A COMPARISON OF THE FATIGUE BEHAVIOUR OF TWO ALUMINUM-ZINC-MAGNESIUM ALLOYS.

Abstract : The torsional fatigue deformation of two aluminium alloys, Al-7.5Zn-2.5Mg and Al-4Zn-5Mg, was examined in various conditions of heat treatment. Two well defined deformation modes were observed, one in which localised transcrystalline slip produced narrow zones of precipitate re-solution, and another in which localised deformation occurred in the precipitate depleted zones at the grain boundaries. An attempt was made to correlate the torsional deformation modes of the two alloys with the fracture modes observed on rotating bending corrosion fatigue test pieces. It was found that test pieces of the Al-7.5Zn-2.5Mg alloy, subjected to torsional fatigue in certain conditions of heat treatment, exhibited localized transcrystalline deformation and precipitate re-solution, and that rotating bending test pieces in these same conditions of heat treatment exhibited extensive shear mode crack growth when fatigued in aqueous 3% NaCl solution. In the Al-4Zn-5Mg alloy there was no correlation between localised transcrystalline torsional fatigue deformation and shear mode failure on rotating bending corrosion fatigue test pieces. In the latter alloy the grain boundaries exhibited a high corrosion susceptibility and the initial shear mode growth stage was replaced by intercrystalline crack growth. (Author)