DEFECT ANNEALING AND DECOMPOSITION IN QUENCHED AL-SC ALLOYS STUDIED BY POSITRON LIFETIME MEASUREMENTS

Positron lifetime tau measurements are performed to study defect annealing and decomposition in Al-(0.003 to 0.36 at%) Sc alloys during isochronal and isothermal heat treatment of water-quenched samples. A two-stage behaviour of the positron lifetime observed below 200-degrees-C is explained as migration and clustering of quenched-in vacancy-Sc atom complexes followed by the annealing of vacancy clusters. In alloys containing 0.18 and 0.36 at% Sc an extra peak appears in the average positron lifetime around 300-degrees-C which is attributed to the precipitation of coherent Al3Sc particles. Although the volume fraction of Al3Sc is very small, f almost-equal-to 0.5%, the precipitates are detected by positrons due to their fine-disperse distribution (diameter almost-equal-to 10 nm). A vacancy-type lifetime component which appears simultaneously with the precipitate formation is attributed to structural vacancies of the Al3Sc phase or to vacancies associated with the Al3Sc particle-matrix interface.