A theoretical-computational model is developed to study irradiation-induced composition patterns and segregation in binary solid solutions under irradiation, which is motivated by the fact that such composition changes alter a wide range of metallurgical properties of structural alloys used in the nuclear industry. For a binary alloy system, the model is based on a coupled, nonlinear set of reaction-diffusion equations for six defect and atomic species, which include vacancies, three interstitial dumbbell configurations, and the two alloy elements. Two sets of boundary conditions have been considered: periodic boundary conditions, which are used to investigate composition patterning in bulk alloys under irradiation, and reaction boundary conditions to study the radiation-induced segregation at surfaces. Reactions are considered to be either between defects, which is called recombination, or between defects and alloying elements, which result in change in the interstitial dumbbell type. Long range diffusion of all the species is considered to happen by vacancy and interstitialcy mechanisms. As such, diffusion of the alloy elements is coupled to the diffusion of vacancies and interstitials. Defect generation is considered to be associated with collision cascade events that occur randomly in space and time. Each event brings about a change in the local concentration of all the species over the mesoscale material volume affected by the cascade. Stiffly-stable Gear's method has been implemented to solve the reaction-diffusion model numerically. Gear's method is a variant of higher order implicit linear multi-step method, implemented in predictor-corrector fashion. The resulting model has been tested with a miscible CuAu solid solution. For this alloy, and in the absence of boundaries, steady state composition patterns of several nanometers have been observed. Fourier space properties of these patterns have been found to depend on irradiation-specific control parameters, temperature, and initial state of the alloy. Linear stability analysis of the set of reaction-diffusion equations confirms the findings of the numerical simulations. In the presence of boundaries, radiation-induced segregation of alloying species has been observed near in the boundary layer: enrichment of faster diffusing species and depletion of slower diffusing species. Radiation-induced segregation has also been found to depend upon irradiation-specific control parameters and temperature. The results show that the degree of segregation is spatially non-uniform and hence it should be studied in higher dimensions. Proper formulation of the boundary conditions showed that segregation of the alloy elements to the boundary is coupled to the boundary motion. With both patterning and segregation investigations, the irradiated sample has been found to recover its uniform state with time when irradiation is turned off. The inference drawn out from this observation is that in miscible solid solutions irradiation-induced composition patterning and radiation-induced segregation are not realizable in the absence of irradiation. / A Dissertation submitted to the Department of Scientific Computing in partial fulfillment of the requirements for
the degree of Doctor of Philosophy. / Summer Semester, 2012. / June 22, 2012. / Binary alloys, Composition patterning, Irradiation, Reaction-Diffusion, Segregation, Stiffness / Includes bibliographical references. / Anter El Azab, Professor Directing Thesis; Per Arne Rikvold, University Representative; Sachin Shanbhag, Committee Member; Gordon Erlebacher, Committee Member; Tomasz Plewa, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_253528 |
| Contributors | Dubey, Santosh (authoraut), Azab, Anter El (professor directing thesis), Rikvold, Per Arne (university representative), Shanbhag, Sachin (committee member), Erlebacher, Gordon (committee member), Plewa, Tomasz (committee member), Department of Scientific Computing (degree granting department), Florida State University (degree granting institution) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource, computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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