Microstructural Degradation and the Effects on Creep Properties of a Hot Corrosion-Resistant Single-Crystal Ni-Based Sup
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loys are widely used in manufacturing of industrial gas turbine (IGT) components that require both good high-temperature strength and hot corrosion resistance.[1–4] The typical microstructures of Ni-based superalloys contain ordered L12 c¢-phase cuboids in a continuous fcc c-phase matrix with completely coherent {100} interfaces between the two phases.[5] The high volume fraction, optimized size, and distribution of c¢-phase cuboids as well as the high c/c¢ lattice mismatch in single-crystal superalloys prevent
X.W. JIANG, D. WANG, DI WANG, X.G. LIU, W. ZHENG, Y. WANG, G. XIE, and L.H. LOU are with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted March 15, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
dislocation movements in the c-phase matrix, resulting in the c¢ cutting or formation of dislocation networks along the c–c¢ interfaces and the excellent creep performance.[6–8] However, the very high volume fraction of c¢ precipitates and c/c¢ lattice mismatch of single-crystal superalloys promote c¢ coarsening and rafting during long-term thermal exposure (LTTE), which may result in fast deterioration of mechanical properties.[8–13] The lattice mismatch between the c matrix and coherent c¢ precipitates induces large internal stresses within single-crystal superalloys. The lattice misfit stresses have a major effect on its mechanical behavior and on the microstructural evolution of the microstructure (e.g., c¢ coarsening and rafting) during the lifetime.[14–16] In the past related studies, the main interests have been concentrated on investigating the stability or instability of the c/c¢ microstructure and structure evolution of dislocation networks under the influence of lattice misfit stresses and different creep conditions, including temperature and applied load, during creep tests. The misfit
stress and the applied stress may be the important driving forces for dislocation motion, and the signs of lattice misfit and applied stress govern the c¢ coarsening and rafting.[17–20] The dislocation motion in c channels may even adopt different modes in the different c/c¢ lattice misfits. As reported by Zhang et al.,[21–23] the larger lattice misfit results in denser c/c¢ interfacial dislocation networks, which is the key factor for small minimum creep rate in the steady creep stage. As the lattice misfit induces coherency stresses, drives rafting of the c/c¢ microstructure at high temperatures and thereby influences the creep properties at the operating temperature. Knowledge of the lattice misfit during LTTE is of great significance for single-crystal superalloys applied in IGT components. Extensive studies have reported that the degradation of c¢ precipitates in the morphology and size as well as a loss of the precipitate coherency result in a significant decrease in strength during LTTE.[13,24–32] In comparison, little has been reported on the evolution processes of dislocation m
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