Point defects and their properties in FeAl and FeSi alloys

Positron annihilation as well as differential dilatometric measurements were performed to measure the vacancy concentration in FeAl and FeSi. The highest vacancy concentration is found at the stoichiometric B2 composition with more than 3.5% at the melting point in FeAl. Additionally, diffusion and isotope effect experiments were carried out in B2 FeAl to obtain information about the diffusion mechanism. The isotope effect decreases when the material transforms from D0₃ to B2. The effective formation volume of the defects in the B2 phases is larger (1.4Ω) than one atomic volume Ω and is related to a defect involving more than one vacancy. The effective migration enthalpy of 0.5–1.8 eV as well as the effective formation enthalpy of 0.7–1.02 eV vary with the concentration of the alloys in an opposite manner. The results for the FeAl system suggest that different defect types may operate as diffusion vehicles in the different phases. The mobility of the defects dominates the thermomechanical behavior of FeAl alloys. The defect production during mechanical deformation of FeAl alloys is divided into two parts. The annealing out of mechanically produced defects occurs on the same time rate as that of thermal defects.