In-reactor creep of Zr-2.5 at % Nb

Abstract

Tensile specimens of heat treated Zr-2.5 at % Nb rod have been tested under constant creep loads both unirradiated and during neutron irradiation. No significant differences were found in creep rates for temperatures of 350, 375 and 400 °C. The in-reactor creep rates were faster at 300 °C and stresses of 25.6, 31.6, and 38.7 kg/mm 2 and at 320 °C and 38.7 kg/mm 2 than the corresponding unirradiated control creep rates. The total creep rate during irradiation under these stresses and temperatures can be expressed by the equation: ε = ε u + ε i , where ε is the measured in-reactor creep rate, ε u is the unirradiated control creep rate, and ε i is the increase in creep rate due to neutron irradiation. The values of ε i show little if any dependence upon stress or temperature. The activation energy for creep Q c , determined for a stress of 38.7 kg/mm 2 is approximately 60 000 cal/ mole. Due to the contribution of ε i there is an apparent decrease in the activation energy at the lower strain rates. There is also a decrease in the activation energy of the control data at lower stresses and temperatures which may be due to measurement of faster primary creep rates instead of minimum creep rates, or a different creep process which has a lower activation energy. The mechanism for the increased creep rate during irradiation observed at 300 and 320 °C is tentatively proposed to be due to growth of stress oriented dislocation loops limited by the generation rate of point defects. The possibility of the increased creep rate resulting from a non-conservative dislocation mechanism limited by point defects produced by irradiation is not excluded. The mechanism of the creep represented by an activation energy of approximately 60 000 cal/mole is most likely a diffusion controlled process such as dislocation climb or non-conservative motion of jogs in screw dislocations.