Thermal vacancies and high-temperature mechanical properties of FeAl

The formation and the kinetics of migration of thermal vacancies in Fe61Al39 have been studied by positron lifetime spectroscopy. From the temperature variation of the equilibrium vacancy concentration an effective vacancy formation enthalpy of H-V(F)=0.98 eV has been determined. From the equilibrium of the thermal vacancy concentration after temperature changes the vacancy migration enthalpy (H-V(M)=1.7 eV) could be derived. As observed recently, vacancy-type defects give rise to an internal friction maximum at 680 K (2 s(-1)) after quenching of FeAl. This reorientation effect as well as its recovery by long-range migration of defects is characterized by the above vacancy migration enthalpy. For the defects of the 680 K internal-friction maximum, a complex of a vacancy on the Fe sublattice and an antisite atom is suggested. A direct relationship between the high-temperature mechanical properties and atomic defects arises from the time dependence of the yield stress anomaly in Fe60Al40 measured after fast heating, which fits perfectly with the equilibration kinetics of the thermal vacancy concentration. From this we conclude that the yield stress anomaly is sensitively affected by thermal defects.