A Reassessment of the Role of Stress in Development of Radiation-Induced Microstructure

Abstract

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.

Irradiation Creep in Materials

“…Saka et al have shown that under stress, loops were elongated in the direction of the tensile tress, and that the results were not consistent with SIPA-I effect (Saka, et al, 1989). It has been argued that the small effect of stress on loop size may not rule out a SIPA effect , since the effect of SIPA-I has been shown to be maximum at intermediate loop size and then to decrease (Garner, et al, 1979). This theoretical result would be worth more investigation given the assumption of infinitesimal loop underlying it .…”

“…There are some experimental evidences that an applied stress induces an anisotropic loop microstructure in ferritic and austenitic steels, with a dependency of the loop density on the normal stress to the loop plane, without significant effect on the loop size (Okamoto & Harkness, 1973) ) (Garner, 1979) (Caillard, et al, 1980) . In particular some results were obtained at low doses, to avoid any effect of the dislocation network on loop distribution (Atkins & McElroy, 1987).…”

“…This model, called stress-induced preferred nucleation (SIPN) ) (Wolfer, et al, 1972), proved to be irrelevant for interstitial dislocation loops. Binding energies of self-interstitials are in general so large that no nucleation barrier exists (Garner, et al, 1979). It was then proposed that small clusters would reorient under stress, which would lead to anisotropic loop distribution (Wolfer, 1980).…”

Frank Loop Formation in Irradiated Metals in Response to Applied and Internal Stresses

“…Even more importantly, however, when these data are plotted against the component of stress (cr 0 N) involved in the description of the SIPA creep process, the data exhibit discernible trends with stress not only het\IJeen planes within a specimen, but between specimens at different hydrostatic stress levels (oH) and fluence. ( 4 ) The stress component aDN = ~aN -crH) preserves the carrel at ion with the normal stress con1' .•:Gnt and allows that some plan~s with_negative values of crDN will suffer : reduction in loop density. Although these data were judged to be reasonably conclusive experimental evidence of the operation of the SIPA creep mechanism on Frank loops, several questions persisted that detracted from the certainty with v1hich this conclusion 1-' Jas stated.…”

Effect of cold work deformationon irradiation hardening of vanadium alloys

“…In addition to that, traditional method, like cold work process in 316 stainless steel (deformation amounts of 20%), is proved to have better void swelling resistance due to the introduction of dislocations [27]. Meanwhile, the effect of cold and subsequent annealing on materials was reported in previous papers [28][29][30]. However, in vanadium alloys, engineered sinks to enhance radiation tolerance have been poorly investigated.…”