Influence of Iridium on the Properties of γ′‐Strengthened Co‐Base Superalloys

Abstract

The influence of iridium on the phase stability, oxidation behavior, and high temperature creep strength of γ′‐hardened Co‐base superalloys is discussed. Ir increases the γ′‐solvus temperature and γ′‐volume fraction since it partitions slightly to the γ′‐phase. However, Ir leads also to a discontinuous precipitation transformation of the metastable γ/γ′‐microstructure into a lamellar microstructure of Co 3 W, CoAl, and a Co‐rich fcc phase for the W‐rich alloys. A reduction of W in presence of Ir leads to a higher phase stability. The constrained lattice misfit between the γ and γ′ phase in the Ir and high W containing alloy is +0.5%. The alloying element Ir strongly increases the creep strength at 850 °C as long as the γ/γ′‐microstructure is not discontinuously transformed. Furthermore, the oxidation resistance is improved by the addition of iridium.