Microstructural Characterization of Ion Irradiated ODS MA956 Alloy

Oxide Dispersion Strengthened (ODS) alloys are considered as promising nuclear materials for the next generation advanced nuclear reactors due to their improved elevated temperature mechanical properties and radiation damage resistance [1,2]. MA956 is a commercial ODS alloy with superior oxidation resistance and creep strength at elevated temperatures. The improved properties for ODS alloys can be attributed to the presence of dispersed nanosized oxide particles. These oxide particles can act as barriers to dislocation motion and provide additional sinks and sites for point defect recombination [3]. Hence, their stability can greatly affect the properties of the irradiated ODS alloys. Besides, irradiation will induce defects into the ODS alloys, and these defects can develop further as irradiation dose increases [4]. The defects propagation can significantly undermine the mechanical properties of the bulk materials, resulting in irradiation hardening and lower ductility. Therefore, it is necessary to have a good understanding of the defect’s evolution with irradiation. Hence, efforts are ongoing to understand the effect of ion irradiation on ODS MA956 with emphasis on oxide particles and evolution of defects as a function of dose (2.5, 50 and 100 dpa). ODS MA956 samples were ion irradiated using 5 MeV Fe2+ at 190°C to 2.5, 50 and 100 dpa at Texas A&M University Accelerator Laboratory. Post irradiation characterization was carried out at Microscopy and Characterization Suite (MaCS), Center of Advanced Energy Studies (CAES). A FEI Quanta 3D FIB was used to prepare TEM (transmission electron microscopy) lamellae and APT (atom probe tomography) tips. A FEI Tecnai G2 F30 STEM was used to characterize the microstructure of MA956 and a CAMECA LEAP 4000X HR Local Electrode Atom Probe (LEAP) was used to study the chemical composition at the oxide particle-matrix interface. A Hysitron TI-950 TriboIndenter was employed to obtain nanoindentation hardness of unirradiated and irradiated MA956 samples. For ion irradiated samples, the irradiation dose is not uniform, and it varies with depth. The peak dose is at around 1 um below the surface. However, the ion implantation is too strong at that depth. Therefore, both the TEM and atom probe characterization for this study was focused at about 500 nm depth from the irradiation surface. At this depth, the actual irradiation doses based on the SRIM plot for the three irradiated samples are 1.25 dpa, 25 dpa and 50 dpa, respectively. BF TEM images of (a) 0 dpa, (b) 1.25 dpa, (c) 25 dpa, (d) 50 dpa ion irradiated samples, and on-zone axis BF STEM images of (e) 0 dpa, (f) 1.25 dpa, (g) 25 dpa, and (h) 50 dpa ion irradiated samples, respectively. Figures 1. (a)-(d) show the BF TEM images, while (e)-(f) show the on-zone axis BF STEM images for the control, and three ion irradiated samples, respectively. The BF TEM images were used to quantify the oxide particles present in MA956, while the BF STEM images were used to quantify the defects. Our study showed that the oxide particles have a slightly larger size and lower number density in the irradiated regions compared with the unirradiated regions, suggesting that irradiation has coarsening effect on it, but the effect is limited. For irradiation induced defects, small dot-like dislocation loops first appear at 1.25 dpa. As the ion irradiation dose increased from 1.25 dpa to 50 dpa, the dislocation loops increased in size and reduced in number density, showing a coarsening phenomenon. In contrast, the dislocation line density did not increase from 1.25 dpa to 25 dpa, but increased from 25 dpa to 50 dpa. No voids were observed in these irradiated samples. These observations suggest that under these irradiation conditions, the major microstructural change is the irradiation induced defects. As the irradiation dose increases, dislocation loop appears and grows first. Once its growth saturates, the dislocation lines propagate. Besides TEM characterization, APT was used to study the chemical composition change in oxide particles. It was found that the major elements are not affected much by irradiation. However, trace elements like Ti diffused into the oxide particle from the interface after irradiation. Finally, it is important to understand the mechanical property changes relative to the microstructure evolution. The nanoindentation experiment showed that the hardness increased after irradiation, and the increment showed a saturation trend. While comparing the hardness with microstructure characterization results, the hardening corresponded well with the irradiation induced defects, especially with the dislocation loops. Microstructure characterization and nanoindentation experiments were performed on unirradiated and ion irradiated ODS MA956. The oxide particles were not significantly affected by these irradiation conditions. But it showed a slight coarsening and chemical evolution after irradiation. The major microstructure evolution after irradiation is the defects. Both dislocation loops and lines increased after irradiation, but no voids were observed. The nanoindentation studies showed that the hardness increased after irradiation and the increment is related with defects evolution, especially with loops [5].