Formation of intergranular phases in precipitation hardening nickel-based alloy 725

From the two possible intergranular phases in nickel alloy 725 (UNS N07725, Inconel 725), i.e., M ₂₃C ₆ and the σ-related F phase, the abundant formation of F phase was found to critically compromise the hydrogen embrittlement resistance of the alloy. Nevertheless, the processing conditions and microstructural features that favor the formation of one phase over the other are yet to be investigated. Herein, intergranular phases in commercial batches in their as-received condition and after selective thermal treatments were studied to better understand the precipitation behavior. The distribution of grain boundary planes of the nickel matrix was assessed using stereological analysis of electron backscatter diffraction data. Results showed a direct correlation between the extent of F phase precipitation and the population of grain boundaries terminated on (111) planes, which provide low-energy nucleation sites for F phase. Our results suggest that while intergranular precipitates formed during aging, the preceding thermomechanical and thermal treatments defined the relative abundance of nucleation sites for each phase by affecting the grain boundary character distribution. Thus, targeted grain boundary engineering approaches could be developed to avoid susceptible microstructures and improve the hydrogen embrittlement resistance of the alloy. Furthermore, the results may help produce samples with different extents of intergranular M ₂₃C ₆ and F phase to evaluate, in future studies, the individual effect of the phases on the properties of the alloy.