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

Abstract

In situ transmission electron microscopic (TEM) mechanical testing techniques enable concurrent TEM-resolution 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. The high defect density of an irradiated material poses two competing effects. First, material strength is controlled by the interaction between moving dislocations and material defects, so the reduced defect spacing due to irradiation enables measurement of yield stress from miniaturized samples comparable to that from macroscopic tests. However, when further reducing sample size, the stress required to operate ever-smaller dislocation sources must overcome the strength introduced by the radiation-induced defects. Thus, there may be a threshold sample size below which yield stress from in situ TEM tests over predicts that from bulk measurements. Determining this threshold is critical for analysis. An Fe-9Cr ODS alloy (Fe-8.67Cr-1.95W-0.28Y-0.23Ti-0.14C-0.048Si-0.06Ni, in wt.%) is studied here. Irradiations were carried out to 100 displacements per atom (dpa) at 500°C with 5 MeV Fe