Defect types and defect properties in FeAl alloys

The thermomechanical behavior of high temperature intermetallic alloys is related to point defects and their concentration. Positron annihilation is a sensitive tool to obtain various information about defect properties. In the FeAl system the formation enthalpy of defects decreases from 1.6 to 2.0 eV in pure α-Fe to 1.37 eV (Fe–7at.%Al) and 0.91eV in the D03 structure. Additionally, the formation volume of the defects increases from 0.88 Ω in Fe–7at.%Al to 1.42 Ω in Fe–40at.%Al (Ω=mean atomic volume). The change of the formation volume indicates different types of defects. In quenching experiments the migration enthalpy is measured using the fast Doppler broadening technique and changes from 0.5 to 1.2 eV. Furthermore, the knowledge of the positron lifetimes gives an additional hint about the main defects and their positions in the lattice. The results suggest that thermal defects in the B2 phase are mainly triple defects at low temperature and double vacancies at higher temperatures. In the A2 as well as in the D03 structure single vacancies are supposed to be the predominant defect types.