloys are investigated in detail. These BMGs all exhibit high thermal stabilities with large Tg (glass transition temperature) and Tx (crystallization temperature) values, among which the [Ni-Ni6Nb5Ta]Ni3=Ni62.5Nb31.25Ta6.25 ternary BMG alloy has the highest Tg value (935 K). Besides, these Ni-Nb based BMGs exhibit high fracture strength and vicker's hardness. Room-temperature compressive curves of [Ni-Ni6Nb5Zr]Ni3 and [Ni-Ni6Nb5Ta]Ni3 alloys show limited plasticity (～0.3%) before failure. The fracture strength of the [Ni-Ni6Nb5Zr]Ni3 and [Ni-Ni6Nb5Ta]Ni3 BMGs are about 3.2 GPa and 3.4 GPa, respectively. Electrochemical tests indicate that Ni-Nb based BMGs have better corrosion resistance properties than common Zr-based BMGs in 1 mol/L HCl at room temperature. Immersion test in 3 mol/L HCl proves that the corrosion resistances of the BMGs are closely related to the alloying elements.The electron concentration (e/a) is an important factor related with formation of BMGs. According to the nearly-free-electron model and spherical cluster packing model, an experimental measured e/a method and a theoretical calculated e/a method are proposed. The e/a value of Ni-Nb BMGs are calculated and then the electron contributions of the alloying elements are analyzed. Zr, Hf additions could increase the e/a value of Ni-Nb based BMGs and Ti, V additions decrease the e/a value of Ni-Nb based BMGs. Ta, Fe, Co, Cu additions show no significant change to the e/a value. The e/a effect on glass formation is discussed and clarified. The e/a value, together with chemical interaction and atom size mismatch, etc., jointly determine the formation of BMG.