Temperature dependence of positron trapping at defects in an Al-Li alloy

The Al-3.7% Li alloy has been thermally treated in order to obtain samples containing vacancy-Li clusters and samples with both clusters and dislocation loops. The positron lifetime parameters have been measured in both cases as a function of the temperature in the interval 10-295 K. For the samples containing only vacancy-Li complexes two lifetimes are observed in the whole temperature range. The average lifetime, the long component and its associated intensity stay roughly constant from 100 up to 295 K and decrease in the interval 100-10 K. In the samples containing two kinds of trap for positrons, two lifetimes are observed only in the temperature region 100-295 K, where the presence of loops leads to a drastic decrease of the long lifetime and an increase in its associated intensity. In the temperature range 10-100 K the only resolved lifetime decreases as observed in the sample containing vacancy-Li complexes. The results are interpreted in terms of a strong temperature dependence of the trapping rate into loops and a reversible `pinning' effect of the Li atoms on the open-volume defects.