| Methods and procedures for composition design of ideal bulk metallic glasses (BMGs) are established via cluster structures. The validity and mechanism of the composition design were analyzed theoretically and were verifyed by experiments.Three-step procedures for BMG composition design were presented basing on the "cluster plus glue atom" structural model. This model dissociates a metallic glass structure into two parts, basic cluster and glue atoms, and the resultant compositions are expressed in a unified cluster formula [cluster]1(glue atom)x, where x is the number of glue atoms. At the first step toward composition design, a suitable cluster should be found from eutectic phases in binary phase diagram to decipher the binary eutectic composition and to constitute the eutectic cluster formula [cluster](glue atom)x, x～1,3. At the second step, the glass-forming abilities are improved by element substitutions in the basic eutectic cluster formula. At the third step, equilibrium of electrochemical potentials of electrons between the cluster and the glue parts should be established to determine the compositions of ideal metallic glasses with high glass-forming ability.Then, Co-B-based B-Co-Si-Ta BMGs were examined to verify the composition design methods. Firstly, the Co-B system eutectic composition C063B37 can be well deciphered with a cluster formula [B3Co7]B=C063.6B36.4, the CN9-B3Co7 cluster being taken from the CoB eutectic phase with the BFe-type structure. In this B3C07 cluster, B occupies the center, six Co atoms take the corners of the trigonal prism, and one Co and two B cap the three quadruple side facets; Secondly, Si, Ta and Nb were selected as substitution atoms in different sites of the cluster formula; Thirdly, the glue atom number of ideal cluster formula was precisely determined by electrochemical potential equilibrium. The designed compositions were all cast into BMGs by copper mould suction casting. The experimental results verify that BMG composition [B3Co7](Si0.62Ta0.38)1.43=B26.2Co61.2Si7.8Ta4.8 possesses the highest glass-forming ability, reaching a critical diameter of 4mm.φ5mm BMG rod at [B3(Co0.65Ni0.35)7](Si0.62Ta0.38)1.43=B26.2(Co0.65Nio.35)61.2Si7.8Ta4.8 could be obtained by using Ni to substitute for Co.B-Ni-Fe-Si-Ta and B-Fe-Si-Nb BMGs were designed by a cluster line approach, which is a phase diagram presentation of the cluster formula [cluster](glue atom)x. The experimental results show that good glass-forming compositions were developed from ideal cluster formulae [B2Nig]Si1.33 and [B2Fe8]Si1.33. Using the same formulae and altering the elemental substitution, BMGs with improved glass-forming abilities were fabricated. For example, the improved BMGs compositions are [B2(Ni0.65Fe0.35)8](Si0.66Ta0.34)1.33 B17.7(Nio.65Fe0.35)70.5Si7.8Ta4 and [B2Fe8](Si0.66Ta0.34)1.33=B17.7Fe70.5Si7.sNb4, respectively.The relationship of reduced glass transition temperature Trg and glass-forming ability was also investigated in these BMGs. It is shown that the largest Trg corresponds to a local high glass-forming ability, but Trg is not a proper glass-forming indicator for BMGs with different cluster structures.In addition, the mechanical, soft magnetic and corrosion resistant properties of the obtained BMGs were investigated. The highest fracture strengthσf=4870 MPa and Vickers hardness Hv=14.5GPa appeared at Co-B-based composition [B3Co7](Si0.62Ta0.38)1.43= B26.2Co61.2Si7.8Ta4.8.Good soft magnetic properties were found at Fe-B-based composition [B2Fe8](Si0.66Nb0.34)1.33=B17.7Fe70.5Si7.8Nb4, with a fairly high saturation magnetization Is= 1.36T and a low corcive force Hc=3.3Am-1. Good corrosion resistant composition was located at [B2(Ni0.65Fe0.35)8](Si0.66Ta0.34)1.33=B17.7(Nio.65Feo.35)70.5Si7.8Ta4.Finally, according to the fact that all the clusters evolved in the present BMGs contain a common fundamental CN6 trigonal prism cluster, the (Co,Ni,Fe)-B-based BMGs compositons were further deciphered by a unified [B-TM6]M1 (TM=Co,Ni,Fe) model. The idealized Co-B-, Ni-Fe-B- and Fe-B-based BMG compositions [BCo5(Si0.62Ta0.38)1]B1= [B(Ni0.65Fe0.35)5(Si0.66Tao.34)1]((Ni0.65Fe0.35)0.67B0.33)1 and [BFe5(Si0.66Nb0.34)1](Fe0.67B0.33)1=B16.4Fe70.9Si78.2Nb4.3 were revealed to possess high reduced glass transition temperature Trg, reaching 0.657,0.615 and 0.604, and high glass-forming abilities with critical diameters 4mm,3mm and 3mm, respectively. The [B-TM6]M1 model can also decipher all the reported typical B-TM-Si-based BMG compositions. In comparison with the reported B-TM-Si-based BMG compositions, our quaternary Co-B-based B-Co-Si-Ta BMGs have simple compositions and higher glass-forming abilities and higher compression fracture strength, Vickers hardness and elastic modulii. The alloy design principle developed in this work may also shed light on designing new bulk metallic glasses in other alloy systems.
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