Doctoral Dissertations
Date of Award
5-2023
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Nuclear Engineering
Major Professor
Steven J. Zinkle
Committee Members
Steven J. Zinkle, Maik K. Lang, Arunodaya Bhattacharya, David T. Hoelzer, Philip D. Edmondson, Jean Henry
Abstract
The operating conditions for fusion reactors are expected to be significantly harsher than existing fission light water reactors. As a result, structural steels need to be developed to meet the demands of higher temperatures, doses, and more energetic neutron damage. Control of secondary phases in a material's microstructure is instrumental towards maintaining acceptable mechanical properties and radiation tolerance. Here we focus on phase stability under irradiation in austenitic stainless steel (AuSS), which is commonly used for LWR core internals and oxide dispersion strengthened (ODS) ferritic alloys, which are actively being developed for fusion reactor applications. First, we discuss which factors affect the ballistic dissolution susceptibility of different solute nanocluster species by comparing the response of Cu- and Ni-Si-Mn-enriched solute nanoclusters in a 304L AuSS to neutron and ion irradiations. It is revealed using the Heinig precipitate stability model that the initial solute concentration gradient adjacent to a cluster's interface affects the thermodynamic driving force for radiation enhanced diffusion solute recovery. Next, we investigate over the fusion-relevant temperature range how phase instabilities contribute to hardening and embrittlement in ODS alloys. Mechanical testing of the ODS alloys after neutron irradiation demonstrates irradiation hardening even at temperatures where reduced activation ferritic/martensitic steels typically recover. There was no evidence of hardening suppression from the high initial sink strength of the alloys. With an eight feature hardening model, we reveal that Cr-enriched nanoclusters are the primary contributor to the observed hardening under irradiation in high-Cr ODS alloys. Lastly, we consider how high dose neutron exposure affects the phase stability of oxide nanodispersoids. Using transmission electron microscopy, we reveal that at suffciently high doses (~ 50 dpa), crystalline oxides are amorphized while Fe and Cr from the matrix enters the particles by ballistic injection and is able to agglomerate and recrystallize forming crystalline islands. These observations challenge the traditional assumption that oxide nanodispersoids are stable under irradiation.
Recommended Citation
Levine, Samara M., "Phase Stability in Nuclear Structural Steels. " PhD diss., University of Tennessee, 2023.
https://trace.tennessee.edu/utk_graddiss/11554