Influence of Variable Temperature Aging on the Microstructure of Nickel-Based Superalloys

Nickel-based alloys undergo complex temperature variations during actual service. Thermal fluctuations may alter elemental diffusion pathways and rates, thereby affecting microstructural homogeneity. Current long-term aging studies predominantly focus on isothermal conditions, while microstructural evolution and performance degradation under non-isothermal aging remain inadequately explored. This study aims to investigate the differences in γ′ phase evolution between isothermal and non-isothermal aging in nickel-based alloys, elucidate the influence of variable temperature sequences on microstructural characteristics, and establish quantitative relationships among microstructure, aging time, and temperature, including equivalent time calculations. The results reveal that the growth kinetics of γ′ phase size in DZ411 alloy under both non-isothermal and isothermal aging follows an identical time-temperature function: the cube of phase diameter exhibits a linear dependence on aging time and temperature, with Feret ratio (FR) fluctuating within comparable ranges. Furthermore, variable temperature sequences systematically govern γ′ phase dimensions. Microstructural observations demonstrate that sequential aging from 900 ℃ to 980 ℃ produces larger γ′ phases than the reverse sequence (980 ℃-900 ℃). Quantitative analysis confirms that the average equivalent diameter (D) for the 900 ℃-980 ℃ sequence is 719 nm, which is larger than that for the 980 ℃-900 ℃ sequence (665 nm). Additionally, variable temperature sequences regulate γ′ phase morphology: rounded rectangular particles dominate in the 900 ℃-980 ℃ sequence, while near-spherical shapes prevail in the 980 ℃-900 ℃ sequence, supported by distinct FR values (1.411 vs. 1.379).