Pressure dependence of positron trapping rate, lifetime and defect formation in metals and alloys

Abstract

In order to determine the pressure dependence of positron annihilation parameters like bulk lifetime and positron trapping rate, positron lifetime and Doppler broadening experiments have been carried out. In aluminium a slight decrease in the bulk lifetime of 3 ps at 530 MPa has been found. Vacancies have been quenched in Fe25at%Al and Fe40at%Al to study the effect of high hydrostatic pressure on the positron trapping rate of vacancies. We like to emphasize that there is no significant pressure dependence of the trapping rate up to 570 MPa shown by these alloys. 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. Additionally, the formation volume of the defects can be determined from measurements varying the pressure at constant temperature. The defect formation volumeV v f has been measured in pure Al and in intermetallic phases of the FeAl system at different temperatures. In the ordered B2 structure a effective defect formation volume of 1.42 Ω was observed, in A2 of 0.88 Ω, in DO3of 0.96 Ω and in pure Al of 0.67 Ω, respectively (Ω=mean atomic volume). While the latter values reflects single vacancy defects, the large formation volume in B2 points to the prevalence of double vacancies and/or, more likely at low temperatures, triple defects.