Effects of small amounts of additional elements on directionality of stress corrosion resistance of the Al-Zn-Mg alloys

Abstract

Effects of small additions of Fe, Si, Cu, Mn, Cr, Ti, Zr and Ag on stress corrosion cracking in the LT-direction and shear cracking of Al-Zn-Mg alloys were investigated with a special consideration of grain shape.(1) Additions of Si, Cu, and Ag increased stress corrosion resistance in both LT- and ST-directions, while Mn, Cr and Zr additions increased stress corrosion resistance in the LT-direction but decreased that in the ST-direction. Effects of Fe and Ti additions on stress corrosion resistance were small.(2) It was found that Fe, Mn, Cr, Ti and Zr additions affected stress corrosion resistance only through their effect on grain shape. However, additions of Si, Cu and Ag increased stress corrosion resistance through some other effects than that on grain shape. It was considered that a Si addition was effective through reduction of solute concentrations and Cu and Ag additions were effective by introducing changes in electrochemical properties of grain boundary regions.(3) Effects of Fe, Si and Cu additions on shear cracking had the same tendency as that on stress corrosion cracking in the ST-direction. However, effects of Cr, Mn and Zr additions on shear cracking corresponded to that on stress corrosion resistance in the LT-direction rather than in the ST-direction. These observations were explained by occurrence of metal flow due to shearing and existence of directionality of stress corrosion resistance.