Effect of Grain Boundary Structure on Defect Absorption and Denuded Zone Formation in Irradiated Nanocrystalline Iron

Abstract

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.

Quantifying grain boundary damage tolerance with atomistic simulations

“…Despite the valuable insight gained from previous work, a deeper correlation between fully relaxed damage accommodation and various GB metrics is still lacking. Such a correlation is valuable to the GB community and might assist in understanding the origin of defect denuded zones in polycrystalline metals [23]. Therefore, the goal of the present work is to provide a complimentary and more in-depth study of damage accommodation and FV evolution within a selection of both symmetric tilt and random high-angle GBs, to elucidate the fundamental mechanisms behind structural rearrangement, FV accommodation, atomic reordering, and correlate damage behavior with GB character.…”

“…In the early efforts to characterize irradiation damage it was discovered that defect denuded zones would form around GBs in irradiated metals. Subsequent studies have shown that some boundaries have higher sink efficiencies than others, leading to the conclusion that GB character plays a role in this behavior [3][4][5][6][15][16][17][18][19][20][21][22][23].…”

Point defect sink efficiency of low-angle tilt grain boundaries

“…The grain boundary sink efficiency, defined as the ratio between the fluxes of point defects into a GB to that into an ideal planar sink (where the point defect concentration is fixed at its thermal equilibirum value), is a measure of the efficacy of the grain boundary to absorb point defects. Many experimental studies and atomistic simulations have been performed to understand the factors that determine the GB sink efficiency [12,13,10,14,15]. For example, Bai et al [16] used accelerated molecular dynamics to investigate the point defect-GB interaction mechanism from picosecond to microsecond time scales.…”

Role of Grain Boundaries under Long-Time Radiation

“…It is widely agreed that interfaces, such as grain boundaries (GBs), can act as sinks to crystalline point defects (PDs), i.e., interstitial atoms and vacancies [1][2][3][4]. Thus enhancing the GB fraction in materials seems a promising way to develop materials serving in radiation environments, where point defects are prevalent [5,6].…”