INTRAGRANULAR SEGREGATION OF BORON IN FE- 40 AT-PERCENT 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 (tran sgranular and intergranular). A low level of boron (few at%) was detec ted by AES in grain boundaries of this hypostoechiometric alloy. Never theless, the kinetic of boron segregation is rapid : even a quench fro m 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 unders tand the mechanism of boron segregation. Dilatometric isothermal exper iments showed that vacancies eliminate more rapidly in B-doped Fe-40 a t%Al than in B-free Fe-40 at%Al. Modelisation of the kinetics of vacan cies elimination predict that complexes boron atoms - vacancies migrat e more rapidly than free vacancies : the difference between their migr ation energies is higher than 0,4 eV. This result could justify the as sumption that at least a part of boron segregation in FeAl is due to a non-equilibrium mechanism.