Coalescence-Ripening Mechanism of γ′−Ni₃(Al, Ti) During Long-Term/High-Temperature Operation of Ni-Based Superalloy Discs

The evolution of the γ′−Ni₃(Al, Ti) phase in superalloys is governed by a coupled coalescence-ripening mechanism driven by elemental diffusion. In this study, thermal exposure experiments were performed at 750°C and 800°C for durations ranging from 100 to 20,000 h. The γ′ phase underwent coalescence-ripening, with its morphology evolving from small spherical particles to elliptical and finally to large spherical particles. During this process, high chemical potential gradients were observed around small-diameter γ′ phases and at the long-axis ends of elliptical γ′ phases. These gradients drove the dissolution of smaller γ′ phases by larger ones and the evolution of elliptical γ′ into spherical ones. Concurrently, the Ni/(Al + Ti) ratio in the γ′ phase decreased from 3.26 to 3.02 during exposure at 800°C. High-resolution scanning transmission electron microscopy (HR-STEM) analysis revealed complex atomic arrangements at the γ/γ′ interface and the dislocation, featuring variations in interplanar distance and abundant edge dislocations, which serve as efficient pathways for atomic diffusion. Moreover, as the γ′ phase coarsened, the γ/γ′ interfaces remained coherent, with the lattice mismatch gradually approaching zero. Geometric phase analysis (GPA) indicated a significant relaxation of elastic strain at these interfaces. Based on the change in Al concentration at the γ/γ′ interfaces, the interfacial energy decreased by approximately 3 times between 100 and 20,000 h of exposure at 800°C. This study provides foundational insights for a deeper understanding of elemental diffusion mechanisms in superalloys.