Influence of carbon on microstructural stability and stress rupture properties of an additively manufactured superalloy

The effects of carbon content (0.05, 0.1, and 0.15, in wt.%) on the microstructural evolution and stress rupture behavior of an additively manufactured Ni-based superalloy (ZGH451) were comprehensively investigated. Long-term thermal exposure tests at 1,000 °C reveal that γ′ precipitates follow a diffusion-controlled coarsening process consistent with the Lifshitz-Slyozov-Wagner (LSW) model, while higher carbon contents increase the effective diffusion coefficient and accelerate coarsening. The evolution of carbides displays a strong dependence on carbon level: low-carbon alloys maintain discrete, fine carbides that stabilize grain boundaries, whereas excessive carbon (>1wt.%) promotes the formation of coarse and continuous carbide films along grain boundaries, serving as preferential crack initiation sites. Stress rupture tests conducted at 760 °C/780 MPa and 980 °C/260 MPa show a pronounced reduction in rupture life with increasing carbon content due to intergranular cracking, carbide-induced stress concentration, and carbon-assisted rafting of γ′ precipitates. Overall, an optimal carbon content near 0.05wt.% provides a balance between microstructural stability and high-temperature strength. These findings provide guidances for the compositional optimization of additive manufactured superalloys intended for long-term high-temperature service.