Effect of Mo on precipitation evolution and mechanical degradation in high-Si nickel-based alloys during long-term aging
摘要
High-Si nickel-based alloys have attracted considerable attention for their potential application in high-temperature molten chloride salt environments of concentrated solar power systems. While Mo addition is employed to further enhance corrosion resistance, its impact on the long-term microstructural stability and tensile properties of these alloys remains a critical concern. This study investigates the precipitation behavior and tensile properties of Ni–25Cr–10Fe–3Si–xMo (x = 0, 1.5, 3 wt%) alloys during aging at 650 °C for up to 5000 h. Microstructural analysis reveals that Mo addition drastically alters the precipitation sequence from solely M23C6 carbide (0 wt% Mo) to a mixture of M₂₃C₆ and brittle TCP phases MoNiSi (1.5 wt% Mo), and finally to dominant TCP phases (MoNiSi and σ) at 3 wt% Mo. Furthermore, Mo markedly accelerates the precipitation kinetics of TCP phases, which leads to severe embrittlement and a transition in fracture mode from fully ductile to a mixed intergranular and transgranular mode in the Mo-containing alloys after aging for 5000 h. After 5000 h aging, the yield strength of Mo-0, Mo-1.5, and Mo-3 alloys reaches 268, 306, and 380 MPa, while the elongation decreases to 46.2%, 42.2%, and 24.3%. Compared with the Mo-free alloy, the yield strength is increased by 14.2% and 41.8%, while the elongation is reduced by 8.7% and 47.4% for the Mo-1.5 and Mo-3 alloys, respectively. Thermodynamic calculations demonstrate that Mo lowers the Gibbs free energy for σ phase formation, rendering it more favorable than that of the M23C6 carbide, thereby rationalizing the shift toward TCP precipitation. This study provides important theoretical guidance for balancing corrosion resistance and mechanical stability in the design of next-generation high-temperature structural alloys for advanced energy systems.