The composition design is of importance for developing high- performance complex alloys and is also the primary step to realize a new mode for material development via theoretical prediction and experimental verification, in comparison with the traditional experience-oriented experiments. Traditional alloy design approaches, including Hume-Rothery rule, electron theories, equivalent method, computer simulation, etc., are first reviewed from the viewpoints of their theoretical basis and applicability to limitations. Almost all the traditional alloys are based on solid solution structures, in which the typical characteristic is the chemical short-range order (CSRO) of the solute distribution. We propose a cluster-plus-glue-atom model for stable solid solutions in light of CSRO. A cluster-formula composition design approach is presented for developing the multi-component high-performance alloys. The cluster- plus-glue-atom model classifies the solid solution structure into two parts, i.e., the cluster part and the glue atom part, where the clusters are centered by solute atoms, showing the strong interactions of clusters with the solvent base and the weak interactions of clusters with solute atoms. The clusters are the nearest-neighbor polyhedrons, being cuboctahedron with a coordination number of 12 (CN12) in FCC structure and rhombic dodecahedron with a CN14 in BCC structure, respectively. Then a uniform cluster-formula of [CN12/14 cluster](glue atom)(x) is achieved from the cluster model. Its wide applications in different multi-component alloy systems confirm its universality as a simple and accurate tool for multiple-component complex alloy composition design. Such alloy systems include corrosion-resistant Cu alloys, high-performance Ni-base superalloys, high-strength maraging stainless steels, Ti/Zr alloys with low Young's modulus, high-entropy alloys, amorphous metallic glasses, quasicrystals, etc.. The specific alloy design steps are incarnated in the beta-Ti alloys with low Young's modulus. Firstly, the necessary alloying elements are chosen according to the service requirements (BCC stability and low Young's modulus). Secondly, the local cluster unit to present CSRO and the corresponding cluster formula of [(Mo, Sn)-(Ti, Zr)(14)](Nb, Ta)(x) are built, in which the occupations of the alloying elements in the cluster formula are determined by the enthalpy of mixing. Delta H between them with the base Ti. Thirdly, these designed alloys are verified experimentally, and the lowest Young's modulus appears at the beta-[(Mo0.5Sn0.5)-(Ti13Zr1)] Nb-1. Finally, a new Mo equivalent formula under the guidance of phase diagram features is proposed to characterize the structural stability of Ti alloy. Thus all the Ti alloy compositions with different structural types can be expressed with a uniform cluster formula, in which the structural types of alloys are determined by the Mo equivalent.
