On the boron effect in FeAl (B2) intermetallic alloys

Small boron additions are known to reduce the intergranular brittleness of some intermetallics, due to their intergranular segregation. Two different origins of this segregation were identified in FeAl (B2) alloys, containing 40–50 at.% Al and less than 50 appm of impurities (O, S, C). Its very fast kinetics, low level and almost no temperature dependence suggest a non-equilibrium mechanism, based on an interaction between boron atoms and the flux of migrating thermal vacancies. Still, a low level (several at.% B) equilibrium segregation is also stable: in fact, only a partial desegregation was observed after a very long (3 months/400 °C) annealing. In the B-doped FeAl alloys, the kinetics of thermal vacancies migration is strongly accelerated compared to ‘pure’ alloys. As this effect is too strong to be explained only by intergranular phenomena, some other aspects of boron addition to FeAl alloys were also studied. A strong decrease of the long range order parameter and modifications of the dislocation structure created by the elimination of thermal vacancies were found.