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 L1 ₂-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.