Voids in neutron irradiated and annealed niobium and niobium-1% zirconium alloy

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Abstract
Transmission electron microscnpe studies of Nb and Nb-1Zr alloy irradiated to a fast neutron fluence of 2.5 × 1022 n/cm2 at 425°, 585°, and 790°C revealed the presence of voids in Nb at all three irradiation temperatures and unusually large voids in the Nb-1Zr irradiated at 790°C. No voids were observed in the Nb-1Zr irradiated at 425°C and 585°C. The voids in Nb irradiated a t 425°C and 585°C were completely annealed after one hour at 1230°C (∼0.55 Tm,) whereas complete annealing of the voids in Nb irradiated at 790°C occurred after one hour at 1380°C (∼ 0.60 Tm). The anneals at 1230°C and 1380°C for the Nb-1Zr alloy irradiated at 790°C served to increase tlie void size and decrease the void concentration. All evidence of the high concentration of small dislocation loops present in the Nb-lZr alloy irradiated at 425°C and 585°C was absent after annealing at 1230°C. Microhiardness measurements of both materials indicated that the significant radiation induced strengthening of Nb and Nb-1Zr at 425°C and 585°C was almost complctely recovered by annealing at 1230°C. Very little radiation-induced strengthening of both materials occurred at 790°C and was partially recovered during annealing at 1230°C. Annealing at 1380°C produced additional recovery for both materials at all irradiation temperatures. From an analysis of the results, it was concluded that vacancy trapping at oversize solute atoms was responsible for the suppression of void formation in the Nb-1Zr alloy during irradiation at 425°C and 585°C. However, the formation of voids in the Nb-121 alloy during irradiation at 790°C was concluded to be the result of simultaneous oxygen and/or nitrogen or carbon and vacancy migration. A reduction of the free vacancy concentration in the Nb-lZr alloy by vacancy trapping was found to enhance the void formation process at 790°C.