Corrosion fatigue of 7075 aluminum alloy.

Abstract

Fatigue tests of 7075-T651 aluminum alloy were carried out in laboratory air and in aerated 3.5%NaCl aqueous solution. In both S-N and da/dN-ΔK measurements, fatigue properties in NaCl were considerably inferior to those in air. A number of laminated cracks, which were intergranular cracks along pan-cake-like grain boundaries, were formed in a lower ΔK region in NaCl, while these were not observed in air at all. In an intermediate ΔK region the fracture pattern was characterized by formation of striations in both environments, but striations developed in air were "the ductile striations", and those in NaCl were "the brittle striations". In a larger ΔK region dimpled fracture area was gradually increased with increasing ΔK, although ΔK value at which dimpled fracture increased rapidly was lower in NaCl than in air. These differences between fatigue and corrosion fatigue can be understood through an effect of hydrogen permeated from the environment, and a lowering of corrosion fatigue properties seems at least partly to result from the localized hydrogen embrittlement. The more direct evidence for the contribution of hydrogen embrittlement to corrosion fatigue was obtained from the fact that the lowered fracture toughness of corrosion fatigued specimen was almost recovered to the original value by the re-heat-treatment after the corrosion fatigue test.