Defect analysis in FeAl and FeSi with positron lifetime spectroscopy and Doppler broadening

Systematic measurements of thermal defect formation in intermetallic compounds of FeAl and FeSi have been carried out by using positron lifetime spectroscopy and Doppler broadening. Temperature dependence of defect formation leads to the effective formation enthalpy H-eff(f), which decreases in D0(3)-ordered FeAl from 0.92 eV (Fe75Al25) to the very small value of 0.73 eV (Fe69Al31) with higher concentrations of Al, in B2-ordered FeAl it is nearly constant about 1 eV. In D0(3)-ordered FeSi effective formation enthalpies of 1.21 eV in Fe82Si18 and 1.04 eV in Fe76Si24 were found. The effective defect formation volume V-eff(f) can be measured by the pressure dependence of the thermal defect concentration. It is found to be smaller than the mean atomic volume Ohm in both D0(3) structures of FeAl and FeSi and A2-structured FeAl. This indicates that single vacancies are the predominant defect type in these structures. In B2-FeAl the formation volume increases to 1.44 Ohm, which can be explained by the formation of bound defects containing more than one vacancy. At temperatures above 1000 K and Al concentrations above 42 at.% an additional increase in the positron defect lifetime lets us suggest a change in the defect type, possibly indicating the formation of vacancies on the Al-sublattice. By comparison of positron lifetime data with differential dilatometric experiments (Kerl R, Wolff J, Hehenkamp Th. Intermettalics, this volume) it is possible to determine the positron trapping rate CL in these systems for the first time. In D0(3) FeSi mu is concentration dependent, rising from 2.9 x 10(+14) s(-1) for Fe82Si18 to 6.6 x 10(+14) s(-1) in Fe76Si24. A very high trapping rate of 56.0 x 10(+14) s(-1) is found in B2-ordered Fe65Al35 (C) 1999 Elsevier Science Ltd. All rights reserved.