Nuclear Engineering Colloquium: Janelle P. Wharry
Title: "Irradiation Tailoring of Deformation Mechanisms in Austenitic Stainless Steel."
The objective of this talk is to understand how irradiation-induced defects control the deformation-induced martensitic phase transformation in austenitic stainless steels (SS). Austenitic SSs are amongst the most widely utilized structural alloys in nuclear energy systems, where SS components must bear mechanical load while also accumulating irradiation damage. Hence, understanding the role of irradiation on deformation mechanisms is critical to ensuring long-term safety and structural integrity of nuclear components. SSs typically deform through dislocation slip, although in the presence of irradiation, localized deformation mechanisms such as twinning and dislocation channeling can become active. Sometimes, a deformation-induced phase transformation can also occur, in which the γ-fcc SS transforms into either α'-bcc or ε-hcp martensite needles. This work will demonstrate how the presence of irradiation-induced cavities (e.g. voids and/or bubbles) can increase the tendency for the martensitic transformation to occur.
Work focuses on AISI 304L stainless steel specimens irradiated in EBR-II to damage doses of 3-20 displacements per atom (dpa) at 415°C, and containing ~0.1-3 atomic parts per million (appm) He. Portions of each specimen undergo post-irradiation annealing in order to alter the cavity microstructure. Micro-scale mechanical testing in situ within a scanning electron microscope (SEM) is carried out in both indentation and compression pillar geometries. Subsequently, site-specific transmission electron microscopy (TEM) enables us to precisely determine deformation mechanisms and their interaction with the microstructure. We find that a high number density of large cavities promotes the direct γ→α' transformation. Post-irradiation annealing reduces the number density of cavities, causing the transformation pathway to change to γ→ε→α'. With an even further reduced cavity number density, twinning occurs instead of the phase transformation. These results are explained by the surface energy contribution from cavities. This work suggests that controlling the irradiated microstructure can enable localized tailoring of the deformation mechanism and phase transformation pathway.
Janelle P. Wharry is an assistant professor in the School of Nuclear Engineering at Purdue University and also holds a courtesy appointment in the School of Materials Engineering. Wharry’s research focuses on understanding microstructure-property relationships in irradiated materials, with an emphasis on deformation mechanisms and mechanical behavior at the nano/micro scale. Her active projects span nuclear structural and cladding alloys, structural materials produced by advanced manufacturing and joining methods, metal and oxide nuclear fuels, and electroceramic materials. She has published more than 55 peer-reviewed articles and proceedings. Wharry is a recipient of the Department of Energy (DOE) Early Career Award, National Science Foundation CAREER Award, and Oak Ridge Associated Universities (ORAU) Ralph E. Powe Junior Faculty Award. She was the General Chair of the 2019 Materials in Nuclear Energy Systems (MiNES) Conference, Chair of ASTM International Subcommittee E10.08 on Procedures for Radiation Damage Simulation, invited member of the NEA-OECD working group on standards for irradiated materials characterization, and former Chair of the American Nuclear Society (ANS) Materials Science & Technology Division. Wharry is also an editor of Materials Today Communications. She received her PhD in Nuclear Engineering and Radiological Sciences from the University of Michigan in 2012.
Wednesday, December 4, 2019 at 1:30pm to 2:30pm
Nuclear Engineering Building, 302
1412 Circle Drive Knoxville TN 37996