Effect of Irradiation on the Microstructure of Zircaloy-4

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Abstract
This paper deals with the irradiation growth and the radiation-induced changes in the microstructure of Zircaloy-type materials. The experimental irradiations that were conducted at 400°C in Siloé (metallurgical test reactor) and Phénix (fast breeder reactor) show that the growth of both alpha-recrystallized and beta-quenched Zy-4FORT (high tin and oxygen contents) is accelerated after a dose of about 4 dpa (displacements per atom). In correlation with this acceleration, a high density of basal-plane c-component dislocations is revealed by transmission electron microscope (TEM) examinations. At this temperature, the structure of precipitates remains fully crystalline, but radiation-induced dissolution occurs and allows the formation of Zr5Sn3 particles evenly dispersed throughout the matrix. To allow comparisons with the microstructural evolutions produced in pressurized water reactor (PWR) conditions, TEM examinations were also performed on standard Zircaloy-4 irradiated as recrystallized guide tube and stress-relieved cladding. In recrystallized Zircaloy-4 irradiated to 10 × 1021 n/cm2 (T ͌ 320°C), a high density of basal-plane c-component dislocations is also observed, the Zr(Fe,Cr)2 Laves phases undergo a partial crystalline-to-amorphous transformation, and their outer rim is severely depleted in iron. In stress-relieved Zircaloy-4 irradiated to the same neutron fluence. irradiation does not produce any significant recovery of the initial dislocation network. Although the Laves phases undergo an amorphous transformation only in the lower part (T ͌ 335°C), a marked dissolution is observed all along the cladding (335 to 380°C).