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Many methods used to produce nanocrystalline (NC) materials leave behind non-equilibrium grain boundaries (GBs) containing excess free volume and higher energy than their equilibrium counterparts with identical 5 degrees of freedom. Since non-equilibrium GBs have increased amounts of both strain and free volume, these boundaries may act as more efficient sinks for the excess interstitials and vacancies produced in a material under irradiation as compared to equilibrium GBs. The relative sink strengths of equilibrium and non-equilibrium GBs were explored by comparing the behavior of annealed (equilibrium) and as-deposited (non-equilibrium) NC iron films on irradiation. These results were coupled with atomistic simulations to better reveal the underlying processes occurring on timescales too short to capture using in situ TEM. After irradiation, NC iron with non-equilibrium GBs contains both a smaller number density of defect clusters and a smaller average defect cluster size. Simulations showed that excess free volume contribute to a decreased survival rate of point defects in cascades occurring adjacent to the GB and that these boundaries undergo less dramatic changes in structure upon irradiation. These results suggest that non-equilibrium GBs act as more efficient sinks for defects and could be utilized to create more radiation tolerant materials in future.

Fe thin films were deposited on sodium chloride (NaCl) substrates using magnetron sputtering to investigate means of texture control in free standing metal films. The Fe thin films were studied using transmission electron microscopy equipped with automated crystallographic orientation microscopy. Using this technique, the microstructure of each film was characterized in order to elucidate the effects of altering deposition parameters. The natural tendency for Fe films grown on (100) NaCl is to form a randomly oriented nanocrystalline microstructure. By careful selection of substrate and deposition conditions, it is possible to drive the texture of the film toward a single (100) orientation while retaining the nanocrystalline microstructure.

The performance of nuclear materials in extreme environments poses important fundamental questions about the behavior of condensed matter under far-from-equilibrium conditions.[1] Nuclear materials are exposed to high heat flux and irradiation that alter their microstructure, mechanical properties, and performance.[2] To mitigate possible damage, the use of UltraFine (UF) and NanoCrystalline (NC) metals has been proposed, due to their high grain boundary densities that thus act as high defect and particle sinks, [3] and improve mechanical properties (strength and ductility).[4] While recent research has focused on the effect of grain boundary character [5] on the radiation tolerance, fundamental questions about the role of non-equilibrium grain boundaries in defect absorption in comparison with equilibrium grain boundaries have yet to be answered. Understanding the effect of the non-equilibrium boundary local strain in irregular defect absorption and denuded zone formation could be crucial in engineering polycrystalline materials of higher radiation tolerance that can sustain the severe environments of future nuclear reactor conditions. In this work, in-situ irradiation was performed in a transmission electron microscope (TEM) on freestanding nanocrystalline Fe samples of less than 100 nm grain size. To form the nanocrystalline samples, Fe films was sputter-deposited on NaCl substrates [6]. TEM micrographs of the samples after annealing showed both equilibrium and non-equilibrium (high local strain manifested by strong extinction bands) boundaries adjacent to each other ( Figure.1), thus enabling the comparison between both boundary types. The in-situ TEM irradiation experiments were performed using the i3TEM facility in the Department of Radiation Solid Interactions at Sandia National Laboratories. Crystallographic orientation microscopy (ACOM) was also performed via NanoMEGAS ASTAR precession diffraction (eg. Figure.2). Effect of defect absorption on grain boundary structure is studied through the in-situ irradiation/ACOM experiments. The findings provide fundamental aspects to be considered in engineering high radiation tolerant nanomaterials.

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