<p>
This dissertation describes the recent advancements in
micromechanical testing that inform how deformation mechanisms in austenitic stainless
steels (SS) are affected by the presence of irradiation-induced defects.
Austenitic SS is one of the most widely utilized structural alloys in nuclear
energy systems, but the role of irradiation on its underlying mechanisms of
mechanical deformation remains poorly understood. Now, recent advancement of
microscale mechanical testing in a scanning electron microscope (SEM), coupled
with site-specific transmission electron microscopy (TEM), enables us to
precisely determine deformation mechanisms as a function of plastic strain and
grain orientation.</p>
<p> </p>
<p>We focus on AISI 304L SSs irradiated in
EBR-II to ~1-28 displacements per atom (dpa) at ~415 °C and contains ~0.2-8
atomic parts per million (appm) He amounting to ~0.2-2.8% swelling. A portion
of the specimen is laser welded in a hot cell; the laser weld heat affected
zone (HAZ) is studied and considered to have undergone post-irradiation
annealing (PIA). An archival, virgin specimen is also studied as a control. We
conduct nanoindentation, then prepare TEM lamellae from the indent plastic
zone. In the 3 appm He condition, TEM investigation reveals nucleation of
deformation-induced <i>α</i>’ martensite in
the irradiated specimen, and metastable <i>ε</i>
martensite in the PIA specimen. Meanwhile, the unirradiated control specimen
exhibits evidence only of dislocation slip and twinning; this is unsurprising
given that alternative deformation mechanisms such as twinning and martensitic
transformation are typically observed only near cryogenic temperatures in
austenitic SS. Surface area of irradiation-produced cavities contribute enough
free energy to accommodate the martensitic transformation. The lower population
of cavities in the PIA material enables metastable <i>ε</i> martensite formation, while the higher cavity number density in
the irradiated material causes direct <i>α</i>’
martensite formation. In the 0.2 appm He condition, SEM-based micropillar
compression tests confirm nanoindentation results. A deformation transition map
with corresponding criteria has been proposed for tailoring the plasticity of irradiated
steels. Irradiation damage could enable fundamental, mechanistic studies of
deformation mechanisms that are typically only accessible at extremely low
temperatures. </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8983583 |
| Date | 02 August 2019 |
| Creators | Keyou Mao (6848774) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/INFLUENCE_OF_IRRADIATION_AND_LASER_WELDING_ON_DEFORMATION_MECHANISMS_IN_AUSTENITIC_STAINLESS_STEELS/8983583 |
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