High temperature strength of Ir-based refractory superalloys

The purpose of this paper is to understand the high-strength and deformation mechanism of new-generation 'refractory superalloys'. Refractory superalloys are defined as alloys that have an fcc and L1₂ two-phase coherent structure and yet considerably high melting points. This paper examines compressive strength up to 1800°C and creep behavior at 1500°C of Ir-V, -Ti, -Nb, -Ta, -Hf, and -Zr binary refractory superalloys. It is indicated that the precipitation-hardening effect depends on L1₂ precipitate morphology. Plate-like precipitates are more effective for precipitation hardening than cuboidal precipitates. Shearing of precipitates was observed in the alloy with plate-like precipitates using TEM. When plate-like precipitates form, a dislocation cannot bypass around a plate, thus being forced to shear a precipitate. However, high-coherency strain energy at the interface prevents the dislocation motion. Therefore, large precipitation hardening appeared in the alloy with plate-like precipitates. On the other hand, the creep resistance of the alloy with plate-like precipitates was smaller than that of the alloy with cuboidal precipitates because discontinuous coarsening occurs and the microstructure becomes coarse in the alloy with plate-like precipitates.