The effect of copper content and deformation mode on the fatigue crack propagation of Al6Zn2Mg xCu alloys at low stress intensities

Abstract

The effect of copper content on the fatigue crack propagation behavior of four Al6Zn2Mg xCu (T7351) alloys was investigated in dry air and distilled water and was compared with a previous study of the T651 condition. The peak-aged treatment exhibited a lower growth rate in dry air over the range of ΔK studied and also displayed a higher threshold and resistance to fatigue cracking near the threshold in the distilled water environment. This suggests that age-hardened alloys with planar slip (dislocation shearing mechanism) will have a higher threshold than those with wavy and/or homogeneous slip. The shape of the growth rate curves can be correlated best with a transition in the fracture surface appearance which is controlled by the deformation behavior. The copper content of the four alloys in the overaged condition did not significantly affect the growth rate in either dry air or distilled water. However, in distilled water there were minor differences in the form of the growth rate curves with copper content. This is associated with electrochemical potential differences at the crack tip among the four alloys. The environmental sensitivity of the peak-aged condition was several times higher than that of the overaged treatment at the higher ΔK values and the sensitivity of the peak-aged condition was much more dependent on the copper content. This behavior is considered to be due to strain localization which occurs in the peak-aged condition and which enhances the metal-environment interactions.