The precipitate evolution in the nickel-based superalloy IN738LC

Abstract

IN738LC is a polycrystalline, nickel-based superalloy, which is widely utilized in a variety of applications such as jet engines and land-base gas turbines. This material provides high performance at temperatures above 650°C. The expected properties at these temperatures are corrosion resistance, optimal thermal properties, creep and fatigue resistance. These required properties are obtained via solid solution hardening and precipitation hardening of face-centered cubic (FCC) nickel (Ni) matrix phase. Precipitates in the Ni-base superalloys are generally in the form of ordered intermetallic compounds. The size, morphology, and distribution of these precipitates determine the properties of the material. Thus, microstructure control and stabilization is very important for effective use of IN738LC. In this study, a number of heat treatments are employed for observation of the life cycle of a precipitate, which starts with nucleation, continues with growth, and ends with dissolution. First part of this thesis explores the formation of cooling precipitates. In the second part, formation of duplex-size microstructure at 1140°C is studied. Coarsening and dissolution mechanisms are discussed in the third part for the data obtained from heat treatments at 1120°C for 12-480 hours. These treatments have given the chance to observe agglomeration of precipitates. Growth features of both fine and coarse precipitates under actual thermal conditions are studied in the fourth part. Heat treatments are conducted at 950°C for 1-480 hours. The results have showed that fine precipitates grow faster than coarse ones at this temperature, but 480 hours are not sufficient to obtain a unimodal precipitate microstructure.