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

Positron lifetime tau measurements are performed to study defect annea ling and decomposition in Al-(0.003 to 0.36 at%) Sc alloys during isoc hronal 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 ato m complexes followed by the annealing of vacancy clusters. In alloys c ontaining 0.18 and 0.36 at% Sc an extra peak appears in the average po sitron lifetime around 300-degrees-C which is attributed to the precip itation of coherent Al3Sc particles. Although the volume fraction of A l3Sc is very small, f almost-equal-to 0.5%, the precipitates are detec ted by positrons due to their fine-disperse distribution (diameter alm ost-equal-to 10 nm). A vacancy-type lifetime component which appears s imultaneously with the precipitate formation is attributed to structur al vacancies of the Al3Sc phase or to vacancies associated with the Al 3Sc particle-matrix interface.