THERMAL-STABILITY OF ADVANCED NI-BASE SUPERALLOYS

Exposures consisting of 1 to 900 h at 1000 and 1100-degrees-C after an ageing treatment of 1 6 h at 870-degrees-C were used to study the the rmal stability of selected gamma'-strengthened Ni-based superalloys re presenting conventional, directional solidification, and single-crysta l castings. Various techniques of microscopy, spectroscopy and diffrac tion were used to characterize the microstructure. Primary MC carbides in the alloys studied were found to be stable toward decomposition in to lower carbides. In the aged condition, the strengthening gamma' pha se assumed a cuboidal morphology; however, all alloys also contained v arying proportions of coarse lamellar gamma' and hyperfine cooling gam ma'. On an atomic scale, the nature of the cuboidal gamma'-matrix inte rface was found to vary from coherent to partially coherent. However, the overall lattice mismatch varied from one alloy to another dependin g upon its composition and the distribution of various elements in car bide phases and lamellar gamma' phase. Directional growth of the cuboi dal gamma' phase upon exposure to higher temperatures was found to be accelerated by a large initial lattice mismatch leading to a considera ble loss of coherency, as indicated by the observation of dislocation networks around the gamma' particles. Although the composition of the gamma' phase remained essentially unchanged, there was a marked change in matrix composition. Sigma phase was found to precipitate in all al loys, but its thermal stability was a function of alloy composition. T he initial decrease in hardness followed by a hardening effect during exposure could be explained in terms of the partial dissolution of the gamma' phase and precipitation of sigma phase.