Oxidation behavior and outward diffusion of Al along oxide grain boundaries of FeCrAl alloys overdoped with Zr and Hf by means of Atom Probe Tomography (APT)

Abstract

Abstract It is generally accepted that the formation of α-Al2O3 scale on FeCrAl alloys is governed by inward oxygen transport along grain boundaries. However, it metal ion outward diffusion plays a role in the development of the oxide scales and their microstructural characteristics. This paper studies oxidation behavior and outward diffusion for iron-chromium alloys containing ~ 10 at. % aluminum, ~ 22 at. % Chromium, occurred when doped with an over-critical concentration of reactive elements, i.e. Zr and Hf. All samples were investigated after thermal exposure at 1100°C by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Atom Probe Tomography (APT). As a result of the overdoping, a considerable increase in oxide growth, an increase in the depth of internal oxidation, and RE-oxide formation near and at oxide Grain Boundaries (GBs) were observed as a consequence of increased inward and outward diffusion. The effect of overdoping manifests itself differently depending on the RE type and amount due to different solubility, ionic size, and electronic structure. The overdoped sample with Zr is the retainment of the adhesion of alumina to the alloy after the first and second thermal exposure, contrary to Hf overdoping which leads to severe spallation after the second thermal exposure. And FeCrAl-Zr overdoped sample formed ZrO at oxide grain boundary and internal Al oxide.