Studying the role of the alloy-grain-boundary character during oxidation of Ni-base alloys by means of the electron back-scattered diffraction technique

Abstract

Most polycrystalline steels and nickel-base alloys exhibit a tendency to preferred oxidation attack along the alloy grain boundaries during exposure to corrosive atmospheres at high temperatures. This is due to the much faster intergranular transport of the reacting elements in combination with easy nucleation of precipitates. In general, Ni-base superalloys exhibit a superior oxidation resistance compared to low-alloy steels due to the presence of elements with very high oxygen affinity, e.g., Cr, Al and Ti, which are often responsible for pronounced intergranular oxidation. Oxidation tests on the Ni-base superalloy IN718 were carried out at temperatures between 850°C and 1000°C using thermogravimetry supported by analytical scanning electron microscopy in combination with EBSD (electron back-scattered diffraction). Evaluation of oxidation kinetics have revealed that special grain boundaries with a high fraction of coincident lattice sites (low Σ values) seem to exhibit a higher resistance to intergranular attack as compared to random high-angle grain boundaries. Hence, grain-boundary engineering might be a promising way to improve high-temperature oxidation resistance.