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

非技术总结：该研究计划将阐明和量化纳米尺度的晶粒细化如何影响抗氧化涂层和合金的抗氧化性和热稳定性。 这将使用能够形成氧化铝鳞片的模型合金来完成，并将利用实验和热力学建模技术的组合来定义晶粒细化对氧化行为的机械影响。 该提案是及时的，因为正在花费大量资源来提高发电系统的运行温度和燃料效率。 这将需要使用的结构材料形成更稳定和保护性的氧化皮。 该基础研究计划将提供基础信息，以稳定和利用纳米晶微结构来提高结构材料的抗氧化性。 该项目与先进涂层技术的相关性非常重要，并为工作提供了强大的实际动力。 所产生的基本知识将广泛适用于其他合金系统进行高温氧化。 结果将通过出版物和演示文稿传播，参与该项目的学生将受益于接触各种最先进的科学技术。技术总结：本提案的研究目标是阐明和量化纳米级晶粒细化如何影响抗氧化涂层合金的抗氧化性和热稳定性。 这一基础性工作将使用模型氧化铝形成合金进行，将提供一些必要的信息，以促进在高温氧化环境中使用纳米晶材料。 据文献记载，晶粒细化促进选择性氧化，这可导致形成保护性氧化皮。 此外，已经表明，这种效果可以通过纳米晶化大大增强，从而导致快速的初始和瞬态阶段（即，阶段I）氧化。 在纳米晶体材料中，假设并普遍接受的是，保护性氧化铬或氧化铝氧化皮的加速生长是由于存在许多晶界，这些晶界为氧化物形成元素提供快速扩散途径，并为连续氧化皮的成核和生长提供丰富的位点。 虽然这一假设似乎是直观的，但人们注意到，还没有进行系统的研究来实验性地建立或验证这种行为背后的内在机制。晶粒尺寸和晶粒取向）和扩散性，并应用这种理解来解释在含有晶粒细化微观结构的多组分涂层系统中发生的选择性氧化和相互扩散过程。 这项研究依赖于：（1）能够提供具有精确控制的组成和微观结构的材料的加工方法;（二）应用适当选择的分析技术与热力学模型相结合，以定量确定和验证晶粒细化涂层中报告的抗氧化性改善的机制。该项目与先进涂层技术的相关性非常重要，并为涂层的开发提供了强大的实际动力。除了为参与的学生提供良好的职业选择外，

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.