Nanostructuring to enhance phase stability of austenitic steels during irradiation

在原子水平上，几乎所有结构金属的原子排列都是高度有序的。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（通常称为晶粒尺寸）也在金属在负载或暴露于各种环境下的行为中起着重要作用。钢是当今最重要、应用最广泛的结构金属之一。奥氏体钢是一种特殊类型的钢，具有面心立方晶体结构，它们被用于许多关键应用，如核反应堆。当受到辐射时，这些钢的晶体结构会由面心立方变为体心立方，从而降低其性能。该项目探讨了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，并寻求通过将晶粒尺寸减小到纳米尺度来减少这种不稳定性的方法。本研究旨在生产纳米结构钢，其在暴露于辐射环境时具有显着增强的稳定性和耐久性。研究合作包括大学、国家实验室和工业界之间的伙伴关系，同时与不同背景的高中、本科生和研究生开展教育和推广活动。技术概述亚稳奥氏体钢在强辐照和高温环境下的相不稳定性是一个长期存在的问题，对钢的性能产生了重大影响。本研究的中心假设是这些金属的纳米结构将增强它们在辐照过程中的相稳定性。研究目的是：(i)将高温辐照过程中微观结构变化引起的辐照和热效应分开；（ii）了解辐照诱导的多晶转变机制和由此产生的晶粒尺寸效应；（三）制定辐照过程中稳定小颗粒的策略。该项目包括原位和非原位离子辐照和热退火实验，结合尖端的微结构表征技术，研究奥氏体304L不锈钢在一系列晶粒尺寸下的辐照诱导缺陷、溶液再分布和相不稳定性。本研究探讨了辐照过程中的相不稳定性，以及利用纳米结构方法设计具有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)提高高中学生对STEM学科的兴趣；（ii）改善物理冶金和材料科学本科生和研究生的教育、培训和多样性。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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.