J. Banhart, M. Liu, Y. Yong, Z. Liang, C. S. T. Chang, M. Elsayed, M. D. H. Lay
In many common Al-Mg-Si alloys (6000 series) intermediate storage at or near
'room temperature' after solutionising leads to pronounced changes of the
precipitation kinetics during the ensuing artificial ageing step at $\approx$
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 and hardness measurement, thermoanalysis and positron
lifetime and Doppler broadening (DB) 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 measure
average lifetimes in below 1 minute. 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, 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 help to
understand the mechanisms involved.
View original:
http://arxiv.org/abs/1006.4778
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