Studies were made on the effects of aging temperature and pre-strain on the stress corrosion cracking of ternary Al-Zn-Mg alloys containing 4.5wt.%Zn-3wt.%Mg, 4.5wt.%Zn-2wt.%Mg, and 4.5wt.%Zn-1wt.%Mg.
The results obtained were summarized as follows:
(1) The alloys aged at low temperature were sensitive to the stress corrosion cracking. The fact would be due to small proof stress and high corrosion rate. The alloys aged at temperatures higher than G. P. solvus were insensitive to the stress corrosion cracking.
(2) When the aging temperature was higher, the alloy having lower content of additional elements promptly turned to be more insensitive to the stress corrosion cracking as compared with that of higher content.
(3) The specimen as pre-strained was more sensitive to the stress corrosion cracking than that as no prestrained. The sensitivity was greater with the increase of pre-strain.

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These studies were made on the shear cracking of aged Al-Zn-Mg alloy plates and on the stress corrosion cracking of step aged alloy sheets.
Plates of aged welded Al-Zn-Mg alloys such as of Al-4.5%Zn-1.2%Mg alloy and Al-4.2%Zn-2.3%Mg alloy were cut with shearing machine. After the pieces of the alloys were immersed in 5%NaCl solution for one month, the shear cracks were observed on the shearing surface. Furthermore, stress corrosion cracking test was performed for stepped aged Al-4.5%Zn-3%Mg alloy.
The results obtained were summarized as follows.
(1) The shear cracking would be a kind of stress corrosion cracking due to the residual stress superimposed on the alloys at shearing. The sensitivity to shear cracks corresponded well with the test results of stress corrosion cracking.
(2) Shear cracks were not observed on the heat-affected zone of weld.
(3) When the pre-aged alloys were further aged at a temperature higher than that of pre-ageing, they became more resistant to the stress corrosion cracking.
(4) Step-ageing treatment improved the resistance to stress corrosion cracking of Al-Zn-Mg alloys.

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Al-Zn-Mg alloys such as compositions of Al-6%Zn-1%Mg (η-alloy) and Al-6%Zn-3%Mg (T-alloy) were subjected to various ageing treatments and their age-hardening mechanism and factors for controlling grain boundary fractures were investigated by tension tests and transmission electron microscopy.
The effect of test temperature on the yield stress was very little in the range between room temperature and 77°K for as-quenched and overaged alloys. However, the effect was appreciable for these alloys containing G. P. zones only at temperatures of about 200°K or lower, or containing intermediate precipitates at below room temperature.
The elongation to rupture was decreased with the increase of strength, independent of the width of precipitate free zone (PFZ) in η alloy: while it was dependent on the width of PFZ in T-alloy, since it was observed to be less in smaller width for the same strength.
As the results of these tests, the age-hardening mechanism and the important factors for controlling grain boundary fractures in these ternary alloys such as the state of slip band and the width of PFZ were discussed.

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Aluminum can effectively be debased by the addition of a small amount of indium or tin.
This paper describes the experimental results for anodic dissolution of high-purity Al-In binary alloys in chloride solutions.
At first, cast alloys having various indium contents were galvanostatically electrolyzed for 300 hrs. in synthetic sea water at a constant anode current density of 1 mA/cm². Anode potential of 0.1% In alloy was constant at-1.1V (SCE) and its anode efficiency was found to be about 90%.
Secondly, alloys, containing 0.010.2% In, were cold rolled to 90% reduction and subjected to solution heat-treatment at temperatures of 470620°C. subsequently, they were galvanostatically electrolyzed at a constant current density of 1 mA/cm² for 1 hr. in 1M-NaCl (pH = 5.5) at 25°C. Stationary anode potential was measured and after anodic dissolution for 1 hr., dissolution morphology of alloys was examined.
The dissolution was pitting and anodic behavior of alloys was found to be classified into two groups.
Anode potential of alloys, solution-treated at temperatures lower than 500°C, was slightly less noble than that of 99.99% Al (-0.7V, SC E). While, alloys, containing more than 0.06% In and quenched from above 500°C, showed less noble anode potential of -1.1V.
The shape of pits produced on pure aluminum was a rugged polyhedron having facets of {100} planes. The pit morphology of thin foils was examined under a transmission electron microscope and it was found that the process of pit growth was tunneling, by which (100) plane as an active front proceeded in [100] direction. Such a crystallographic growth of pits by tunneling was more remarkable in alloys showing dissolution at a potential more noble than -w0.9 V (A-type dissolution). Facets of the pits developed on the alloys, dissolved at less noble potential of -1.1 V (B-type dissolution), included {111} and other planes as well as {100} planes.
Anodic behavior of the alloy having air-formed film was compared with that of the alloy treated in pure water at 30°C for 72 hrs. and very little difference was found in pitting behavior in the chloride solution between the both.

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This paper describes the experimental results for anodic dissolution of high-purity Al-Sn alloys and discusses the dissolution mechanism of this binary aluminum alloy system in comparison with that of Al-In alloys.
Alloys, containing 0.04, 0.1, and 0.2% Sn, were cold rolled to 90% reduction and subjected to solution heat treatment at 400620°C. Subsequently, they were galvanostatically electrolyzed at a constant current density of 1 mA/cm² for 1 hr. in 1 M NaCl (pH = 5.5) at 25°C. Stationary anode potential of the alloys, solution-treated at temperatures lower than 500°C, was slightly less noble than that of pure aluminum. While, alloys containing more than 0.1% of Sn and quenched from higher temperatures, showed less noble anodic dissolution at -1.4 V (SCE). The dissolution was pitting and pit morphology of the binary alloys was similar to that of Al In alloys.
The dissolution mechanism of these two systems of aluminum alloys in the chloride solutions was investigated. At first, the critical concentration of additional elements in aluminum matrix was determined by Hardy's solubility curves on the assumptions that the alloys reach an equilibrium state during heat treatment;, and the state is perfectly quenched to room temperature, at which indium or tin does not diffused in the solid solution. Critical concentration of indium or tin in the solid solution needed for effective debasing of aluminum was found to be about 0.04wt.% for the both binary alloys. Then, mechanisms of two types of dissolution (that is, pitting produced at more noble than -0.9 V and that at -1.1 or -1.4 V) were discussed.
The process of pit growth in pure aluminum was tunneling, by which close peaked (100) plane as an active front proceeded in [100] direction. This characteristic of tunneling was kept by the pitting of alloys dissolved at rather noble potential (A type dissolution). As for alloys, showing dissolution at less noble potentials (B-type dissolution), pitting was also crystallographicall and the facets of these pits included {111} and other planes as well as {100} planes.
Aluminum alloys behave as non-plarizable electrodes under the potential exceeding the critical value for pitting when they are anodically polarized in chloride solutions. The above fact is explained by the increase in the area of active fronts which support the current.
As it is well known, indium and tin are special additional elements to aluminum. The binding energy between a quenched vacancy and an atom of solute, indium or tin, will extraordinarily be large. Vacancies in the close packed surface planes can promote active dissolution of the planes due to step-nucleation. The fact that the tunneling growth of pits is likely to be promoted during A-type dissolution would be explained by the presence of frozen vacancies bound with the solute atoms. The critical concentration of the solute necessary for B-type dissolution is about 0.01 at.%, which corresponds to the concentration of the solute required to make the quenched acvancies "all bound". The explanation of the increase in kind of facets by the presence of "all bound" vacancies remains still unsolved, because of the lack of information on facetting dissolution and repassivation of active fronts.

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