FATIGUE CRACK GROWTH STUDIES ON AGED ALLOYS BASED ON AL–CU–MG SYSTEM

Abstract

The addition of 0.4 wt% of silver or cadmium to the alloy Al-4%Cu-0.3%Mg, which has a high Cu:Mg ratio changes the nature, morphology and dispersion of the precipitates that from on age – hardening at medium temperatures such as 150°C – 200°C. The purpose of this paper is to report the effects of these micro structural changes on the tensile and fatigue behavior of the alloys. The tensile properties of the alloys aged at 170°C increased in the order Al-Cu, Al-Cu-Mg, Al-Cu-Mg-Cd and Al-Cu-Mg-Ag. Despite differences in their microstructures and tensile properties, the fatigue performance of the alloys was found to be relatively unaffected. Fatigue deformation was similar in each case and the alloys showed identical fatigue limits (± 70 MPa). It is concluded that microstructure rather than tensile properties has the dominant effect on the fatigue properties of the alloys. Moreover, providing the precipitates are capable of dispersing dislocations generated by fatigue stressing, their precise nature is unimportant so far as the nucleation of fatigue cracks is concerned. Some differences were observed in the fatigue crack growth behaviour between silver – containing alloy and other alloys. In all cases, cracks propagated along grain boundaries and the fact that the particles there were closer in the silver – containing alloy may explain this result.