Deformation Structure in Highly Irradiated Stainless Steels

Abstract

Surface steps and deformation microstructure in cold-worked SUS316 stainless steels irradiated to 4 and 35 dpa (displacements per atom) were examined after being deformed by uniaxial tensile stress at 320 C at a slow or fast strain rate. Dislocation channeling was the predominant mode of deformation near the surface at the slow strain rate. Twinning was dominant at the fast strain rate whereas twinning and nanotwin formation occurred in the locally stressed area at the slow strain rate. Deformation heterogeneity measured using the spacing of coarse surface steps induced by dislocation channels increased with increasing dose from 4 to 35 dpa. Grain boundary separation occurred when dislocation pileups and high normal stress on the grain boundary plane coexisted, which likely was a precursor of intergranular cracking without any environmental factor.

Angular Distribution of Slip Steps by Three-Dimensional Polycrystalline Model for Stainless Steel

“…In previous studies, 7,8) in order to quantify the degree of localization, the spacing of steps and the step angle were measured by optical microscopy. Figure 1 shows an example of the surface image of a deformed specimen made of Type 316 stainless steel irradiated in a pressurized water reactor as flux thimble tubes.…”

“…7,8,23) Figure 8 shows the angular distribution of step angle obtained using ðp s Þ, which is normalized by the sum of step angles, together with the experimental results shown in Fig. 3.…”

“…Therefore, the angular distribution of step angle can be estimated from evaluating RSS. In the NC model, which was used in previous studies, 7,8,23) the crystal was assumed to slip in an arbitrary direction once the slip plane was specified. The maximum RSS on a slip plane under a tensile stress is given by…”

“…The angle can be geometrically obtained from ' and p s using Since it is difficult to identify the angle ' by surface observation, a uniform angular distribution of ' was assumed under an angle p s in the previous studies. 7,8,23) Then, for each step angle p s , the average RSS on the slip plane can be estimated by averaging the values for all possible ' values as…”

“…The angular distribution of step angle has been obtained experimentally in previous studies. 7,8) On the other hand, it is possible to evaluate an expected angular distribution from the mechanical investigation of the slipping behavior of each crystal. Since there are only four slip planes in a face-centered cubic crystal, the possible step angle on the surface is limited depending on the crystallographic orientation.…”