Precipitate Formation and Evolution in the Superalloy IN738LC

IN738LC is a polycrystalline, nickel-based superalloy, which provides a high performance in aggressive environments at temperatures above 650°C. At this high temperature, the expected properties are good corrosion resistance, optimal thermal properties, and creep and fatigue resistance. These required properties are obtained via solid solution hardening and precipitation hardening of a face-centered cubic (fcc) Ni matrix phase. The size, morphology and distribution of precipitates determine the properties of the material. Thus, microstructure control is very important for effective use of IN738LC. In this study, a number of heat treatments were conducted to observe the evolution of precipitates. The formation of bimodal microstructure at 1140°C is explored. The data shows formation of a duplex size precipitate microstructure after 5 min of the aging. Formation mechanism of the duplex structure is discussed. Coarsening and dissolution mechanisms at 1120°C are discussed. Precipitates are found to grow up to a critical size in coherent and cubical morphology, beyond which dissolution sets in.