The effect of irradiation on diffusion controlled creep processes

At temperatures below about half the melting point and under moderate stress, below the post-irradiated yield stress, a metal will, in a fairly high fast neutron flux ($\text{/}$ > 10¹³ n/cm² sec), suffer an enhanced creep compared to an unirradiated control held at the same temperature and state of stress. Ever since Schoeck's initial suggestion that the excess vacancies introduced by irradiation should cause an acceleration of the diffusion-controlled climb of edge dislocations over obstacles in their glide paths, there has been lively discussion on what conditions must exist (or indeed if there are any conditions) under which irradiation-accelerated diffusion will lead to an enhancement of the creep rate. It is shown here, considering both vacancies and interstitials, that for irradiation-accelerated climb of edge dislocations to enhance creep two basic conditions are necessary. First, the climb must be followed by glide and only that deformation produced by the glide is increased by irradiation. Second, there must exist a net difference in the flux of irradiation-produced interstitials and vacancies into the dislocations. This difference must be provided by other sinks which preferentially absorb one type of defect. This difference cannot be simply caused by the interaction of dislocations of differing orientations with the applied stress field or a preferential attraction between one type of defect and the dislocations. Creep rates computed using a diffusion-controlled climb model are compared with creep rates of a zirconium-base alloy at 300 °C, measured both during and following fast neutron bombardment. The model is shown to be reasonably consistent with such measurements.