Hoffman, Andrew. Advanced microstructural characterization of irradiation-induced phase transformation in 304 steel

Principal Investigator
First Name:
Andrew
Last Name:
Hoffman
Institution:
Missouri University of Science and Technology
Title:
Graduate Research Assistant
Team Members:
Name: Institution: Expertise: Status:
Lingfeng He Idaho National Laboratory Nuclear Fuels, TEM, radiation effects, Ceramics Other
Haiming Wen Missouri University of Science and Technology TEM, Mechanical Properties, FIB, APT, electron backscatter diffraction Faculty
Xiang Liu Idaho National Laboratory TEM, ODS, Nanoindentation, X-Ray Diffraction, APT, Austenitic, dislocation loops, Irradiated Microstructure, Ferritic Martensitic Steels, EDS, Radiation Induced Segregation, Advanced Alloys, Post-Irradiation Examination Post Doc
Experiment Details:
Experiment Title:
Advanced microstructural characterization of irradiation-induced phase transformation in 304 steel
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.):
Two samples of 304 steel neutron irradiated at different temperatures will be compared using advanced electron microscopy. One sample will be 304L steel irradiated in EBR-II at 370°C to ~6.25x10^22 n/cm2, and the other will be 304H irradiated at 500°C to ~3 dpa in a University of Wisconsin experiment; both samples will be requested from the NSUF sample library. Because less segregation occurs at 370°C, the cause of phase transformation is more likely to be due to strain, and at 500°C the segregation and precipitation is expected to have a large effect on the phase transformation. Although one sample is 304L and the other is 304H, it is anticipated that the higher carbon content in the 304H will not affect the formation of the Ni enriched phases that are primarily responsible for the austenite instability, and we anticipate 304H to have similar chemically driven transformation as 304L. To look for initial phase transformation, x-ray diffraction (XRD) will be performed to estimate the fraction of the ferrite phase in the irradiated samples. TEM liftouts will then be prepared via focused ion beam (FIB) and thinned to thicknesses under 100 nm, and electron backscatter diffraction (EBSD) will be conducted prior to FIB to identify the sample regions with desired phase compositions. A precession electron diffraction-based technique using ASTAR system on TEM will be utilized to provide a more accurate measurement of the fractions of austenite and ferrite phases. ASTAR will also be used to produce stain mapping and determine if orientation relations characteristic of deformation induced martensite exist. EDS will also be performed, in scanning TEM (STEM) mode, in combination with ASTAR on the same sample regions, producing correlative chemical and phase maps. EDS line scans will be performed across grain boundaries of interest, especially those near ferritic grains that show Ni depletion/Cr enrichment, to determine the role of segregation in the formation of ferrite. Furthermore, electron energy loss spectrometry (EELS) will be used to determine the exact thickness of TEM lamellas.
Technical Abstract
The γ-austenite (FCC) to α-ferrite (BCC) phase transformation has been observed in 300 series austenitic steels in the past decades. The mechanism for this transformation has been largely attributed to Ni segregation and precipitation of Ni enriched phases, however, strain induced diffusionless phase transformation from γ-austenite to α’-martensite has also been observed during irradiation of high Ni austenitic steels. We hypothesize that both strain induced diffusionless γ-austenite to α’-martensite transformation as well as instability of the γ phase from precipitation and segregation of Ni contribute to the γ to α phase transformation. In this study we propose to use precession electron diffraction combined with super-X energy dispersive X-ray spectroscopy (EDS) to study two samples of 304 steel neutron irradiated at different temperatures. This advanced technique will allow us to study to what extent each mechanism plays a role in the phase transformation, and how temperature affects the phase transformation. The results of this study will help to develop austenitic steels that are resistant to phase transformation during irradiation. Because the ferritic phase has significantly different mechanical properties as well as poor corrosion resistance, prevention of this phase transformation will have a significant impact on extending the lifetime of these materials.