ENVIRONMENTAL-EFFECTS ON CRACK CLOSURE DURING FATIGUE IN AN AL-ZN-MG-MN ALLOY

The fatigue crack propagation (FCP) behaviour of peak aged Al-Zn-Mg-Mn alloy has been investigated with reference to that of peak aged Al-Zn-Mg alloy by comparing crack closure in 0.5M Na2SO4+0.5M NaCl and 0.5M Na2SO4 solutions. Anodic studies were complemented by potentiostatic current transient measurements. For both alloys, the intrinsic FCP rate in the Cl- containing solution exceeded that in dry air; however, the apparent FCP rates were similar. Crack closure decreased in the order: 0.5M Na2SO4 + 0.5M NaCl solution, 0.5M Na2SO4 solution, dry air. Irrespective of environmental conditions, the addition of manganese to the Al-Zn-Mg alloy reduced the intrinsic FCP rate and clack closure. Specimens in the Cl- containing solution were more slowly repassivated than those in the sulphate solution. In the Cl- containing solution the Al-Zn-Mg alloy was more slowly repassivated than the Al-Zn-Mg-h in alloy, in good agreement with the trend of intrinsic FCP. The enhanced intrinsic FCP rate and crack closure in the Cl- containing solution resulted from an increase in localised corrosion attack on the slip step at the crack tip. (C) 1995 The Institute of Materials.