Four equi- or near equi-molar platinum-group-metals-containing high entropy alloys (PGM-HEAs) have been developed with the aim to identify promising materials for ultra-high temperature applications. Experimental results indicate that IrNiPtRh and IrNiPtRhCo HEAs can form single phase fcc solid-solution, while secondary phases are also formed within the fcc matrix in IrNiPtRhAg0.5 and IrNiPtRhAl0.5 alloys, which are identified as (Ag, Pt)-rich fcc2 and (Pt, Ni)3Al-L12 phases, respectively. For mechanical performance, the room to high temperature hardness of PGM-HEAs is outstanding, especially for IrNiPtRhAl0.5, which shows significantly improved hot hardness than those of Ni-based superalloys, such as IN718 and Udimet 720Li, at temperatures above 800 °C, while it also exhibits much higher melting tolerance. It is related to the contributions from the advantages of platinum-group-metals, strong solution hardening, and L12 precipitation strengthening, thus providing superior resistance against thermal softening. It is believed that the current findings will shed light toward a promising research direction for ultrahigh-temperature applications, which is hard to achieve for the traditional platinum-group-alloys. In addition, the design space of compositionally complex alloys for future investigations would be significantly broadened.
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