Initial oxidation of Ni-based superalloy and its dynamic microscopic mechanisms: The interface junction initiated outwards oxidation

Understanding the high-temperature initial oxidation mechanism and its dynamic processes is generally important because a small amount of initial oxidation can induce rapid corrosion and cause catastrophic failure. We reveal the mechanism of initial oxidation in a third-generation Ni-based superalloy by in situ visualizations and investigations of the nano- and atomic-scale dynamic processes from room temperature to 900 degrees C with a Cs-corrected environmental transmission electron microscope. The initial oxidation starts from the gamma/gamma' interface at low temperatures. The high-temperature oxidation process with deficient oxygen prefers oxidation sites at the cross-junctions of the gamma/gamma' interfaces. The growth rate of the oxide nanoparticles depends on the oxidation temperatures, with a low rate below 600 degrees C and a rapid-growth speed at temperatures higher than 700 degrees C. Mass transfer from the gamma' and gamma phases to the gamma/gamma' interface, particularly at the cross-junctions of these interfaces, is observed to cause oxide accumulation. This study provides a direct observation of the interphase, interface-junction-initiated outward oxidization including Al and Cr elements. These results shed light on the initial oxidation mechanisms of materials containing a second phase with interphase interfaces and the materials at deficient oxygen conditions as well as those with protection coating layers. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.