Characterization of deformation structure using ion-irradiated stainless steels

Full text of publication follows: Deformation behavior of irradiated stainless steels (SS) was investigated using ion-irradiated 304SS and 316SS. Macroscopic deformation and microscopic structure were analyzed, and relationships between deformation behavior and damage structure, strain rate were studied. Small tensile test specimens were prepared from solution annealed 304SS and 316SS. After mechanical and electropolishing, one side of gage section was irradiated with 200 keV He+ ions at 300 deg. C up to 20 dpa at damage peak depth. Hardness in a damage region was measured using an ultra-micro hardness tester. Damage structures were observed by TEM. The specimens were tensile at 300 deg. C in argon gas at strain rates of 10-4/s and 10-7/s to 2% plastic strain. After the tensile tests, surface step angles to the tensile axis and step spacing were measured using an optical microscope. Deformation microstructures were observed by TEM. The hardness increased with dose up to 2 dpa and saturated. The increase of the hardness of 316SS was larger than that of 304SS. Dislocation loops and cavities were observed as damage structures, whose average diameters and number densities were different between 304SS and 316SS. Increase of shear stress due to the formation of defects calculated using the Orowan model was well in proportion to the increase of the hardness. The distributions of the measured surface step angles showed the same trend to the calculated distribution. This indicated that the surface steps were formed by slip deformation. The average step spacing increased with dose, but the tendencies were depending on strain rates and materials. As for effect of strain rates, the spacing on fast deformation were smaller in unirradiated specimens and increased sharply at lower doses and saturated to smaller value than those on slow deformation. As for effect of materials, 304SS showed sharp increase at lower doses and smaller saturated value than those of 316SS, whereas the spacing in unirradiated specimens of both materials were almost the same. TEM observation results showed that a part of planer slips was interrupted at the damage region and that the surface steps corresponded to dislocation channeling. The details for the interaction between defects and dislocation channeling will be also discussed. (authors)