Nitridation of a Fecral Alloy in H2+N2 Environment at 900oc: A Microstructural Study

Abstract

When exposed to O2 and/or H2O at high temperature, iron-chromium-aluminum (FeCrAl) alloys form an adherent a-Al2O3 scale that grows very slowly. Because of the excellent protective properties of this scale, the FeCrAls resist high temperature corrosion and oxidation very well and they are used in a wide range of high temperature applications up to 1300oC. However, when exposed to nitrogen-containing environments with low oxygen activity, e.g. H2+N2 mixtures, the FeCrAls become sensitive to nitridation. Such environments are often used in heat treating and sintering operations. This paper investigates the nitridation behavior of the FeCrAl alloy Kanthal APMT at 900°C. In order to provoke nitridation, the material was exposed to a N2+H2 mixture with very low water vapour content. Microstructural analyses after 24h exposure revealed that a continuous Al2O3 scale covered almost all of the alloy surface. However, the oxide scale contained a distribution of Cr2O3 patches with diameters of 10-40 mm. Cross-sectional studies using Focused Ion Beam (FIB) milling showed internal nitridation of the alloy, displaying a variety of precipitates below the Cr2O3 patches, including needle shaped AlN, equiaxed Cr2N and CrN particles. In contrast, the alloy beneath the Al2O3 scale was not nitrided. To investigate possible reasons for the formation of chromia patches we kept track of specific regions before and after exposure. Hence, several analytical techniques were employed to fully characterize the alloy microstructure prior to the exposures, using Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD) and Energy Dispersive X-ray (EDX) analysis. Some markers were used to enable identification of specific areas during post-exposure analysis and to enable correlation of chromia formation and internal nitridation with the alloy microstructure.