Nanostructuring to enhance phase stability of austenitic steels during irradiation

非技术总结在原子水平上，几乎所有的结构金属都有高度有序的原子排列。这些原子的排列被称为“晶体结构”，它们对材料的性质有很大的影响。除了结构外，金属中的晶体尺寸(通常被称为晶粒度)也对金属在加载或暴露在各种环境下的行为起着重要作用。钢是当今最重要和最广泛使用的结构金属之一。奥氏体钢是一种具有面心立方晶体结构的特殊类型的钢，它们被用于核反应堆等许多关键应用。当受到辐射时，这些钢的晶体结构会从面心立方转变为体心立方，这可能会降低其性能和性能。该项目探索奥氏体钢的晶体结构在辐照过程中如何以及为什么会发生变化，并寻求通过将晶粒度减小到纳米级来减少这种不稳定性的方法。这项研究旨在生产在辐射环境中具有显著增强的稳定性和耐久性的纳米结构钢。研究合作包括大学、国家实验室和行业之间的伙伴关系，同时与不同背景的高中生、本科生和研究生开展教育和外联活动。技术综述亚稳奥氏体钢在强辐射和高温环境中的相不稳定一直是一个长期存在的问题，对性能和性能造成重大影响。这项研究的中心假设是，这些金属的纳米结构将增强其在辐照过程中的相稳定性。研究的目标是：(I)分离高温辐照过程中微观结构变化引起的辐照和热效应；(Ii)了解辐照诱导的多晶性转变和由此产生的晶粒度效应的机制；以及(Iii)开发一种在辐照过程中稳定小颗粒的策略。本项目包括原位和非原位离子辐照和热退火实验，结合尖端微结构表征技术，研究奥氏体304L不锈钢在不同晶粒度范围内的辐照缺陷、溶液再分布和相不稳定。这项研究探索了辐照过程中的相不稳定，以及一种利用纳米结构方法设计在辐照过程中具有较强相稳定性的奥氏体钢的新策略。教育和推广部分的目标是：(I)促进高中生对STEM学科的兴趣；以及(Ii)改善本科生和研究生在物理冶金和材料科学方面的教育、培训和多样性。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARYAt an atomic level, nearly all structural metals have a highly ordered arrangement of atoms. These arrangements of atoms are known as “crystal structures” and they have a large impact on material properties. In addition to structure, crystal size in metals (often referred to as grain size) also plays a significant role in how metals behave under loading or when exposed to various environments. Steels are one of the most important and widely used structural metal today. Austenitic steels are a particular type of steel having a face-centered cubic crystal structure and they are used in many critical applications such as nuclear reactors. When exposed to radiation, the crystal structure of these steels can change from face-centered cubic to body-centered cubic which can degrade its properties and performance. This project explores how and why the crystal structure of austenitic steels change during irradiation, and seeks to develop ways to reduce this instability by reducing the grain size to the nanometer scale. This research seeks to produce nanostructured steels with significantly enhanced stability and durability when exposed to radiation environments. Research collaborations include partnerships among universities, national laboratories and industry while educational and outreach activities are conducted with high-school, undergraduate and graduate students of different backgrounds. TECHNICAL SUMMARYPhase instability of metastable austenitic steels in service environments involving intense irradiation and high temperature has been a long-standing problem, posing significant impact to properties and performance. The central hypothesis of this research is that nanostructuring of these metals will enhance their phase stability during irradiation. The research objectives are to: (i) separate irradiation and thermal effects caused by microstructural changes during elevated-temperature irradiation; (ii) understand the mechanisms for irradiation-induced polymorphic transformations and the resulting grain size effect; and (iii) develop a strategy to stabilize small grains during irradiation. This project involves in-situ and ex-situ ion-irradiation and thermal annealing experiments combined with cutting-edge microstructural characterization techniques to study irradiation-induced defects, solution redistribution and phase instability in austenitic 304L stainless steel across a range of grain sizes. This research explores phase instability during irradiation as well as a novel strategy to utilize the nanostructuring approach for designing austenitic steels with robust phase stability during irradiation. The objectives of the educational and outreach components are to: (i) promote interest in STEM disciplines among high school students; and (ii) improve education, training and diversity of undergraduate and graduate students in physical metallurgy and materials science.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.