Vacancy trapping at tin atoms during the recovery of a fast-quenched dilute aluminium-tin alloy and its effect on the isomer shift of the 119Sn Moessbauer isotope

Vacancy trapping at tin atoms during stage III and stage IV recovery as well as tin precipitation were studied in a fast-quenched Al-0.014 at.% Sn alloy by parallel positron lifetime and Mossbauer spectroscopy. The results show that the substitutional 119Sn isotope with a trapped vacancy has 2.275 mm s-1 isomer shift. Trapping of more vacancies may cause significant line shift and leads to the appearance of a line at 2.86 mm s-1 in the Mossbauer spectra. The origin of this line position was interpreted as the relaxation of substitutional tin atoms to tetrahedral and octahedral interstitial positions in the centre of the vacancy clusters nucleated at the tin atoms. The defect recovery taking place in the alloy was found to be dominated by the strong tin-vacancy interaction. In stage III annealing, both vacancy annihilation and the formation of vacancy clusters independent of tin atoms are considerably suppressed by the strong trapping of vacancies at tin atoms. In stage IV recovery, the dissolution of dislocation loops, the migration of tin-multivacancy complexes and the formation of tin-atom clusters were observed. Precipitation of metallic tin exhibits several substages. The positron trapping observed upon precipitation was ascribed to the mismatch free volumes at the incoherent particle-matrix interface of the precipitates. (author)