Fundamental Influences of Grain Size on Oxidation Behavior of Nanocrystalline Alumina-Forming Alloys

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Non-Technical Summary:This research program will elucidate and quantify how grain refinement to nanometer length scales can influence the oxidation resistance and thermal stability of oxidation resistant coatings and alloys. This will be accomplished using model alloys capable of forming aluminum oxide scales, and will utilize a combination of experimental and thermodynamic modeling techniques to define the mechanistic influences of grain refinement on oxidation behavior. The proposal is timely in that significant resources are being expended to increase operating temperatures and fuel efficiencies in power generating systems. This will require that the structural materials being used form more stable and protective oxide scales. This fundamental research program will provide fundamental information that will allow to stabilize and exploit nanocrystalline microstructures to improve the oxidation resistance of structural materials. The relevance of the project to advanced coating technologies is important and provides a strong practical motivation for the work. The fundamental knowledge generated will be broadly applicable to other alloy systems subjected to high temperature oxidation. The results will be disseminated through publications and presentations and the students in the project will benefit from exposure to a variety of state of the art scientific techniques. Technical Summary:The research objective of this proposal is to elucidate and quantify how grain refinement to nanometer length scales can influence the oxidation resistance and thermal stability of oxidation resistant coating alloys. This fundamental work, which will be conducted using model alumina-forming alloys, will provide some of the necessary information to facilitate the use of nanocrystalline materials in high temperature oxidation environments. It is well documented that grain refinement promotes selective oxidation which can lead to the formation of a protective oxide scale. Furthermore, it has been shown that this effect can be greatly enhanced by nanocrystallization resulting in a rapid initial and transient stage (i.e., stage I) of oxidation. In nanocrystalline materials, it is hypothesized and generally accepted that the accelerated growth of protective chromia or alumina scales results from the existence of numerous grain boundaries which provide rapid diffusion pathways for oxide forming elements and abundant sites for the nucleation and growth of a continuous oxide scale. Though this hypothesis seems intuitive, it has been noted that no systematic studies have been conducted to experimentally establish or verify the intrinsic mechanisms underlying this behavior.The intellectual challenge to be addressed in this research is to provide quantitative understanding of the relationships between microstructure (i.e., grain size and grain orientation) and diffusivity and to apply this understanding to explain the selective oxidation and interdiffusion processes that occur in multicomponent coating systems containing grain refined microstructures. This research relies on: (1) processing methods capable of delivering materials with precisely controlled compositions and microstructures; (2) the application of appropriately selected analytical techniques coupled with thermodynamic modeling to quantitatively determine and validate the mechanisms underlying the improved oxidation resistances reported in grain refined coatings.The relevance of the project to advanced coating technologies is important and provides a strong practical motivation to the work in addition to offering good career options for the students involved.