Influences of composition and grain size on creep–rupture behavior of Inconel® alloy 740

Creep–rupture experiments were conducted on multiple heats of the nickel-based superalloy Inconel® 740 at temperatures between 923 and 1123 K (650 and 850 °C). The interactions between chemistry, microstructure, and creep performance were evaluated by analysis of creep data, optical microscopy, electron microscopy, and computational thermodynamics. The data show that grain size has a modest effect on the creep–rupture strength. Computational thermodynamics verified experimental observations of the formation of eta phase as a function of temperature and alloy chemistry, but the kinetics for the precipitation of eta phase did not agree with the experimental findings. Despite the formation of eta phase and the concomitant reduction in volume fraction of gamma prime, the creep resistance of the alloy is insensitive, within the range of chemistries tested, to the volume fraction of gamma prime. The creep ductility was found to increase with test temperature. Precipitation of a large volume fraction of eta phase (greater than 7%) appears to reduce the creep–rupture ductility, but smaller amounts do not produce adverse effects.