Study of ageing in Al-Mg-Si alloys by positron annihilation spectroscopy

摘要

In many common Al-Mg-Si alloys (6000 series) intermediate storage at or near'room temperature' after solutionising leads to pronounced changes of theprecipitation kinetics during the ensuing artificial ageing step at \approx180{\deg}C. This is not only an annoyance in production, but also a challengefor researchers. We studied the kinetics of natural 'room temperature' ageing(NA) in Al-Mg-Si alloys by means of various different techniques, namelyelectrical resistivity and hardness measurement, thermoanalysis and positronlifetime and Doppler broadening (DB) spectroscopy to identify the stages inwhich the negative effect of NA on artificial ageing might appear. Positronlifetime measurements were carried out in a fast mode, allowing us to measureaverage lifetimes in below 1 minute. DB measurements were carried out with asingle detector and a 68Ge positron source by employing high momentum analysis.The various measurements show that NA is much more complex than anticipated andat least four different stages can be distinguished. The nature of these stagescannot be given with certainty, but a possible sequence includes vacancydiffusion to individual solute atoms, nucleation of solute clusters, Mgagglomeration to clusters and coarsening or ordering of such clusters. Positronlifetime measurements after more complex ageing treatments involving storage at0{\deg}C, 20{\deg}C and 180{\deg}C have also been carried out and help tounderstand the mechanisms involved.