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3/1/2007 9:30 AM
1) Characterization of Nanostructural Features in Reactor Pressure Vessel Steels and Model Alloys
Irradiation embrittlement in RPV steels occurs due to the presence of Cu nano precipitates. The Nano precipitates grow with increasing fluence. Cu-rich precipitates are the dominating hardening mechanisms. Presence of Ni refines the precipitates. Introduction to positrons, where they go in the material and what happens to them was discussed as a background to the project. Atom probe tomography confirmed the presence of Mn-Ni enriched precipitates in alloys with no or low Cu. Also large volume density of late blooming phases cause severe embrittlement.
2) Kinetic Monte Carlo Simulations of Substitutional Helium Diffusion
Ferritic-martensitic steels experience severe radiation induced degradation of many important performance sustaining mechanical properties. Also potential dimensional instabilities in fusion environment are driven by production of defects and high concentration of Helium. Among the major problems is radiation induced segregation, diffusion and cluster segregation. Atomistic based modeling is used to investigate He fate upon slowing down and thermal He diffusion in BCC occurs by a vacancy mechanism. He is considered to be born interstitial, although trapping with self-interstitial atom can occur and substitutional He at about 20% of the time.
3) The Effect of Nickel on Neutron Irradiated Copper-Bearing RPV Alloys
Role of Cu in embrittlement of A533B and related steels as pressure vessels on nuclear reactors is subject of atom probe study. Copper rich precipitates rich in Mn, Ni were observed at dislocations and grain boundaries. Phosphorous also segregates to these boundaries and dislocations. Yield and ultimate tensile strength increase after neutron irradiation with increasing copper content. High temperature and low fluence irradiation treatment produce significantly smaller increase in Y.S, U.T.S. Cu rich precipitates diffuse in low T and high fluence condition. Microstructure of Cu RPV was studied by local electrode atom probe.
4) The Effects of Helium and Hydrogen in Irradiated BCC Iron
The aim is to utilize experimental and modeling techniques to understand defect evolution and its effect on the microstructure and resulting mechanical and physical properties. Linear increase in bubble size with temperature was observed. Bubble density went down with increase in temperature. Bubble migration was activated at higher temperature leading to bubble coalescence. 250KeV proton irradiation of polycrystalline Fe was performed. Black dots were observed. No evidence of irradiation damage was observed at higher radiation temperature. Also annealing effect was seen.
5) Thermal Helium Desorption of Helium Implanted Iron
F-M steels degrade in severe fusion environment due to embrittlement. Crucial aspects worth looking at are to understand the migration and trapping of He. Thermal He desorption spectroscopy was used to understand the kinetics of He in Fe. Specimens were irradiated with 5-20KeV He. More He atoms desorp at higher temperature with increasing implantation energy. Series of desorption events in each spectra were divided into T and activation energy ranges. Impurities were identified as traps for He. Broad release peaks at low and medium temperature were understood by using diffusion model kinetics.
6) Micromechanical Testing of Ion-Irradiated Fe and Fe-Cr Alloys.
Ion irradiation is used to mimick the damage created by fast neutrons. The depth of damage is about 2-3 microns. Till now majorly nanoindentation was used as a technique to characterize the damage. Focussed ion beam (FIB) is a new method to manufacture cantilever beams inside single grains. Microbeams are imaged using a nanoindenter using AFM scanning mode. Simple beam theory is used to convert the force-displacement curves into stress-strain curves. Micromechanical testing seems to be a feasible technique to study thin irradiated regions. Plastic response was observed to be complex and both work hardening and plastic zone expansion contributed to results.
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