Grain refinement strengthening mechanism of an austenitic stainless steel: critically analyze the impacts of grain interior and grain boundary

Abstract

In order to probe the superior nano/micron scale mechanical behaviors of ultra-fine grain (UFG) austenitic stainless steel as compared with coarse grain (CG) counterpart, systematic nanoindentation tests were carried out together with post-mortem EBSD characterizations. Relatively higher nanohardness (Hn) was obtained in UFG in both grain interior and grain boundary. This phenomenon had a relationship with the grain refinement. In the “grain interior” nanoindentation, the matrix strength in UFG was higher than CG, resulting from the pre-existing dislocations in UFG were lower, leading to higher stress close to the ideal strength was required to nucleate dislocations and cause plastic deformation. In the case of “on grain boundary”, the average Pc for UFG grain boundary was higher, corresponding to the higher shear stress required for dislocation activation, revealing higher grain boundary effect, attributed to the higher strain surrounded the grain boundary. The activation volume (v) obtained from nanoindentation had weak dependence on grain orientation but strong dependence on grain size. The relatively higher v was obtained (greater than 1 b3, but smaller than 103 b3), indicating that neither only diffusional creep processes nor only conventional dislocation segments passing through dislocation forests governed plastic deformation in our cases. The grain boundary effect coefficient k of UFG grain boundary was higher than that of CG. Therefore, the higher macron-strength of the UFG was mainly determined by the higher grain boundary strength, and the fine grain size with less contribution from the grain interior. The mechanical behaviors could be governed by different grain boundary effects in microstructure.