Effects of Heat Treatment and Chemistry on the Long-Term Phase Stability of a High Strength Nickel-Based Superalloy

Abstract

Phase stability of UDIMET@ alloy 720 (U720), a high strength, hot corrosion resistant alloy was studied as a function of chemistry, processing, heat treatment, and service temperature. U720 is relatively stable with respect to sigma phase when given a supersolvus heat treatment which results in a coarse grain creep resistant microstructure. When U720 is given a subsolvus heat treatment to preserve a fine grain high strength microstructure rapid precipitation of a globular grain boundary sigma phase occurs during 760°C exposure. This precipitation is associated with a degradation in tensile ductility, creep resistance and toughness. As much as a 60% reduction in initial tensile ductility was observed after 1000 hours of exposure while room temperature charpy v-notch impact strengths were reduced from 22 to 6 ft. lb. A modified version of UDIMET alloy 720LI (U72OLI) was found to be substantially less prone to precipitation of sigma phase in fine grain product. U72OLI maintains tensile properties and only suffers minor degradation in ductility after 1000 hours exposure at 760°C.