Date of Award
Open Access Thesis
ODS alloys, or oxide dispersion strengthened alloys, are made from elemental or pre-alloyed metal powders mechanically alloyed with oxide powders in a high-energy attributor mill, and then consolidated by either hot isostatic pressing or hot extrusion causing the production of nanometer scale oxide and carbide particles within the alloy matrix; crystalline properties such as creep strength, ductility, corrosion resistance, tensile strength, swelling resistance, and resistance to embrittlement are all observed to be improved by the presence of nanoparticles in the matrix. The presented research uses various methods to observe and characterize the microstructural and microchemical properties of two experimental ODS alloys, 18Cr ODS and 9Cr ODS. The results found aid in assessing the influence of chemical and structural variations on the effectiveness of the alloy, and further aid in the optimization of these advanced alloys for future use in nuclear cladding and structural applications in Generation IV nuclear reactors. Characterization of these alloys has been conducted in order to identify the second-phase small precipitates through FESEM, TEM, EDS, Synchrotron X-ray diffraction analysis, and CuKα XRD analysis of bulk samples and of nanoparticles after extraction from the alloy matrix. Comparison of results from these methods allows further substantiation of the accuracy of observed nanoparticle composition and identification. Also, TEM samples of the two alloys have been irradiated in-situ with 1 MeV Kr and 300 keV Fe ions to various doses and temperatures at the IVEM-Tandem TEM at Argonne National Laboratory and post-irradiated characterization has been conducted and compared to the pre-irradiated characterization results in order to observe the microstructural and microchemical evolution of nanoparticles under irradiation. Overall in the as-received state, the initial Y2O3 is not found anymore and in addition to oxide particles the alloys contain carbides. In both alloys a good correlation between the EDAX and XRD results is found both before and after extraction of the nanoparticles from the matrix. Both alloys show the presence of Y-Ti-O particles as well as Al-containing compounds, and Cr- carbides of the M23C6 type. The oxide particles in 18Cr ODS appear to be stable under irradiation up to 20dpa at 500°C. There may be a decrease in density and increase in average size in both particle families. In the 9Cr ODS alloy, comparison of results from both methods of analysis also shows the presence (in the as-received state) of a combination of carbides and Y- containing oxides, and Al-containing compounds are also found. After irradiation small Y-Ti-O particles appear to be enriched with Cr and C, and larger particles show indications of amorphization at 25°C. Further irradiations on both alloys will provide deeper insight into the structural and chemical stability of the oxide and carbide nanoparticles and the merit of both alloys as structural and cladding materials in future nuclear applications.
Goddard, J. K.(2013). Characterization of Two ODS Alloys: 18Cr ODS and 9Cr ODS. (Master's thesis). Retrieved from http://scholarcommons.sc.edu/etd/1666