The development of aluminium-zinc-magnesium alloys for superior stress corrosion resistance
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Date
2015-04-30
Authors
Dauskardt, Reinhold Horst
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Abstract
A thorough literature survey has been undertaken to provide
the necessary understanding of:
i) the general metallurgy and microstructure of
Al-Zn-Mg alloys,
ii) the stress corrosion cracking (SCC) of Al-Zn-Mg
alloys (including accelerated SCC test methods), and
iii) the influence of composition, microstructure and
heat treatment on SCC properties.
Three accelerated SCC test methods were evaluated using
existing commercial alloys in different temper conditions.
These were the notched rod load relaxation, the
electrochemical acceleration and the slow strain rate SCC
tests. The slow strain rate method gave the most reliable and
reproducible results. This was therefore chosen for all
subsequent testing. Baseline SCC test data was obtained from
existing alloys in order to facilitate comparison of new alloy
compositions *
The microstructure of a representative Al-Zn-Mg alloy was
examined using optical, scanning and transmission electron
microscopy. The effects of quench rate from solution heat
treatment, and ageing time and temperature on both the
microstructure and SCC properties were investigated.
Decreasing quench rate produced a moderate increase in
resistance to SCC. The characteristic increase in resistance
to SCC found by overageing was, however, associated with an
unacceptable loss of mechanical properties,
ABSTRACT
A thorough literature survey has been undertaken to provide
the necessary understanding of:
i) the general metallurgy and microstructure of
Al-Zn-Mg alloys,
ii) the stress corrosion cracking (SCC) of Al-Zn-Mg
alloys (including accelerated SCC test methods), and
iii) the influence of composition, raicrostructure and
heat treatment on SCC properties.
Three accelerated SCC test methods were evaluated using
existing commercial alloys in different temper conditions.
These were the notched rod load relaxation, the
electrochemical acceleration and the slow strain rate SCC
tests. The slow strain rate method gave the most reliable and
reproducible results. This was therefore chosen for all
subsequent testing. Baseline SCC test data was obtained from
existing alloys in order to facilitate comparison of new alloy
compositions..
The raicrostructure of a representative Al-Zn-Mg alloy was
examined using optical, scanning and transmission electron
microscopy. The effects of quench rate from solution heat
treatment, and ageing time and temperature on both the
raicrostructure and SCC properties were investigated.
Decreasing quench rate produced a moderate increase in
resistance to SCC. The characteristic increase in resistance
to SCC found by overageing was, however, associated with an
unacceptable loss of mechanical properties.
Melting, casting and hot working techniques were developed in
order to fabricate defect-free small scale experimental alloy
compositions.
Seven experimental casts were made to cover a wide
compositional variation {2n:4~6 wt*t, and Mg:0.8-2.5 wt.%).
Slow strain rate SCC testing revealed the beneficial effects
of having a zn:Mg ratio of 3:1 (wt.%).
Description
Thesis (M.Sc.(Engineering))--University of the Witwatersrand, Faculty of Engineering, Department of Metallurgy, 1983.