Nanoscale Mechanisms in Alloy Oxidation: Binary and Ternary Ni-Based Alloys

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

Non-technical Corrosion is in the most general terms the breakdown of a material due to chemical reactions at the surface, and is often seen in daily life as rust on steel and iron. Corrosion is everywhere, in cars and water pipes to name a few, and most metals will break down over time which can lead in the most extreme case to catastrophic failure. The annual cost incurred in the U.S. by corrosion is more than 3% of the gross national product. Protecting materials from corrosion is critical to the design of new materials. How fast a material corrodes and degrades depends strongly on the environmental conditions, such as temperature, and humidity, salinity, noxious fumes, and radiation. Often, coatings are placed on materials that protects it from rusting rapidly. For example, Ni-based superalloy are used in turbine blades for energy production and propelling airplanes, yet they degrade rapidly in the extreme high temperature environment. Small additions of elements are often added, such as W, Mo, Cr, to these alloys to dramatically reduce corrosion. This work combines forefront experimental and computational tools to gain understanding at the atomic scale on how rust develops from the initial oxygen striking the surface of a material to the development of a protective oxide in Ni-based superalloys, and the beneficial role of adding certain elements to control and reduce corrosion. Such knowledge will benefit the design of future alloys, especially those used in extreme environments. This work will contribute to graduate and undergraduate research experiences. Furthermore, a video-based portfolio of on-demand lectures will be developed through a collaboration between University of Virginia and James Madison University. Technical Dry and aqueous corrosion/oxidation are leading causes of materials loss, and catastrophic failure. Corrosion resistance is therefore included as a design criterium in alloy engineering. The proposed work will advance understanding of oxidation mechanisms in Ni-based superalloys with Cr, Mo, and W as alloying elements. Mechanistic understanding of the initial reaction sequence where the alloy surface transforms into an oxide layer, is achieved by probing the geometric, electronic, and chemical structure in alloy and oxide at the nanometer scale. This work advances fundamental understanding of oxidation, and will infuse the development of computational methods to design protective coatings and better alloys. This proposal aims to develop a detailed mechanistic understanding of the initial steps in alloy oxidation of binary and ternary Ni-based alloys. The formation of oxide layers on Ni-Cr, and Ni-Cr-Mo or Ni-Cr-W alloys is studied with methods traditionally used in surface science and catalysis and afford the requisite geometric, electronic, and chemical information at the nanoscale. This includes scanning tunneling microscopy, and electron spectroscopies at synchrotron facilities with in-situ and operando observations starting with the clean alloy all the way to the closed oxide layer for a wide range of alloy composition and processing conditions (T, p(O2)). The majority of experiments are positioned in a regime where NiO and chromia formation compete. Monte Carlo methods, and density functional theory are used to model the element distribution in the pristine alloy and combined with experimental studies of segregation, and diffusivity to understand the near-surface Cr inventory in various alloys. Surface phase transformations, reconstructions, and element-specific nucleation pathways contribute to the development of oxide heterogeneity and are dramatically modified by minor alloying elements (W, Mo). The propagation of heterogeneity into the Cabrera-Mott regime is tested with operando experiments (ambient pressure x-ray photoelectron and absorption spectroscopies). The materials knowledge developed in this proposal will provide an understanding of mechanisms at the atomic scale, which are critical to infuse computational approaches, including machine learning approaches, and to achieve future predictive capabilities. A video-based portfolio of on-demand Surface Science lectures will be developed integrating textbook knowledge and pertinent literature. These videos build the knowledge base in surface science, corrosion, catalysis, thin film growth and analytical techniques for undergraduate and graduate students at UVa and James Madison University (Prof. Baber), a primarily undergraduate institution. The PI is committed to outreach and diversity, and will recruit UG students for the proposed research project. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.