Oxidation of an inward grown aluminide diffusion coating studied using SEM and TEM

Journal article, 2005

Oxidation of an inward grown aluminide diffusion coating on René N4 after isothermally oxidation in air at 1050°C, between 0.5 and 2000 h was studied using SEM and TEM. For preparation of TEM foils an improved method using focused ion beam (FIB) milling, combined with the lift-out technique, was applied. Studies of the microstructure and chemistry of the oxide scale revealed existence of three different types of voids; (a) interfacial voids, (b) a row of voids dividing the oxide scale in two parts with different morphology and structure, and (c) internal voids within the α-Al2O3 grains. It is believed that the different types of voids are the result of different mechanisms. TEM/EDS analyses show that the outermost part of the scale is enriched in Cr and has different morphology than the inner part. This is partly explained by the change in oxygen partial pressure (pO2) at the metal/oxide interface during the oxide growth.