Ab initio DFT plus U predictions of tensile properties of iron oxides

Stress-corrosion cracking of steel is a ubiquitous phenomenon in which steel is chemically corroded, followed by fracture induced by applied loads. The products of steel corrosion in moist air at high temperature (>570 degrees C) are alpha-Fe2O3, FeO, and Fe3O4. Here we employ an ab initio density functional theory + U method to predict the tensile properties of these oxides, to gain insight into failure mechanisms. The universal binding energy relationship of Hayes et al. is employed to extrapolate atomic scale data to macroscopic sample sizes. The extrapolated macroscopic predictions are consistent with experimental measurements. The ordering of tensile strengths is shown to be FeO < Fe3O4 < alpha-Fe2O3, which correlates with increasing ionicity (as given by the formal charge on the Fe cations) in the three oxides. The direction that has weakest tensile strength is predicted to be along [01 (1) over bar (3/2)(a/c)(2)2] for alpha-Fe2O3 and along [001] for both FeO and Fe3O4. The direction dependence of tensile properties for these three iron oxides can be understood via a local bond strain analysis. We also predict that loading FeO along [110] and Fe3O4 along [001] or [111] produces a plastic response prior to brittle fracture at high temperature.