Effects of applied stress on microstructural evolution in 316SS under ion irradiation

To obtain fundamental knowledge about the stress effects on microstructural behavior in irradiated materials, we developed a tensile loading apparatus which is to be mounted on the dual-ion irradiation stage of the High-fluence Irradiation Facility of the University of Tokyo and established a corresponding experimental method. In the first part of this paper, a description of the facility and the experimental technique are given. As an initial experiment using this facility, austenitic Type 316 stainless steels in solution annealed condition was irradiated by 4 Mev nickel ions at 773 or 873 under tensile stresses between 0 and 200 MPa. The irradiation dose was 3 or 50 dpa. Faulted loop microstructure and cavity microstructures were examined with a transmission electron microscope. The effects of applied stress were apparent in both cases, which proved the validity of the facility and the experimental method. Relative number density of the interstitial type faulted loop appeared to increase when they have such an orientation that relaxes the tensile stress. The degree of such deflection of loop orientation was similar for 50 and 100 MPa of tensile stress. The size distribution of the faulted loops was not clearly affected by external stress. As for cavity microstructure, an increasing tensile stress increased their mean size but did not clearly influence the number density. The observed stress dependence of cavity size distribution showed that a larger stress advances both the beginning and the end of cavity nucleation. Further microstructural examination suggested a possible effect of stress-enhanced precipitation on cavity nucleation.