Correlation between grain boundary character distribution and δ-phase precipitation in nickel-based superalloy 718

Abstract

The nickel-based superalloy 718 after processing through three thermomechanical routes, i.e., forged, hot rolled, and cold rolled, was investigated considering its δ-phase (Ni3Nb–DOa) intergranular precipitation. The samples were subjected to different combinations of annealing and aging heat treatments to obtain different grain boundary character distribution (GBCD) as well as different δ-phase volume fractions and morphologies. While δ-phase precipitation can be beneficial to avoid grain growth during recrystallization, an increase in volume fraction can be detrimental in several aspects. Deliberate manipulation of GBCD through thermomechanical processing was found to be a key to improving Ni-based superalloy properties. The present study quantified δ-phase occurrences along the GBs based on electron backscattering diffraction analysis to identify preferential precipitation sites in consonance with GB misorientation angles. The results show that random high-angle boundaries with Ʃ > 29 are more populated with δ-phase than low coincidence site lattices (CSL), Ʃ < 29, special boundaries, focusing specifically on Ʃ3n class of boundaries. Nonspecial triple junctions (0- and 1-CSL) are also preferential nucleation sites when compared to 2- or 3-CSL triple junctions. Keywords: Ni-based superalloys, δ-phase, Grain boundary character distribution, Electron backscattered diffraction, Intergranular precipitation