Thermal stability of advanced Ni-base superalloys

Exposures consisting of 1 to 900 h at 1000 and 1100 °C after an ageing treatment of 16 h at 870 °C were used to study the thermal stability of selected γ′-strengthened Ni-based superalloys representing conventional, directional solidification, and single-crystal castings. Various techniques of microscopy, spectroscopy and diffraction were used to characterize the microstructure. Primary MC carbides in the alloys studied were found to be stable toward decomposition into lower carbides. In the aged condition, the strengthening γ′ phase assumed a cuboidal morphology; however, all alloys also contained varying proportions of coarse lamellar γ′ and hyperfine cooling γ′. On an atomic scale, the nature of the cuboidal γ′-matrix interface was found to vary from coherent to partially coherent. However, the overall lattice mismatch varied from one alloy to another depending upon its composition and the distribution of various elements in carbide phases and lamellar γ′ phase. Directional growth of the cuboidal γ′ phase upon exposure to higher temperatures was found to be accelerated by a large initial lattice mismatch leading to a considerable loss of coherency, as indicated by the observation of dislocation networks around the γ′ particles. Although the composition of the γ′ phase remained essentially unchanged, there was a marked change in matrix composition. Sigma phase was found to precipitate in all alloys, but its thermal stability was a function of alloy composition. The initial decrease in hardness followed by a hardening effect during exposure could be explained in terms of the partial dissolution of the γ′ phase and precipitation of sigma phase.