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

more »

... ted 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 ++ ions. Both unirradiated and irradiated ODS have defect densities ~5.7 x 10 23 m -3 , or ~12.1 nm obstacle spacing calculated from ref. [3] . Focused ion beam was used to machine compression pillars having rectangular cross-sections (following the technique in ref. [4]) with varying minimum dimensions between 75 nm and 600 nm. Pillars (Fig. 1) were tested using a Hysitron PI95 fitted with a diamond flat punch. The compressive yield strength is determined using the 0.002 strain offset method from the stress-strain curve measured during each pillar test. Seventeen unirradiated pillars and three irradiated pillars are tested. Macroscopic measurements provide a range of 1000-1200 MPa [5], [6] for the unirradiated ODS yield stress, and 1100-1600 MPa [5], [6] for the irradiated yield stress. Thirteen of the unirradiated pillars have yield stresses with error bars that fall within or overlap the expected range, while only four have yield stresses greater than the expected range. All three of the irradiated pillars have yield stresses with error bars that fall within or overlap the expected range. Yield stresses are shown as a function of minimum pillar dimension (Fig. 2 left) and pillar volume (Fig. 2 middle) . These plots show that there is no apparent size threshold below which pillars deviate from bulk-like yield stress. Ongoing work aims to identify the source of the wide variation in yield stresses measured from small-volume pillars. The elastic modulus is also obtained from each compression test. Measurements on the unirradiated pillars are ~20-50 GPa, which is an order of magnitude lower than expected values of ~200 GPa from macroscopic tests [7] . This difference arises because the measurement does not account for deformation 1478