Exact equi-atomic senary alloys including three elements from 3d, 4d and 5d transition metals (TMs) were investigated for their ability to form solid solutions as high-entropy alloys (HEAs). Three alloys of CoCuPdTiZrHf, CoCuFeTiZrHf and AgAuCuNiPdPt were selected by focusing on (Ti, Zr, Hf) from Early-TMs, and (Cu, Ag, Au) and/or (Ni, Pd, Pt) from Late-TMs based on an alloy design with a help of Pettifor map for binary compounds with several stoichiometries and binary phase diagrams, together with a marginal Al4CoNiPdPt alloy. The XRD analysis revealed that the CoCuPdTiZrHf alloy was formed into a bcc, whereas both the CoCuFeTiZrHf and Al4CoNiPdPt alloys were a CsCl, and the AgAuCuNiPdPt alloy was dual fcc structures. The observations with optical and scanning-electron microscopes and analysis with energy dispersive X-ray for chemical composition revealed the homogeneous morphologies of these alloys in micrometer scale. The types of crystallographic structures of the CoCuPdTiZrHf, CoCuFeTiZrHf and AgAuCuNiPdPt HEAs and the Al4CoNiPdPt alloy can be principally explained by valence electron concentration. Three constituent elements from TMs in the same group enhance the increase in the number of complete solid solutions in the constituent binary systems, leading to forming these HEAs.
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