Effect of overheating on the microstructure of DD5 superalloy

The second-generation nickel-based single crystal superalloy DD5 serves as a key material for aero-engine hot-section components. Its microstructural stability and performance are critically challenged under overheating conditions, which occur when the service temperature exceeds its maximum design limit of 1100 °C. Although considerable attention has been given to topologically close-packed (TCP) phase formation during conventional service, far less is known regarding the microstructural evolution under extreme overheating conditions (≥1150 °C). This work systematically investigates the microstructural response of DD5 alloy under short-term (30 min) and long-term (100 h) overheating treatments at 1100-1200 °C. The results reveal that short-term overheating induces non-equilibrium dissolution and reprecipitation of the γ′ phase, which results in limited degradation in microhardness. In contrast, long-term overheating promotes Ostwald ripening, leading to significant γ′ coarsening and a pronounced reduction in area fraction. Notably, characteristic acicular M 6 C carbides precipitate via re-nucleation rather than in-situ transformation at 1100-1150 °C after 100 h, forming a semi-coherent interface with the γ matrix that follows the orientation relationship: [0,-1,1] γ // [0,-1,1] M6C and (1,-1,-1) γ //(1,1,1) M6C . At higher temperatures (1150-1200 °C), interfacial pores nucleate through thermal-stress-driven vacancy aggregation. The observed microhardness degradation directly correlates with γ′ deterioration, demonstrating that the microstructural damage from prolonged overheating is irreversible and adversely affects service reliability. Consequently, this study elucidates the microstructure evolution and damage mechanisms in the DD5 alloy under extreme overheating, providing critical theoretical and experimental support for the condition assessment of turbine blades.