| Ti and Ti-alloys possess excellent properties including high specific strength, high toughness, good resistances to high temperature and corrosion, and good forgeability, etc., whih are ideal materials in the fields of areo-space, navigation, automobile, petroleum, chemical industry and medicine. Among them, (3-Ti solid solution alloys with BCC structure are applied widely with respect to the mechanical properties, processabilities. Compared withα-Ti and (α+(3)-Ti alloys,β-Ti solid solution alloys exhibt the highest specific strengths, lowest Young's modulus, high ductilities and good corrosion-resistances.β-Ti solid solution alloys with low Young's modulus are generally obtained in multi-component systems by trial-and-error tests. Although several empirical methods were proposed to guide composition design, they all lack a quantitative description of solid solution structures in complex systems, which is essential for the understanding on the composition rules of the seemingly complex alloys. we propose a cluster-plus-glue-atom model to solve the composition design of multi-component complex alloys from the structure viewpoint. This model dissociates an alloy structure into a cluster part and a glue atom part, i.e. isolated clusters linked with glue atoms, where the clusters are near-neighbor coordination polyhedra. The alloy composition is then expressed with [cluster](glue atom)x according to the model, x denoting the number of glue atoms matching one cluster. We analyzed the composition rules of typicalβ-Ti solid solution alloys with low Young's modulus by using this model. According to the enthalpy of mixingΔH between the solute atom and the base Ti, Solute atoms in the cluster model are classified into three parts:solute atoms with negativeΔH with Ti positions at the cluster center site; solute atoms with positiveΔH with Ti serve as glue atoms; and solute atoms similar with Ti can substitute for Ti. The knownβ-Ti solid solution alloy compositions satisfy cluster formula [CN14 cluster](glue)x, and the common characteristics of the cluster formulas are:there must be a BCC stabilizer such as Mo and Nb; the glue site is occupied by low-modulus elements; the cluster center site is occupied by an element with a large negativeΔH with Ti and preferably with a low Young's modulus. According to above rules, the present work designed serial (Ti,Zr)-(Mo,Sn,Al)-Nb multi-component cluster formula alloys based on Ti-Mo binary system with the cluster-plus-glue-atom model. Cluster formula alloy rods with diamenter of 3 mm and 6 mm were prepared by using copper-mould suction-casting method. Alloy strucutures were identified with XRD and optical microscopy (OM), and mechanical tests were finally carried on to the BCC alloys. The experimental results indicated that a series ofβ-Ti solid solution alloys with low Young's modulus and good synthesized mechanical properties can be obtained with the cluster model, where the [CN14 cluster](glue)1 alloys given by of 1:1 cluster model have the optimum properties. The parameters of [SnTi14]Mo1 alloy are respectively E=70 Gpa,σ0.2=835 MPa,σb=814 MPa andε=18%; and those of [Mo(Ti14-xZrx)]Nb1 alloy are respectively E=68 Gpa,σ0.2=739 MPa,σb=760 MPa andε= 8.7 %. Therefore, the 1:1 cluster model simplifies composition complexity in multi-component alloy systems. In addition, all these results also shew that it is difficult to ensure that an alloy has not only BCC structure but also low Young's modulus and good mechanical properties in simple alloy systems, which can be realized in multi-component alloy systems.
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