Bubble Stability and Mobility in Nickel under In Situ Ion Irradiation

Materials within nuclear reactors undergo irradiation that results in adverse microstructural changes, including the formation of point defects, defect clusters, voids, dislocation loops, segregation, and precipitation. The Intermediate voltage electron microscope (IVEM)-Tandem Facility at Argonne National Laboratory is a user facility for <i>in situ</i> TEM study of the radiation effects on material microstructures. It interfaces a 500 kV ion accelerator and a 20 kV helium ion source to a 300 kV Hitachi H-9000NAR TEM, allowing real-time microscopy under dual-beam ion irradiation damage/implantation with well-controlled conditions including specimen orientation, temperature, ion type, ion energy, dose, dose rate, applied strain. <i>In situ</i> TEM has unique capability to capture videos, offering quantitative insights into the dynamic process that cannot be obtained with <i>ex situ</i> experiments.<br/>Our previous study in IVEM observed voids growing or shrinking in nickel at various temperatures from 525^○C to 650^○C under 1 MeV krypton ion irradiation with a flux of 6.3x1011 ions-cm⁻²-s⁻¹. Initially, 6 samples underwent irradiation at 600^○C to achieve a damage level of 0.5 dpa, which resulted in the formation of voids with an average size of about 15 nm within the TEM foil. Subsequently, the temperature was individually adjusted to 525^○C, 550^○C, 575^○C, 600^○C, 625^○C, and 650^○C. Following this temperature adjustment, irradiation continued for an additional 1 dpa. The stability of the voids at these new temperatures was monitored through <i>in situ</i> videos in regions where the foil thickness was approximately 100 nm. A total of about 5000 frames of videos were recorded. We applied deep learning-based semantic segmentation model (IoU score = 0.8135) to automatically analyze the videos and showed that voids shrank at low temperatures and grew at high temperatures under irradiation, where the transition occurred at 575^○C (~0.5 TM).<br/>This presentation will focus on the behavior of small Kr-containing bubbles, which had a size smaller than 10 nm. Unlike larger, immobile voids, these small bubbles are mobile under irradiation. Notably, the bubbles were often found tagging along with the irradiation-induced dislocations loops. The movement of those bubbles was confined to the path where dislocation loops expanded or shrank. For bubbles not associated with dislocations, on the other hand, their movements appeared to be random. Besides, while large voids tended to grow steadily at temperatures above 575^○C, the bubbles were unstable, switching between growth and shrinkage. The behavior of individual bubbles may be affected by its own unique surroundings such as nearby voids, bubbles, dislocations, and concentration of point defects. Understanding how these individual bubbles exhibited heterogeneous behavior and added up contributing to the resultant macroscopic property requires tracking capability.<br/>We developed a tracking algorithm using Trackpy, and showed that, in average, void stability increases with void size, while void mobility decreases with void size. Nonetheless, there is significant variability in growth or mobility across voids of the same size and for the same void at different times. Besides, the analysis allows us to identify the threshold size at which bubbles transition from instability to steady growth. The temperature dependence on the bubble stability and mobility will also be discussed.