An evaluation of the effect of stress on the swelling behavior of 20% cold-worked AISI Type 316 stainless steel was conducted. Beam specimens strained in a four-point bending mode during fast neutron irradiation in Experimental Breeder Reactor-II (EBR-II) were selected for this study. Peak elastic fiber stresses in the beams ranged from 140 to 320 MPa during irradiation to 8 × 1022 neutrons (n)/cm2 (E > 0.1 MeV) at temperatures of 685 and 697 K. Regions of the beams corresponding to tensile, compressive, and neutral loading were examined by transmission electron microscopy (TEM) to evaluate the type, morphology, and distribution of irradiation-induced voids.
TEM revealed a fairly uniform distribution of voids throughout the lower temperature beam with little difference noted in void size, void density, or swelling. In the higher temperature beam, void densities increased by as much as a factor of two from the plane of zero stress—the central portion of the beam—to the tension and compression surfaces. This produced an increase in swelling relative to the neutral axis in both tension and compression regions, the latter showing a slightly lower swelling as the result of a somewhat smaller average void size. The general microstructure was that of a cold-worked material, with similar dislocation arrays in the three regions of the beams. Widely scattered faulted loops were detected but, because of their small size and density, no assessment regarding the influence of stress on such distributions was possible. However, there appeared to be no interaction between voids and dislocations and no encircling of voids with dislocation loops was noted.
Although the irradiation conditions in the present work produced extremely low levels of swelling with associated large uncertainties, the results suggest that swelling is enhanced by a tensile stress such as is produced in bending. Moreover, the results also indicate that a compressive stress similarly produced may also contribute to an increase in swelling.