Doctoral Dissertations

Orcid ID

http://orcid.org/0000-0003-1093-3958

Date of Award

12-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Nuclear Engineering

Major Professor

Steven J. Zinkle

Committee Members

Sudarsanam Suresh Babu, Maik K. Lang, Kurt A. Terrani, Brian D. Wirth

Abstract

One of the major obstacles in the realization of advanced nuclear reactor concepts is the lack of materials that can withstand the extreme conditions associated with these environments. High-temperatures, irradiation damage, and environmental degradation all limit the viability of conventional materials and requires the development of advanced materials to overcome these challenges. Fortunately, alloys containing a high number density of dispersed second phase particles, termed oxide dispersion strengthened (ODS) alloys, have been under investigation due to their high temperature strength, their exceptional oxidation resistance in the presence of steam, and their irradiation resistance.Although there exists substantial data on the optimization of ODS FeCr alloys in the literature, the increasing demand for improved oxidation resistance in both current and advanced nuclear reactors has resulted in renewed interest in ODS FeCrAl alloys that have competitive properties in comparison to ODS FeCr alloys while simultaneously increasing the resistance to corrosion and oxidation in nuclear environments. However, there are still substantial gaps in knowledge pertaining to the effects of alloy composition and thermomechanical treatments on the nanoprecipitate distributions in these ODS FeCrAl alloys.In this study, ODS Fe-based alloy powders with alloying additions CrAZY (Cr-Al-Zr-Y) were fabricated using a mechanical alloying approach followed by either powder heat treatment or alloy consolidation treatments to study the nucleation and growth kinetics for nanoscale precipitation. Using a multi-scale approach combining the near-atomic scale resolution of atom probe tomography (APT) with macro-scale small-angle neutron scattering (SANS) powder experiments, it has been shown that the composition and structure match closely to the yttrium aluminum garnet (YAG) or yttrium aluminum perovskite (YAP) phase depending on thermal annealing temperature and time. It was also determined that Zr preferentially sequesters impurity C and N elements within the alloy microstructure to form carbonitrides.Using long-term coarsening data for annealed CrAZY powders, a power law coarsening model was developed for representative thermomechanical processing temperature ranges. Using the results of this model, an innovative two-step thermal annealing treatment for alloy consolidation is proposed that maximizes alloy irradiation resistance through an increase in sink-strength while also allowing for the optimization of alloy strength and ductility.

Comments

Portions of this document were previously published in journal articles, or will be submitted as manuscripts in the near future.

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