Yuanzineng kexue jishu (Dec 2023)
Microstructure Evolution during Fabrication of Zr-2.5Nb Pressure Tube (Ⅰ)—α/β Phase Transformation
Abstract
Zr-2.5Nb pressure tube is one of the most important components of the fuel channel in the CANDU reactors. The α/β phase transformation of Zr-2.5Nb during pressure tube fabrication is much more complex than that in single α phase zirconium alloys for fuel cladding tube, since Zr-2.5Nb is a α+β dual-phase alloy. In consideration of the α/β phase transformation effect on the microstructure evolution of Zr-2.5Nb pressure tube, microstructure characterization of Zr-2.5Nb alloy in VAR ingot, forged billet, hot-extruded tube was carried out. In addition, heat treatments of β quenched billet at 950-1 070 ℃ for 5-120 min and at 780 ℃ for 3 h were performed as well for a better understanding on the microstructure evolution of the Zr-2.5Nb pressure tube during fabrication. Microstructures were mainly characterized by OM, SEM/ECC, and EBSD. Microstructures of Zr-2.5Nb alloy are various with fabrication processes, such as α+β Widmanstatten structure in ingot and forged billet, α′ martensite and α laths in β quenched billet, α+β lamellar structure in hot-extruded tube. Those differences are corresponding to different α/β phase transformation types, α/α′ nucleation and growth mechanisms. It is demonstrated that the cooling rate plays an important role on the α/β phase transformation types as well as microstructure characteristics of Zr-2.5Nb alloy. The microstructure of β quenched billet is linked to the depth in the radius direction. Owing to the fast-cooling rate, the outer surface of β-quenched billets undergoes β→α′ transformation and consists of α′ lath. The internal location of β-quenched billets suffers a slower cooling rate and performs β→α′+α transformation, leading to a microstructure of α′+α lath. Compare to the quenched billet, ingot and forged billet suffer a much slower cooling rate and have α+β Widmanstatten structure resulting from β→α+β phase transformation. The relatively slow cooling rate of hot-extruded tube makes β→α+β transition happened, where transformed α grows on the basis of primary α. The width of transformed α lath increases with the decrease of the cooling rate, which leads α lath with about 1 μm width exists in the ingot while that in β-quenched is less than 300 nm. Based upon the interaction of dynamic recrystallization, α/β phase ratio, strain, and cooling rates, forged billet exhibits different microstructures in the radius direction. In addition to the β→α+β and β→α/α′ transformations during cooling, α/α′→α+β and α+β→β appeare in the heating process. Metastable and supersaturated α/α′ laths in β quenched billet would be decomposed into α+β lamellae under heating treatment at 780 ℃ for 3 h. And those α+β lamellae will transform equiaxial β phase when the temperature increases to above α+β/β transition line.
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