Experimental and numerical assessment of grain boundary energies in polycrystalline uranium dioxide

Abstract

Uranium dioxide ceramics are widely used as nuclear fuels. Thus, it is important to understand the role of the grain boundaries (GBs) which decisively govern the properties of these polycrystalline materials and subsequently determine their performances. Here, we report a coupled numerical-experimental approach enabling to assess GB energies. Firstly, GB formation energies (γgb) were computed for 34 symmetric tilt GBs in UO2 with molecular dynamics simulations at 1700 K. The surface energies (γS) relative to the respective planes of these GBs were calculated as well. The Herring relation was then used to assess the dihedral angles Ψ of the corresponding GB grooves. Secondly, a UO2 ceramic sample was annealed at 1673 K to obtain GB grooves. The CSL GBs of interest were identified by EBSD and their Ψ angles determined by AFM. Computed and measured Ψ values were found to be very close.

Molecular dynamics study of UO2 symmetric tilt grain boundaries around [001] axis

“…We showed that the cleavage energy seems to decrease with the misorientation angle of CSL grain boundaries 19 . Using the thermal grooving method, we have also measured the formation energies of several CSL grain boundaries on a polycrystalline sample 8 . Measured energies appear to be in good agreement with calculated energies, which demonstrates the reliability of the calculations and the relevance of our coupled approach.…”

“…In a previous work, we simulated the most common grain boundaries observed in the UO 2 polycrystalline materials using four empirical potentials. As a result of this study, the CRG potential was selected to assess the grain boundary energy mainly for two reasons: The energies calculated using this potential at 300 K were consistent with the classification of the grain boundaries as a function of the linear fraction measured by EBSD 19 and in good agreement with measured energies for various grain boundaries 8 ; This potential yielded good results with regard to the evolution of thermodynamic properties with temperature (see Ref. [30] and references therein). …”

“…In this work, we calculated formation energies of the grain boundaries, surface energies of the grain boundary planes, cleavage energies, formation energy of Schottky defects near the STGB, and incorporation energies of fission gases atoms in Schottky defects. Formation energies of grain boundaries, surface energies, and cleavage energies were calculated as presented in our previous works 8,19 . The formation energy is the excess energy resulting from the presence of an interface compared to the energy of the corresponding single crystal.…”

“…Experimental measurements on grain boundaries are common on nonradioactive materials such as metals or yttria stabilized zirconia (YSZ) 6,7 . Concerning UO2, far fewer results are available, 10 but our Institute recently proposed the first measurements of formation energies and toughness of grain boundaries in UO 2 8‐10 . Numerical simulations are a good alternative to this small amount of experimental data.…”

“…In recent works, 8,19 we have studied UO 2 grain boundaries following an original approach coupling experiments and numerical simulations. First, we used 2D‐EBSD measurements to determine the distribution of grain boundaries of several polycrystalline UO 2 samples regarding their CSL index 19 .…”