Environmental effects on crack closure during fatigue in an Al—Zn—Mg—Mn alloy

Abstract

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 crack 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—Mn 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.