Effect of Grain Boundary Character on Intergranular Precipitation and Coverage in Ni–Cr Alloys

Abstract

Abstract Grain boundary precipitation strengthening is an effective strategy for improving the high-temperature mechanical properties of heat-resistant alloys. However, the grain boundary coverage fraction of intergranular precipitates cannot be controlled simply by thermodynamic factors, such as solute supersaturation, because precipitation behavior is also governed by grain boundary character, including both the crystallographic relationship between adjacent grains and the grain boundary plane orientation. In this study, the effect of grain boundary character on α -Cr precipitation along γ / γ grain boundaries was investigated in Ni–Cr binary alloys using electron backscatter diffraction and focused ion beam cross-sectional observations. The α phase precipitated preferentially along grain boundaries at low Cr supersaturation, whereas intragranular and discontinuous precipitation occurred with increasing supersaturation. Even under conditions where the average grain boundary coverage fraction became nearly saturated, a large boundary-to-boundary scatter was observed, indicating the importance of local grain boundary character. Notably, among grain boundaries with the same crystallographic relationship between adjacent γ grains, the precipitation morphology and coverage fraction varied systematically with grain boundary plane orientation. Based on the local crystallographic orientation analysis, grain boundary α precipitates were classified into three types: Type I, satisfying orientation relationships with both adjacent γ grains; Type II, satisfying an orientation relationship with only one adjacent γ grain and consisting of a single variant; and Type III, satisfying an orientation relationship with only one adjacent γ grain but consisting of multiple variants. The coverage fraction decreased in the order Type I > Type II > Type III. For Types II and III, the coverage fraction increased with decreasing angle ϕ between the grain boundary plane and the {111} γ habit plane. This trend was interpreted in terms of the reduced nucleation barrier for faceted γ / α interfaces. These results suggest that controlling both the crystallographic relationship between adjacent γ grains and the local grain boundary plane is important for enhancing grain boundary coverage in Ni–Cr alloys.