Defect analysis with positron annihilation ? Applications to Fe aluminides

Abstract

Measurements of positron annihilation at various temperatures at constant pressure give information about the formation of thermal defects and their enthalpy in metals and alloys. In the FeAl system the formation enthalpy decreases from 1.6-2.0eV in pure α-Fe to 1.37eV (Fe7at%nAl) and 0.91eV in the D0 3 structure. Additionally, the formation volume of the defects can be determined from measurements varying the pressure at constant temperature. The formation volume increases from 0.88Ω in Fe7at%Al to 1.42Ω in Fe40at%Al(Ω = mean atomic volume). The change of the formation volume indicates different types of defects. With the further knowledge of the positron lifetimes in these materials we suggest triple defects as the main defects in this system. In quenching experiments the migration enthalpy is measured using the fast Doppler broadening technique. In Fe 3 Al we observe a migration enthalpy of 0.5 eV. which is influenced by an apparing change of order in a temperature range above the DO 3 B 2 transition as determined from the phase diagram by Massalski. Thickness reduction of Fe6.3at%Si single crystals changes positron lifetimes with increasing deformation, particularly by rolling less than 5%.