MICROSTRUCTURE EVOLUTION DURING ISOTHERMAL AGING OF MULTIMODAL NICKEL-BASED SUPERALLOYS

The morphological evolution of secondary gamma' precipitates under the coarsening process was investigated for commercial wrought Ni-based superalloys, which can be classified into two processes, i.e. "localization process" and "aggregation process". The localization process was defined as a phenomenon in which cuboidal gamma' precipitates were arranged in the <100> direction for superalloys. In contrast, the aggregation process was defined as a phenomenon in which neighboring spherical gamma' precipitates coarsen while overlapping their interfaces for superalloys. All the wrought Ni-based superalloys could be classified into the above two processes based on their volume fraction and lattice misfit. The coarsening of y' precipitates follow the aggregation process when the misfit is smaller than 0.05%, and it follows the localization process otherwise.