High-entropy alloys (HEAs) with a single hexagonal close-packed (hcp) structure were found in Ir26Mo20Rh22.5Ru20W11.5 and Ir25.5Mo20Rh20Ru25W9.5 alloys annealed at 2373 K for 1 h. These HEAs were designed based on a sandwich strategy for valence electron concentration (VEC) of the constituent elements and by referring to their crystallographic structures in the periodic chart. The initial component and composition of these alloys were determined by considering exact and near equiatomic sub binary phase diagrams, followed by optimizing the composition by Thermo-Calc with the TCHEA3 database for HEAs as well as the SSOL5 database for solid solutions. The X-ray diffraction analysis, scanning electron microscopy observation, and energy-dispersive X-ray spectroscopy analysis revealed that the Ir26Mo20Rh22.5Ru20W11.5 and Ir25.5Mo20Rh20Ru25W9.5 alloys annealed at 2373 K for 1 h were formed in an hcp single phase. The formation of the hcp structure was principally evaluated by the VEC values of 7.855 and 7.865 for Ir26Mo20Rh22.5Ru20W11.5 and Ir25.5Mo20Rh20Ru25W9.5 alloys, respectively, such that these values were close to VEC = 8 for pure elements Ru and Os with an hcp structure. It is considered that Ru is a strong hcp-forming element under this strategy because the other pure constituent elements have different crystallographic structures. The formation of Ir26Mo20Rh22.5Ru20W11.5 and Ir25.5Mo20Rh20Ru25W9.5 HEAs with an hcp structure is unique because these alloys, which consist of only transition metals, can be produced without applying high pressure, similar to a CrMnFeCoNi HEA with an hcp structure.
- High-entropy alloys
- Transition metals
- Valence electron concentration