Characteristics of the gamma ' precipitates at high temperatures in Ni-base polycrystalline superalloy IN738LC

IN738LC is a Ni-base cast superalloy used in land-base gas turbines and aer ospace applications. As in other superalloys gamma' precipitates contribute to strengthening of this alloy at high temperatures. In this study, the au thors investigate the characteristics and mechanisms of precipitate dissolu tion into the matrix solid solution. The precipitates grow in cuboidal shap e up to 1130 degrees C, above that a duplex-size precipitate microstructure sets in upon quenching from the temperature range 1140-1150 degrees C. The duplex-size precipitate microstructure consists of two very distinct sizes of precipitates (fine and coarse). Holding for longer times in the tempera ture range 1140-1150 degrees C does not coarsen the fine precipitates of th e duplex microstructure. The source for the formation of the fine precipita tes in the duplex microstructure is the dissolution of the newly grown smal ler-sized precipitates when the agings start from fine size precipitates an d the "corner dissolution" of coarse precipitates when the starting microst ructure consists of the maximum-sized cuboidal precipitates. At and above 1 160 degrees C, the duplex as well as the coarse precipitate microstructures dissolve to form a single-size fine precipitate microstructure upon quench ing from any temperature up to 1225 degrees C. A single-phase solid solutio n with no precipitates is obtained only upon quenching from 1235 degrees C or above. The dissolution of coarse precipitates and formation of the fine ones are found to be very fast processes in the corresponding temperature r anges. The fine precipitates are postulated to form during quenching from t he temperature range 1140-1225 degrees C and are considered to be of the "c ooling" type. (C) 2000 Kluwer Academic Publishers.