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

非技术概述：本研究计划将阐明和量化纳米级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氧化膜的模型合金来完成，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理影响。这项提议是及时的，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。这将要求使用的结构材料形成更稳定和更具保护性的氧化膜。这一基础研究计划将为稳定和开发纳米晶微结构以提高结构材料的抗氧化性提供基础信息。该项目与先进涂层技术的相关性很重要，并为这项工作提供了强大的实践动力。所产生的基本知识将广泛适用于其他合金体系的高温氧化。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到各种最先进的科学技术。技术综述：本建议的研究目标是阐明和量化纳米级的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。这项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，以促进纳米晶材料在高温氧化环境中的使用。众所周知，晶粒细化促进了选择性氧化，而选择性氧化可导致保护性氧化膜的形成。此外，研究表明，纳米晶化可以大大增强这一效应，从而导致氧化的快速初始和瞬时阶段(即阶段I)。在纳米材料中，人们假设和普遍接受的是，保护性铬层或氧化铝膜的加速生长是由于大量晶界的存在，这些晶界为氧化物形成元素提供了快速的扩散途径，并为连续的氧化膜的形核和生长提供了丰富的位置。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从实验上建立或验证这一行为背后的内在机制。这项研究中需要解决的智力挑战是提供对微观结构(即晶粒度和晶向)与扩散系数之间关系的定量理解，并应用这一理解来解释在包含细化微观组织的多元涂层系统中发生的选择性氧化和互扩散过程。这项研究依赖于：(1)能够提供精确控制成分和微观结构的材料的处理方法；(2)应用适当选择的分析技术，结合热力学模型，定量地确定和验证颗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很重要，除了为参与的学生提供良好的职业选择外，还为该工作提供了强大的实践动机。

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.