Coalescence‐Ripening Mechanism of γ′−Ni ₃ (Al, Ti) During Long‐Term/High‐Temperature Operation of Ni‐Based Superalloy Discs

ABSTRACT The evolution of the γ′−Ni 3 (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.