Fracture properties of austenitic stainless steel grain boundaries oxidized in PWR environment

In this study, the fracture properties of a FeCr$₁₂$Ni$₂₆$Si$_3$ (wt %) austenitic stainless steel grain boundaries (GB) oxidized in Pressurized Water Reactor (PWR) environment are assessed. The alloy chemical composition mimics the composition of 304/316 stainless steels GBs after irradiation, making the results obtained relevant for Irradiation-Assisted Stress Corrosion Cracking (IASCC). Oxidation tests have been performed in nominal PWR environment for 3680 h and 7470 h. The oxidized samples have been characterized through Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) to quantify the chemical compositions and thicknesses of bulk and intergranular oxides. Micro-cantilevers beams containing a single grain boundary have been milled on oxidized samples using Focused Ion Beam (FIB) and tested in-situ SEM at room temperature. Intergranular cracking has been observed in 12 micro-beams, either within the GB oxide or at the oxide/metal interface. The Coupled Criterion (CC) theoretical framework for crack nucleation has first been used to obtain estimates of the fracture properties. These estimates have then been refined by comparing the experimental results to Finite Element (FE) simulations using Cohesive Zone Model (CZM). A good agreement for the macroscopic fracture load is obtained with a fracture energy $\gamma_c$ $\approx$ 11 $\pm$ 3 J m$^-2$ and a strength $\sigma$$_c$ $\approx$ 1000 $\pm$ 250 MPa of the oxide. These values are finally compared to the results available in the literature, as well as critically assessed with respect to their dependence to the experimental and numerical parameters.