Influence of Iridium on the Properties of Gamma′-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 Co3W, 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.