TECHNICAL: The advanced energy systems push operating environments to more severe and aggressive extremes, including high temperature, high radiation dose, and high mechanical stresses. A new generation of materials is required to meet these challenges since current materials, which were designed using knowledge developed as long as 50 years ago, will not operate safely, reliably, and economically under these conditions. This program explores a fundamentally new approach for the design of materials that would permanently resist radiation. PIs plan a new approach whereby alloy microstructures are designed to include a high density of point-defect traps that are dynamically stable under irradiation. This goal is achieved by taking advantage of PIs? past work on nanoscale compositional patterning induced by irradiation, and learning how to use these compositional heterogeneities as effective traps for point defects. By design, the nanostructured materials would thus be radiation-insensitive. PIs plan to apply this approach to selected Cu-base and Fe-base model alloys, as well as to similar alloys strengthened by nanoscale oxide dispersion. In the latter case, nanocomposite oxide-metal thin films are grown by combining cluster beam deposition with magnetron sputtering. The thin films are characterized before and after ion irradiation with a combination of techniques, including XRD, TEM, and atom probe tomography. A particular emphasis is put on the latter since it achieves sub-nanometric chemical resolution in three dimensions, and thus makes it possible to fully characterize nanostructured compositional patterns. The point-defect trapping efficiency of these nanostructures is assessed by radiation-enhanced diffusion and swelling measurements. A computational effort would identify the conditions for compositional patterns to remain dynamically stable even as the microstructure is slowly drifting. To achieve this goal, in collaboration with Lawrence Livermore National Laboratory, PIs will implement a new parallel kinetic Monte Carlo algorithm that speeds up simulations by several orders of magnitude, thus making it possible to follow the complex evolution of the microstructure in the simulations. NON-TECHNICAL: The research has broad scientific impact for the development of alloy design strategies for new materials that are critical to advanced energy production systems. Besides publishing widely the results from this research, PIs plan to organize, in the US, a summer school on Materials under Irradiation. The objective is to educate the next generation of scientists and engineers required to maintain or even expand the share of nuclear energy in the US energy production portfolio. In addition, the two graduate students working on this project will be trained on the most advanced instruments of materials characterization. PIs will hire undergraduate assistants, in particular women and underrepresented minorities. The present work will be integrated into the PIs teaching activities, exposing undergraduate students to the potentials and the challenges offered by nanostructured materials. PIs will also expand ?Materials Mobile? high-school visit program so as to reach a much larger student population.
Project Outcomes Materials often operate in the presence of external forces that drive them into non-equilibrium states, in turn modifying their properties and thus their performance. For instance, materials used in nuclear reactors or in space applications are continuously subjected to a flux of energetic particles. While this continuous irradiation is often detrimental and limits the useful material’s life, it has been recognized that, under suitable conditions, it can in fact be used to design materials that become radiation-resistant by developing self-organized microstructures. In order to implement this approach for the design of advanced technological materials, it is first necessary to learn how to trigger and control these self-organization reactions. In this research we investigated the fundamentals of microstructural self-organization in model Cu-base alloys subjected to ion irradiation, by combining experiments on alloy thin films and atomistic simulations. It has been found that ion irradiation at room temperature can result in the precipitation of highly immiscible elements such as Mo and W, leading to the formation of nanoprecipitates that are very strong and very stable during exposure to further irradiation or thermal annealing. Furthermore, by using two alloying elements, for instance using Cu1-x-yNbxWy, core/shell nanostructures can be stabilized, producing microstructures that do not coarsen even at temperatures as high as 650?C (≈ 1200 ?F). We have developed and used atomistic simulation techniques to better understand these nanoscale self-organization processes. These simulations, for instance, revealed that irradiation can stabilize unique non-equilibrium nanostructures, where precipitates contain small clusters of the matrix atom, similarly to Russian dolls. These nanostructures are spatially and temporally self-organized, going through cycles of nucleation, growth and absorption by the matrix. Remarkably similar structures have been observed experimentally in Cu-Fe and Cu-V alloys subjected to ion irradiation. The research has advanced considerably our understanding of nanostructuring under irradiation and it has shown how to design alloy systems that possess outstanding resistance to coarsening during exposure to elevated temperatures. More broadly, the research program has supported two PhD graduate students, who will graduate in 2014, one Masters graduate student, who graduated in 2011, and two undergraduate students, who graduated in 2011 and 2012, and have enrolled in graduate school . These students have been trained on multiple advanced techniques for the synthesis, the characterization, and the modeling of materials. During this research, an outreach program has also been developed, which includes show-and-tell visits to local high schools to encourage K-12 students to pursue STEM careers, and in particular in materials science and engineering.
