The Impact of Crystal Orientation on Initial Stage Oxidation of Polycrystalline Nickel and Chromium

Processing techniques that promote particular crystallographic orientations with low oxidation rates can potentially enhance and increase the durability of components subject to damage by oxidation, and in turn can improve the properties and performance of a diverse subset of products in aerospace, power generation, and nuclear energy industries. For example, if nickel alloy turbine blades could be engineered to resist oxidation, the performance of the engine could be improved and might translate into an increased cost savings and improved energy efficiency. However, before these advanced materials can be developed, it is necessary to understand the science at the atomic level and how the microstructure affects surface phenomena. In the present work, nickel and chromium were examined at the microstructural level in order to better understand how surface orientation affects high temperature oxidation. In particular, this research focuses first on nickel (fcc), the major component in many high-strength and high-temperature alloys, and continues with chromium (bcc) , which is typically added to these alloys to improve corrosion resistance. While previous studies on oxidation and corrosion have focused on either: (1) the bulk material without attention to the microstructure, or (2) crystal structures with one or a few orientations, this research explores oxidation behavior as a function of the continuum of surface orientations. Specifically, polycrystalline materials were examined because there are hundreds or even thousands of unique orientations present at the surface. This approach allows for the effects of surface processes like oxidation and corrosion to be determined for metals over all surface orientations. The oxide is grown on the metal at an elevated temperature for a predetermined amount of time. The topography and relative thickness of the oxide layer are then characterized using optical profilometry. Next, the microstructure of the metal surface is characterized by mapping grain orientations using electron backscatter diffraction (EBSD). EBSD is a technique, which allows crystallographic information to be obtained from samples in the scanning electron microscope (SEM). By correlating the results from EBSD and optical profilometry, the oxide thickness can be determined for each orientation in the specimen and a map of the oxide thickness is generated as a function of surface orientation. The map of oxidation thickness is then correlated with the underlying microstructure and crystal orientations that are most resistant to oxidation are identified. As a result of using this technique, a more fundamental understanding of the role of surface orientation on oxidation behavior is gained; our research has identified a distinct effect each unique crystal orientation has on oxidation. Specifically, in nickel oxidized at 700 °C, we found that the oxide height was correlated to the angular deviation of the surface normal away from the \u3c111\u3e direction, whereas in chromium oxidized at 950 °C, the oxide height was correlated to the angular deviation of the surface normal away from the \u3c001\u3e direction. Ultimately, knowledge of oxidation behavior as a function of surface orientation will provide valuable insight into the role of microstructure in material properties and will pave the way to develop and engineer microstructures more resistant to these phenomena