Nanostructuring to enhance phase stability of austenitic steels during irradiation

Missouri University Of Science And Technology, Rolla MO

Investigators

Abstract

NON-TECHNICAL SUMMARY At 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 SUMMARY Phase 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.