Fluence and flux dependence of void formation in pure aluminum

Abstract

Void formation in high purity aluminum resulting from irradiation to fluences between 1.5 × 10 19 and 1.6×10 22 neutrons/cm 2 ( E ⪢ 0.1 MeV) at a temperature of 55 ± 5°C was studied, primarily by means of transmission electron microscopy. Void size distribution curves were obtained for all fluences, and from these the mean void radius was found to increase in proportion to the irradiation time raised to the one-sixth power. The void concentration displayed a fluence dependence best described by a power law, N ~ ( φt) a , in which the exponent decreased from 2.0 at 10 19 neutrons/cm 2 down to only 0.1 at 10 22 neutrons/cm 2. Treating the swelling with an analogous power relation, ΔV/ V~( φt) b , a similar saturation effect was observed, with the fluence exponent b decreasing roughly from 5 2 to 1 2 over the range effluence studied. Increases in the microhardness of irradiated specimens were found to be consistent with an expression based upon void contributions to the impeding of dislocation motions. Irradiation at a factor-of-ten lower flux produced effects upon the void morphology (e.g., half as many but larger and more elongated voids) much like those resulting from irradiation at a higher temperature. In each case, the lowering of the vacancy supersaturation during irradiation is a consequence. When material degassed prior to irradiation was compared with similarly irradiated as-received material (containing 15–20 atomic ppm hydrogen), no difference in the void concentrations or sizes was evident. The experimental observations are compared with the current models for void formation. Two models most consistent with the experimental evidence both involve transmutation-produced helium playing a crucial role in void nucleation — one involving heliumstabilized spikes and the other small helium bubbles as the nuclei of voids.