Wei Ying Chen

Characterization of dislocation loops in CeO2 irradiated with high energy Krypton and Xenon

In order to fully characterize the structure of dislocation loops in CeO2, ion irradiations have been performed at 800 °C on CeO2 single-crystal thin films individually using 1 MeV Kr ions and 150 keV Xe ions, both to a dose of 5 × 1014 ions/cm2. Post-irradiation TEM examination, diffraction contrast imaging and high-resolution transmission electron microscopy (HRTEM), has confirmed that the irradiation-induced dislocation loops in CeO2 were Frank loops, having an interstitial nature with {1 1 1} habit planes and a 1/3 Burgers vector. Dislocation loops were confirmed to be a stacking fault in nature through TEM observations of the interference fringes inside the loop periphery.

Crack Growth Rate and Fracture Toughness of CF3 Cast Stainless Steels at ~3 DPA

Cast austenitic stainless steels (CASS) used in reactor core internals are subject to high-temperature coolant and energetic neutron irradiation during power operations. Due to both thermal aging and irradiation embrittlement, the long-term performance of CASS materials is of concern. To assess the cracking behavior of irradiated CASS alloys, crack growth rate (CGR) and fracture toughness J-R curve tests were performed on two CF3 alloys. Miniature compact tension specimens were irradiated to ~3 dpa, and were tested at ~315 °C in simulated LWR coolant environments with low corrosion potentials. No elevated cracking susceptibility was observed at this dose in the test environments. The power exponents of the 3 dpa J-R curves were much lower than that of unirradiated or irradiated specimens at lower doses, indicating a significant decline in fracture resistance. A preliminary microstructural study revealed irradiation-induced microstructural changes in both austenite and ferrite, suggesting an embrittlement mechanism involving both phases at this dose level.

Microstructure and Deformation Behavior of Thermally Aged Cast Austenitic Stainless Steels

Cast austenitic stainless steels (CASS) consist of a dual-phase microstructure of delta ferrite and austenite. The ferrite phase is critical for the service performance of CASS alloys, but can also undergo significant microstructural changes at elevated temperatures, leading to severe embrittlement. To understand thermal aging embrittlement, fracture toughness J-R curve tests were performed on unaged and aged CF8 specimens at 315 ℃. The microstructure of CF8 was also examined before and after thermal aging with transmission electron microscopy and atom probe tomography. While no microstructural change was observed in the austenite after thermal aging, a high density of G-phase precipitates and a phase separation of alpha/alpha prime were detected in ferrite. To study the deformation behavior, tensile tests were performed at room temperature with in situ wide-angle X-ray scattering measurements. The differences in lattice strains between ferrite and austenite were much higher in the aged than in the unaged samples, suggesting a higher degree of incompatible deformation between ferrite and austenite in the aged samples.