Effect of Nb content on the precipitation behaviors of Fe-Cr-Al fuel cladding alloys during long-term aging at 475 °C

Highlights: • Large amounts of (Nb, Mo)-Al-rich phase precipitated and distributed uniformly within the grains of 0.5Nb and 1.0Nb alloys. • The dominant precipitation of the μ-phase on the grain boundaries transited to the Laves phase with Nb content. • The μ-phase and secondary Laves phase on the grain boundaries led to transition from ductile to brittle fracture. The precipitation behavior and tensile properties of Fe-13Cr-6Al-2Mo-xNb (x = 0.15, 0.5, 1.0) alloys for fuel cladding materials were investigated during aging at 475 °C up to 8000 h. During the aging process, the precipitation of the metastable (Nb, Mo)-Al-rich phase was promoted with Nb content. Meanwhile, the dominant precipitation of the μ-phase on the grain boundaries transited to the Laves phase with Nb content. With aging, the yield strength of the alloys increased firstly and then kept almost unchanged while the elongation decreased, owing to the precipitation and growth of different second phases. The grain boundary μ-phase and secondary Laves phase led to the transformation from ductile to brittle fracture. After long-term aging at 475 °C, the 0.5Nb alloy possessed the best combination of yield strength and elongation, making it more suitable for application as fuel cladding materials in light-water reactors.