Yaqiao Wu

EBSD and TEM Analysis of the Heat Affected Zone of Laser Welded AISI 304/308 Stainless Steel

The objective of this study is to utilize a range of advanced microscopy techniques to characterize the heat affected zone (HAZ) of an AISI 304 stainless steel (SS) laser weld. Laser weldments of 304 SS, made with Type 308 SS as a filler material, are of interest to the nuclear power industry because of its low energy input as compared to conventional welding techniques. Laser welds are especially relevant for mid-life weld repairs of light water reactor (LWR) internals, which contain helium, irradiationinduced voids, and other irradiated microstructural phenomena [1]. Conventional welding processes such as gas tungsten arc welding (GTAW) introduce thermal stresses, which leads to the formation of helium bubbles on the grain boundaries [2]. This phenomenon directly results in disastrous heliuminduced cracking on the weld boundary. Laser welding, however, is hypothesized to reduce the helium coalescence and cracking at the weld boundary, due to the reduced heat input that inhibits surface cracking and subsurface defects [3]. Nevertheless, no studies have characterized the laser welded microstructures at resolutions greater than scanning electron microscopy (SEM) level resolution. In this work, we utilize a combination of SEM and transmission electron microscopy (TEM) to understand the laser welded microstructure of a laser weld on unirradiated and irradiated 304 SS.

In situ TEM Mechanical Testing: An Emerging Approach for Characterization of Polycrystalline, Irradiated Alloys

In situ transmission electron microscopic (TEM) mechanical testing techniques enable concurrent TEMresolution imaging/video and mechanical testing of sub-micron-sized electron-transparent specimens. Because nuclear materials are often volume-limited, due to constraints imposed either by radioactivity levels or near-surface ion irradiation damage layers, in situ TEM mechanical testing presents great potential for analyzing these small specimen volumes. But thus far, only a few studies have conducted in situ TEM mechanical tests on irradiated or engineering alloys. Irradiated alloy work [1] focused on single-crystal Cu that was irradiated after the TEM specimen was fabricated, while the oxide dispersion strengthened (ODS) alloy tested [2] was unirradiated. The objective of the present study, then, is to extend the use of in situ TEM mechanical tests to ODS alloys that have previously been irradiated in bulk form, which is a conventional specimen configuration amongst the nuclear materials community.