Atomic scale analysis of grain boundary segregation in advanced steels

Metallic alloys can nowadays be understood and manipulated down to near-atomic scale. This applies particularly to interfaces in high strength steels: Their structure and composition can be decisive for brittle or ductile response, respectively.

Fig. 1: Example of a site specific joint STEM-APT analysis of a grain boundary in a Fe-C-Mn martensite alloy. The dots indicate carbon positions measured by APT. The analysis reveals strong segregation of carbon at the lath martensite grain boundary. The misorientation of this lath martensite interface amounts to about 7°. The shown depth of the evaporated volume was approximately 180 nm. — **Fig. 1:** Example of a site specific joint STEM-APT analysis of a grain boundary in a Fe-C-Mn martensite alloy. The dots indicate carbon positions measured by APT. The analysis reveals strong segregation of carbon at the lath martensite grain boundary. The misorientation of this lath martensite interface amounts to about 7°. The shown depth of the evaporated volume was approximately 180 nm.

Max-Planck-Institut für Eisenforschung GmbH

**Fig. 1:** Example of a site specific joint STEM-APT analysis of a grain boundary in a Fe-C-Mn martensite alloy. The dots indicate carbon positions measured by APT. The analysis reveals strong segregation of carbon at the lath martensite grain boundary. The misorientation of this lath martensite interface amounts to about 7°. The shown depth of the evaporated volume was approximately 180 nm.

Max-Planck-Institut für Eisenforschung GmbH

In this context segregation of solutes to grain boundaries plays an essential role. Here we give an introduction to the current state of chemical grain boundary analysis in steels by use of atom probe tomography (APT). The challenge in coupling atomic scale interface characterization by APT with structure analysis via electron microscopy of the same material region is discussed and examples for pearlite and maraging steel are presented. Theoretical analysis of grain boundary segregation is conducted on the basis of the Gibbs and McLean isotherms.