The effect of S-phase distribution on fatigue crack growth in peak-aged AlLiMgCuZr alloys

Abstract

Curves of da dN against ΔK have been compared in CT specimens of two AlLiMgCuZr alloys of differing Cu content aged to peak hardness at 170 °C. Changes in the S-phase distribution caused by stretching or duplex ageing have been employed as a means of influencing slip distribution, and fatigue crack growth (FCG) curves at R-values of 0.1 and 0.7 have been compared in order to evaluate crack closure effects. At R=0.7 crack closure appears to be negligible, and stretched materials show a lower threshold than unstretched ones due to slip homogenization. The alloy with higher Cu content exhibited lower thresholds than the other alloy, since the associated higher volume fraction of the S phase gave greater slip homogenization. Duplex-aged (DA) alloys show the greatest crack roughness and the highest thresholds; it is suggested that their high planarity of slip arises from the absence of a dislocation substructure and from the shearing of the finer S-phase particles. At R = 0.1, roughness- and debris-induced crack closure contribute to produce higher thresholds in all specimens. When FCG curves are expressed in terms of the effective stress intensity range (by allowing for crack closure) the data obtained at R = 0.1 and R = 0.7 are comparable.