Please consider attending the following: MATERIALS ENGINEERING SEMINAR "Microstructural Characterization and Micromechanical Testing of Laser Weld Repairs on Neutron Irradiated 304 Stainless Steel" By Keyou Mao Purdue MSE Ph.D. Preliminary Exam Advisors: Professor Janelle P. Wharry and Professor Xinghang Zhang ABSTRACT This study is intended for the assessment of microstructural and mechanical integrity of the laser weld repairs to highly neutron-irradiated, helium (He) containing AISI 304 Stainless Steel (SS). To satisfy the demands of the life extensions of Light Water Reactors (LWRs), welding is needed to repair the structural components of the reactor vessels. Conventional welding techniques such as gas tungsten arc welding has high-heat-input, leading the He coalescence into bubbles on the grain boundaries, which exacerbates cracking. The laser welding limits such issue by greatly reducing the heat input. There is consequently a critical need to assess the integrity of these welds for over-extended irradiation service. In this work, single laser welds are made on different plates of 304 SS irradiated in Experimental Breeder Reactor-II (EBR-II) reflector over a range of dose and He concentration conditions: ~1-24dpa with ~0.2-8 appm He, corresponding to ~0.5-2.5% swelling. A combination of transmission electron microscopy and electron backscattered diffraction is utilized to characterize the microstructure of the base, Heat Affected Zone (HAZ) and weld metal, for instance: voids, dislocations, grain structures, precipitates and chemical composition etc. Mechanical integrity in these regions is assessed by three major micromechanical testing methods: 1) Ex-situ and in-situ nanoindentation measures the hardness across the weldment; 2) Scanning Electron Microscopic (SEM) in-situ micropillar compression quantifies the dislocation- -grain-boundary interactions and identifies the deformation and slip transfer mechanisms. 3) SEM in-situ microcantilever beam bending test compares the fracture toughness of single grains dependent on irradiation and welding conditions. Date: Thursday, April 19, 2018 Time: 1:00 P.M. Place: ARMS 3115