Systematic measurements of thermal defect formation in intermetallic compounds were performed by means of positron lifetime spectroscopy and Doppler broadening. The temperature-dependence of defect formation indicated that the effective formation enthalpy decreased, in D03 ordered FeAl, from 0.92eV for Fe75Al25 to 0.73eV for Fe69Al31. In the case of B2 ordered FeAl, the effective formation enthalpy was nearly constant at about 1eV. In D03 ordered FeSi, an effective formation enthalpy of 1.21eV was found for Fe82Si18 and a value of 1.04eV was found for Fe76Si14. The effective defect-formation volume could be deduced from the pressure-dependence of the thermal defect concentration. It was found to be smaller than the mean atomic volume in the D03 structures of FeAl and FeSi or the A2 structure of FeAl. This indicated that single vacancies were the predominant defect-type in these structures. In B2-FeAl, the formation-volume increased to 1.44O. This was attributed to the formation of bound defects which contained more than one vacancy. At temperatures above 1000K, and Al concentrations above 42at%, an additional increase in the positron defect lifetime suggested that there was a change in the defect type with, perhaps, the formation of vacancies on the Al sub-lattice. By comparing positron lifetime data with differential dilatometry data, it was possible to determine the positron trapping rate in these systems.

Defect Analysis in FeAl and FeSi with Positron Lifetime Spectroscopy and Doppler Broadening. A.Broska, J.Wolff, M.Franz, T.Hehenkamp: Intermetallics, 1999, 7[3-4], 259-67