Microstructure and mechanical properties of HCP-structured IrRhRuReFe high-entropy alloys

Hexagonal close-packed (HCP) high-entropy alloys (HEAs) based on platinum-group metals (PGMs) are promising candidates for extreme-temperature applications, but the links between solidification-induced chemical heterogeneity and high-temperature deformation kinetics remain insufficiently understood. In this study, three as-cast Ir–Rh–Ru–Re–Fe alloys with a single HCP solid-solution matrix were investigated. Scanning electron microscopy combined with energy-dispersive spectroscopy revealed dendritic/interdendritic chemical partitioning in all alloys. A key finding is that Fe addition changes the segregation behavior of Ru from slight interdendritic enrichment in the Fe-free alloy to dendrite-region enrichment in the Fe-containing alloys. This trend is consistently supported by both liquidus-near liquid/solid partition coefficients and Scheil–Gulliver solidification simulations. X-ray diffraction confirmed a single-phase HCP structure within the instrumental detection limit. In situ strain-rate-jump compression tests at room temperature and 1500 °C showed clear positive strain-rate sensitivity, and the alloys retained substantial strength at 1500 °C; for example, the Ir20Rh20Ru20Re20Fe20 alloy reached ~270 MPa at 3 × 10⁻⁴ s⁻¹. Among the investigated compositions, 20Fe exhibited the highest strength at both temperatures. Apparent stress exponents and activation volumes derived from the strain-rate-jump tests indicate a transition from short-range, thermally activated glide at room temperature to a high-temperature regime involving larger effective activation events. These results clarify how Fe modifies solidification partitioning and the retained chemical heterogeneity of HCP Ir–Rh–Ru–Re-based HEAs, and how these changes are reflected in their extreme-temperature mechanical response.