Arivu, Maalavan. Correlative Transmission Electron Microscopy and Atom Probe Tomography Study of Radiation Induced Segregation and Precipitation in Nanostructured SS304

Principal Investigator
First Name:
Maalavan
Last Name:
Arivu
Institution:
Missouri University of Science and Technology
Title:
Graduate Research Assistant
Team Members:
Name: Institution: Expertise: Status:
Andrew Hoffman Missouri University of Science and Technology TEM, Mechanical Properties, FIB, X-Ray Diffraction, SEM, Atom Probe Tomography, EBSD Graduate Student
Haiming Wen Missouri University of Science and Technology TEM, Mechanical Properties, FIB, APT, electron backscatter diffraction Faculty
Experiment Details:
Experiment Title:
Correlative Transmission Electron Microscopy and Atom Probe Tomography Study of Radiation Induced Segregation and Precipitation in Nanostructured SS304
Describe the work that you are proposing in detail. Please include as many specifics as possible (e.g., dose, dose rate, ion energy, types of ions, beam line x-ray energy, irradiation temperature, analysis temperature, atmosphere, etc.):
This work includes the study of three samples of SS304: one conventional coarse grained (CG) sample, one ultra-fine grained (UFG) sample prepared using equal channel angular pressing (ECAP), and one nano-crystalline (NC) sample prepared using high pressure torsion (HPT). Samples were previously irradiated using 3.7 MeV Fe2+ ions up to a max of 50 dpa at 500C at the University of Wisconsin-Madison. Sample preparation will first include EBSD of the CG and UFG samples in order to target specific grain boundaries in the CG and UFG material. APT tips will be prepared using FIB liftout along identified grain boundaries and select intragranular regions, and tips will be mounted on transmission electron microscope (TEM) grids. APT tips will be examined in TEM, using a precession electron diffraction based technique (ASTAR), to investigate segregation in relation to grain size and GB character. This approach will enable to separate the effects of grain size and GB character on radiation induced segregation. Selected area electron diffraction (SAED), high resolution TEM (HRTEM), and energy dispersive X-ray spectroscopy (EDS) will be used to identify and characterize structural data for any precipitates that may be found. APT will be performed using laser mode to obtain more data and to prevent fractures at grain boundaries. Analysis will focus on grain boundary chemistry to compare segregation behavior as well as precipitate composition, volume fraction, size, and number density.
Technical Abstract
Fe-Cr-Ni austenitic stainless steels are vital core internal structural materials in light water reactors (LWRs) and used as pressure vessel cladding due to especially their corrosion resistance. Even though these steels are widely used, they suffer from embrittlement, swelling, and stress corrosion cracking (SCC) during irradiation. In order to increase the sustainability of LWRs and advance the development of Gen IV reactors, advanced steels that can tolerate the radiation, temperature, and corrosion environment need to be developed. Increasing the volume fraction of grain boundaries can enhance irradiation tolerance in nanostructured materials as grain boundaries serve as effective sinks for irradiation induced defects. While UFG steels have been shown to possess reduced void swelling and embrittlement after irradiation, there have been limited studies on the radiation induced segregation (RIS) and precipitation in nanostructured materials. Because RIS is driven by vacancy flux towards defect sinks through the inverse Kirkendall effect, a decrease in the total number of defects due to nanostructuring is expected to lower the vacancy flux and therefore reduce RIS. This project will systematically study the effect of grain size on RIS by comparing segregation behavior after ion irradiation in CG, UFG and NC SS304. A correlative technique using procession electron diffraction via TEM-ASTAR on atom probe tips will allow for comparing grain boundaries with the same or similar character but different grain sizes, and this work will be the first to use a correlative technique consisting of APT and TEM-ASTAR to understand RIS separately as a function of grain size and as a function of grain boundary character. Additionally, this experiment will compare the effects of grain size on intragranular segregation and precipitation. It is anticipated that results of this project will lead to development of strategies to reduce irradiation induced segregation and precipitation through nanostructuring and accordingly improve the lifetime of structural materials and cladding.