Strain-Induced Asymmetric Line Segregation at Faceted Si Grain Boundaries

The unique combination of atomic-scale composition measurements, employing atom probe tomography, atomic structure determination with picometer resolution by aberration-corrected scanning transmission electron microscopy, and atomistic simulations reveals site-specific linear segregation features at grain boundary facet junctions. More specific, an asymmetric line segregation along one particular type of facet junction core, instead of a homogeneous decoration of the facet planes, is observed. Molecular-statics calculations show that this segregation pattern is a consequence of the interplay between the asymmetric core structure and its corresponding local strain state. Our results contrast with the classical view of a homogeneous decoration of the facet planes and evidence a complex segregation patterning.

Figure 1

Atomic structure and chemistry of a faceted $\sum 9$ tilt grain boundary analyzed on a single specimen. (a) Schematic illustration of the faceted interface with asymmetric line segregation. (b) Overview ABF-STEM image of an APT/TEM specimen resolving the faceted structure of the grain boundary superimposed by APT reconstruction. Extended magnification of the highlighted area showing a strain sensitive LAADF-STEM micrograph, a 2D density map of impurities, and a 3D atom map. (c) Corresponding projection of 3D atom map with reconstructed lattice planes and GB topology. (d) High resolution HAADF-STEM image resolving the atomic structure in the vicinity of the right-hand GB junction.

Figure 2

Atomic structure of a faceted $\sum 3$ tilt grain boundary resolved by HR-STEM. (a) HAADF-STEM image of a TEM specimen resolving the faceted structure of the grain boundary. (b) Strain sensitive LAADF-STEM micrograph of the same area. (c) High resolution HAADF-STEM image of the GB segment highlighted in (a), resolving the structural motifs of the two facets and the facet junction cores (I) and (II).

Figure 3

Asymmetric line segregation observed by atom probe tomography at a faceted $\sum 3$ tilt grain boundary. (a) 3D atom map of the faceted interface viewed along the common tilt direction [110] showing co-segregation of C, Fe, and N impurities. Fe and N have been identified with the help of a ${\mathrm{FeN}}^{2+}$ molecular species. (b) Lattice planes can be resolved in the magnified volume. (c) 3D atom map of the grain boundary region, where the impurity atoms are plotted thicker in close proximity to the center of the segregation line for better visualization. (d) Corresponding atomic density plot resolving nine individual peaks.

Figure 4

MEAM potential simulations of the complex strain state and its impact on the C segregation at a faceted $\sum 3$ tilt grain boundary. (a) Local atomic strain at a nanoscale $\sum 3\{111\}$ twin segment embedded between two asymmetric $\sum 3\{112\}$ facets. Blue regions are under compressive, red regions under tensile strain. (b) C concentration with respect to the bulk. Inset middle: Atomic strain at a flat asymmetric, incoherent $\sum 3\{112\}$ segment. Color code is the same as in (a). Inset left and right: Atomic geometries at the junctions (I) and (II), respectively, with blue (red) $>1\%$ ($<-1\%$) atomic strain than at the symmetry equivalent site at a flat asymmetric $\sum 3\{112\}$ segment. Vertical solid lines enclose the $1\times 1$ unit cell of the asymmetric $\sum 3\{112\}\mathrm{GB}$. Dashed lines indicate the $\sum 3\{111\}$ twin. (c) Comparison of experimental STEM image and corresponding atomic strain at the core of facet junction (I). (d) Same comparison of facet junction core (II). The colored regions in the experimental images highlight the facet junction cores, respectively.