Grain boundary diffusion in a compositionally complex alloy: Interplay of segregation, precipitation and interface structures in a Ni–Cr–Mo alloy

Grain boundary complexions in the Hastelloy C-22 are modified to unravel the structure–property interplay of grain boundary precipitation (carbides), segregation, and grain boundary diffusion rates. Annealing treatments are used to create distinct grain boundary complexions with specific segregation-precipitation-structure states. Two grain boundary states were compared: those induced by low- (at 873 K, mainly for segregation) and by high-temperature (at 1123 K, for carbide precipitation) annealing treatments. A correlative analytical microscopy–tracer diffusion analysis approach is followed to elucidate the influence of the carbide formation, element segregation, and structure evolution on grain boundary diffusion. The tracer measurements with the 51Cr and 63Ni radioisotopes reveal a strongly non-linear dependence of the grain boundary diffusion rates on the pre-annealing time. Via in-depth characterization of the grain boundary microstructure, the evolution of Mo segregation and of incoherent metallic carbides at the interfaces is quantitatively related to the captured monotonic grain boundary diffusion behavior in Ni–Cr–Mo system.