In-situ compression videos on laser welds of irradiated 304L SS

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By Keyou Mao

Purdue University

In-situ compression videos for Mao's Ph.D. dissertation.

Version 1.0 - published on 30 May 2019 doi:10.4231/G2WP-M551 - cite this Archived on 30 Jun 2019

Licensed under CC0 1.0 Universal

Description

In-situ micropillar compression videos on the base metal and HAZ of laser welds of irradiated AISI 304L SS of 1 dpa and 0.2 appm He. 

These videos accompany a dissertation describing recent advancements in micro-mechanical testing that inform how deformation mechanisms in 304L stainless steels (SS) are affected by the presence of irradiation-induced defects. Austenitic 304L 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 relatively poorly understood. Now, recent advancement of micro-scale 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.

We focus on AISI 304L stainless steel irradiated in EBR-II to 20 displacements per atom (dpa) at 415°C, and contains ~3 atomic parts per million (appm) He amounting to 1.5% swelling.  A portion of the specimen is laser welded in-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. TEM investigation reveals nucleation of deformation-induced α’ martensite in the irradiated specimen, and metastable ε 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 sufficient free energy to accommodate the martensitic transformation. The lower population of cavities in the PIA material enables ε martensite formation, while the higher cavity number density in the irradiated material causes α’ martensite fromation. SEM-based micropillar compression tests confirm nanoindentation results. Irradiation damage could enable fundamental, mechanistic studies of deformation mechanisms that are typically only accessible at extremely low temperatures.

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