On the influence of step aging on the stress corrosion cracking of Al-Zn-Mg alloy

Abstract

These studies were made to examine the effects of two-stepped aging on the stress corrosion cracking of Al-Zn-Mg alloys.Al-6Zn-3Mg alloy was aged in two steps under several conditions. The stress corrosion cracking test were performed on the above two-stepped aged alloys to pursure factors affecting the stress corrosion cracking.The results obtained were summarized as follows:(1) The stress corrosion cracking tests were performed on the alloy which had the maximum strength on the final aging curve. The stress applied was 75% of the proof stress of the specimens. The resistance to the stress corrosion cracking was lower when the preaging temperature was higher and its time was longer.(2) In the alloy which was sensitive to the stress corrosion cracking on the above tests, the size of the precipitated particles was fine and the change in surface roughness due to the tensile deformation was great.The sensitivity to the stress corrosion cracking did not depend only on the width of precipitate free zone on the grain boundary, but also on other factors.(3) It has been thought that the two-stepped aging reduces the sensitivity to the stress corrosion cracking of Al-Zn-Mg allys. It may be due to the following two reasons.In general, the stepped-aged alloy has higher strength than the alloy ordinally aged. Then, the former specimen will have higher resistance to deformation and also to stress corrosion cracking than the latter when the both specimens were subjected to the stress corrosion cracking tests under the load of the same weight.The other reason would be due to the difference in aging stage. When the final aging of the both specimens was performed under the same conditions, the former specimen will be more advanced in aging than the latter. The sensitivity to stress corrosion test is generally low in over-aged alloys. Then, the former specimen, which is more advanced in aging, will have higher resistance to stress corrosion cracking than the latter.(4) The electrode potential and the weight loss by normal corrosions were not closely related with the stress corrosion cracking. As well known, pitting corrosions often occurred on the specimens sensitive to the stress corrosion cracking.