ACKNOWLEDGMENTThe authors are pleased to acknowledge the assistance of R. M. Kniefel with the tensile testing and cyclotron irradiations, and the contributions of R. A. Spurling and D. R. Warner to the metallography.

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Data are now accumulating which clearly demonstrate that the stress state plays a strong role in the development of void and dislocation microstructure in metals during neutron irradiation. In these experiments the application of a tensile biaxial stress state at constant fluence and temperature has been found to lead to a progressively decreasing metal density with increasing stress. The effect of stress on the concurrent development of voids, Frank interstitial loops, and dislocation networks has been studied with transmission electron microscopy. The results of these experiments clearly show that the densities of both Frank loops and voids are enhanced by a tensile stress field, with the relevant operating variable being the hydrostatic stress. More importantly it appears that any anisotropy in the stress field is reflected in a corresponding anisotropy that develops in the number of Frank loops that form on the various (111) planes. The loop density that develops on each plane exhibits a clear and direct dependence on the resolved normal stress component at each plane. Although the data from these experiments have been interpreted previously to support the existence of stress-assisted nucleation mechanisms for both loops and voids, further analysis has shown both of these explanations to be deficient in one or more respects, and both models have been replaced. Whereas a previous analysis of these data invoked as the dominant process an effect of stress on the rate of thermal re-emission of vacancies by voids, this process has been found to be too slow at the temperature at which the experiments were conducted to account for the magnitude of the observed perturbation. The current model employs as the dominant process the effect of stress on changing the capture efficiency of voids for interstitials. The stress-assisted nucleation model for Frank loops has been replaced with a stress-assisted growth model arising from the stress-induced preferential absorption (SIPA) mechanism of irradiation creep. Several previously unresolved problems arising from the earlier interpretation of the data set have been reconciled with the new model. The current analysis of this data field yields two additional significant results that were not part of the original experimental objectives. For the first time the existence of the proposed SIPA mechanism of irradiation creep has been validated by a convincing microstructural record of its existence. The initially perplexing but consistent anisotropy of planar loop populations in the absence of externally applied stresses has also revealed the existence and magnitude of internal stresses generated in polycrystalline materials during irradiation.

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