Ir–Hf–Zr ternary refractory superalloys for ultra-high temperatures—Phase and microstructural constitution

A phase and microstructural evaluation of Ir–Hf–Zr ternary alloys with a composition below 30 mol% (Hf + Zr) was conducted by microstructural observation using scanning electron microscopy (SEM), composition analysis using electron probe microscopy analysis (EPMA), and phase identification using X-ray diffraction analysis (XRD). Partial isothermal sections of the Ir–Hf–Zr ternary system close to the Ir corner at 1800 °C and 2000 °C were determined. Research revealed that the f.c.c. and L12–Ir3(Hf, Zr) two-phase regions, which are shown in the Ir–Hf and Ir–Zr binary systems, were connected from the Ir–Hf side to Ir–Zr side in the Ir–Hf–Zr ternary system at Hf + Zr contents of less than 25 mol%. The L12–Ir3Hf and L12–Ir3Zr phases were fully soluble with each other. An Ir3(Hf, Zr)/Ir(Hf, Zr) two-phase structure was found in the Ir–15Hf–15Zr alloy with the Hf + Zr contents of 30 mol%. The potential of the Ir–Hf–Zr ternary alloys as ultra-high-temperature structural materials is discussed from the viewpoints of the microstructure and the lattice misfit between the f.c.c. and the L12 phases. •Ir–Hf–Zr ternaries with different f.c.c. to L12 fractions were prepared.•Partial isothermal sections of Ir–Hf–Zr ternary close to Ir corner were determined.•L12–Ir3Hf and L12–Ir3Zr phases were fully soluble with each other and formed a complex L12–Ir3(Hf, Zr) phase.•f.c.c. and L12–Ir3(Hf, Zr) two-phase region existed at Hf + Zr content less than 25 mol%.•The potential of Ir–Hf–Zr alloys as ultra-high-temperature structural materials was discussed.