296 Andreas Stoffers et al.


Figure 5. APT reconstruction (LEAP 5000 XS) showing C segregation along two different GB facets (a). Detector den- sity maps were plotted at the position of blue plane in the upper grain (b) and at the position of the red plane in the lower grain (c) enabling the identification of several pole directions. The APT reconstruction was optimized for the volume corresponding to the dashed circles until the lattice distances fitted the expected values (d, e).


Figure 6. APT reconstruction (LEAP 5000 XS) showing strong differences in the segregation behavior of the observed impurity species at the two facets.


This phenomenon can now be interpreted in correlation to the HRSTEMdata. The GB consists mainly of the two facets shown in Figure 4, which profoundly differ in atomic arrangement. Most likely, these differences in atomic struc- ture influence the impurity segregation. The known direc- tions of the <110> zone axis in the APT specimen and the reconstructed GB planes, allow us to determine that facet B


Table 1. Calculated Gibbs’ interfacial excess values. at./nm2


ΓSiO


Facet A 0.17 —


Facet B Kink


0.30 ΓC


0.08 0.09 0.09


ΓFeN


0.02 0.15


ΓO 0.23 ΓCu


0.04 —— —


0.03 0.06


most likely has a (122) habit plane and thus consists of the structure observed in Figure 4. Our observation in the present study suggests that segregation is sensitive to the local GB facet and thus to the local atomic arrangements. Preferential segregation might be caused by the fact that the (122) structure of facet B offers not much space for interstitial impurities. Hence, only substitutional C atoms and small amounts of interstitial Cu and (Fe+N) are segregating. In contrast, a relatively high amount of (interstitial) O impurities can be observed at facet A, which seems to have much more complex atomic structure with a higher probability to offer open sites for impurity segrega- tion. Furthermore, the different impurity species might mutually influence their segregation behavior. Recent computational work on GB segregation revealed complicated


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