Intragranular segregation of boron in Fe-40 at% Al with B2 structure: Influence of the interaction of thermal vacancies and boron atoms

Boron segregation to grain boundaries was studied in ordered (B2) FeAl containing 40 at% Al. It was observed that small additions of boron (200 at ppm) change the fracture mode from intergranular to mixed (transgranular and intergranular). A low level of boron (few at%) was detected by AES in grain boundaries of this hypostoechiometric alloy. Nevertheless, the kinetic of boron segregation is rapid : even a quench from 950 degrees C promotes it. As FeAl alloys are known to retain large concentrations of thermal vacancies, the possibility of an interaction between boron atoms and vacancies was investigated in order to understand the mechanism of boron segregation. Dilatometric isothermal experiments showed that vacancies eliminate more rapidly in B-doped Fe-40 at%Al than in B-free Fe-40 at%Al. Modelisation of the kinetics of vacancies elimination predict that complexes boron atoms - vacancies migrate more rapidly than free vacancies : the difference between their migration energies is higher than 0,4 eV. This result could justify the assumption that at least a part of boron segregation in FeAl is due to a non-equilibrium mechanism.