Dependence of intergranular precipitation on grain boundary characteristics in Ni–Cr–Fe–Mo–Cu alloy

Hastelloy G30 is a typical representative of Ni–Cr–Fe–Mo–Cu alloys, which have excellent corrosion resistance and mechanical properties. High alloying-element content induces a risk of intermetallic precipitation, which is poorly eliminated in steel/nickel bimetallic composite manufactured by hot forming or welding. In this study, aged material was observed using transmission electron microscopy, electron backscatter diffraction, and scanning electron microscopy. The results show that the precipitates formed in Hastelloy G30 after aging at 800°C comprised Cr- and Mo-rich σ phase. More than 95% of Σ1 and Σ3 boundaries were free from σ phase, and about half of the other low-Σ boundaries were immune to the formation of σ phase. All Σ9, Σ27, and random high-angle grain boundaries exhibited low intergranular precipitation resistance of approximately 5%–10%. The (301) planes of the σ phase and the (−220) planes of the matrix showed semicoherent lattice matching. Higher grain-boundary energy and serrated grain boundaries provided opportunities for σ phase precipitation to occur.