Characteristics of the γ′ precipitates at high temperatures in Ni-base polycrystalline superalloy IN738LC

IN738LC is a Ni-base cast superalloy used in land-base gas turbinesand aerospace applications. As in other superalloys γ′ precipitates contribute to strengthening of this alloy at hightemperatures. In this study, the authors investigate thecharacteristics and mechanisms of precipitate dissolution into thematrix solid solution. The precipitates grow in cuboidal shape up to1130°C, above that a duplex-size precipitate microstructure sets inupon quenching from the temperature range 1140–1150°C. Theduplex-size precipitate microstructure consists of two very distinctsizes of precipitates (fine and coarse). Holding for longer times inthe temperature range 1140–1150°C does not coarsen the fineprecipitates of the duplex microstructure. The source for theformation of the fine precipitates in the duplex microstructure isthe dissolution of the newly grown smaller-sized precipitates whenthe agings start from fine size precipitates and the “corner dissolution” of coarse precipitates when the startingmicrostructure consists of the maximum-sized cuboidal precipitates.At and above 1160°C, the duplex as well as the coarse precipitatemicrostructures dissolve to form a single-size fine precipitatemicrostructure upon quenching from any temperature up to 1225°C. A single-phase solid solution with no precipitates is obtained onlyupon quenching from 1235°C or above. The dissolution of coarseprecipitates and formation of the fine ones are found to be very fastprocesses in the corresponding temperature ranges. The fineprecipitates are postulated to form during quenching from thetemperature range 1140–1225°C and are considered to be of the “cooling” type.