Relative grain boundary energies from triple junction geometry: Limitations to assuming the Herring condition in nanocrystalline thin films

Grain boundary character distributions (GBCD) are routinely measured from bulk microcrystalline sam-ples by electron backscatter diffraction (EBSD) and serial sectioning, and this data can be used to recon-struct relative grain boundary energy distributions (GBED) based on the 3D geometry of triple lines, as-suming that the Herring condition of force balance is satisfied. These GBEDs correlate to those predicted from molecular dynamics (MD); furthermore, the GBCD and GBED are found to be inversely correlated. For nanocrystalline thin films, orientation mapping via precession enhanced electron diffraction (PED) has proven effective in measuring the GBCD, but the GBED has not been extracted. Here, the established rel-ative energy extraction technique is adapted to PED data from four sputter deposited samples: a 40 nm-thick tungsten film and a 100 nm aluminum film as-deposited, after 30 and after 150 min annealing at 400 degrees C. These films have columnar grain structures, so serial sectioning is not required to determine boundary inclination. Excepting the most energetically anisotropic and highest population boundaries, i.e. aluminum E3 boundaries, the relative GBED extracted from these data do not correlate with energies calculated using MD nor do they inversely correlate with the experimentally determined GBCD for either the tungsten or aluminum films. Failure to reproduce predicted energetic trends implies that the con-ventional Herring equation cannot be applied to determine relative GBEDs and thus geometries at triple junctions in these films are not well described by this condition; additional geometric factors must con-tribute to determining triple junction geometry and boundary network structure in spatially constrained, polycrystalline materials.

Automated summary

Grain boundary character distributions (GBCD) can be measured from microcrystalline samples using electron backscatter diffraction (EBSD) and can be used to reconstruct relative grain boundary energy distributions (GBED) based on the 3D geometry of triple lines, assuming the force balance condition is satisfied. However, for nanocrystalline thin films, the GBED cannot be extracted effectively using orientation mapping via precession enhanced electron diffraction (PED). In this study, the relative energy extraction technique was adapted to PED data and the results showed that the GBED extracted from these films do not correlate with energies calculated using molecular dynamics (MD) or with the experimentally determined GBCD. This suggests that additional geometric factors contribute to determining the triple junction geometry and boundary network structure in these films.