Abstract
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.
Highlights
Nanocrystalline materials possess a high volume fraction of grain boundaries that can act as defect sinks to reduce the amount of damage retained after irradiation
Descriptions of grain boundaries in nanocrystalline materials have ranged from a highly disordered grain boundary structure described as frozen gas-like or amorphous, to the crystalline description adopted for microcrystalline materials[21,22,23,24,25,26,27]
The structural disorder retained in a nanocrystalline material is present in the form of disordered regions, steps, ledges, extrinsic defects, and more complex dislocation content that arises due to excess defect concentrations and extensive grain boundaries (GBs)/defect interactions during processing
Summary
Nanocrystalline materials produced by severe plastic deformation (SPD) contain large fractions of non-equilibrium grain boundaries, as do thin films produced by physical vapor deposition methods, such as sputtering, which are highly non-equilibrium processes These boundaries contain excess free volume and higher energy than their equilibrium counterparts. Materials produced by SPD techniques such as mechanical alloying[38], high energy ball milling[38], equal channel annual pressing[39,40], high pressure torsion[40], and accumulative roll-bonding[41] are susceptible to the formation of these distorted grain boundaries Such grain boundaries have been termed ‘non-equilibrium’ partly due to their relative high energy, excess free volume, dislocation/disclination content, disordered atomic structure, and energetically metastable state as compared to their equilibrium counterparts. The results of this evaluation are presented with a comparison to atomistic simulations of various model boundaries that were performed in tandem to understand the effect of free volume and grain boundary equilibrium on damage accommodation/ annihilation
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