Physics – Condensed Matter – Materials Science
Scientific paper
2011-09-09
Physics
Condensed Matter
Materials Science
Scientific paper
10.1103/PhysRevB.83.014101
In many common Al-Mg-Si alloys (6000 series) intermediate storage at or near 'room temperature' after solutionising leads to drastic changes of the precipitation kinetics during the ensuing artificial ageing step at 180{\deg}C. This is not only an annoyance in production, but also a challenge for researchers. We studied the kinetics of natural 'room temperature' ageing (NA) in Al-Mg-Si alloys by means of various different techniques, namely electrical resistivity, hardness measurement, thermoanalysis and positron lifetime and Doppler broadening spectroscopy to identify the stages in which the negative effect of NA on artificial ageing might appear. Positron lifetime measurements were carried out in a fast mode, allowing us to acquire one lifetime value every 45 s. DB measurements were carried out with a single detector and a 68Ge positron source by employing high momentum analysis. The various measurements show that NA is much more complex than anticipated and at least four different stages can be distinguished. The nature of these stages cannot be given with certainty, but a possible sequence includes vacancy diffusion to individual solute atoms, homogeneous nucleation of solute clusters, Mg agglomeration to clusters and coarsening or ordering of such clusters. Positron lifetime measurements after more complex ageing treatments involving storage at 0{\deg}C, 20{\deg}C and 180{\deg}C have also been carried out and could help to understand the mechanisms involved.
Banhart John
Chang C. S. T.
Elsayed M.
Hill Allen J.
Lay M. D. H.
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