Fatigue deformation in the interior of aged ternary aluminium-magnesium-zinc alloys

Abstract

Metallographic observations have been made of deformation, associated with the fatigue process in the interior of several aged ternary aluminiumzinc-magnesium alloys. It has been found that the deformation occurring in these relatively complex alloys differs from that observed in pure metals. With alloys which had a high zinc content the fatigue process was essentially a grain boundary phenomenon and cracking was intercrystalline. As the magnesium: zinc ratio was increased deformation occurrcd within the grains but was concentrated in certain slip bands along which cracking could occur. Cavities have been observed in these slip hands and appeared to precede the formation of an actual crack. Zones which were depleted of precipitate were found adjacent to slip bands in heavily deformed regions and these zones are considered to form by a process of re-solution of the precipitate. As the magnesium: zinc ratio was increased still further the fatigue process was accompanied by little visible evidence of duformation. This structural stability is attributed to the relatively high magnesium content and these alloys had the best fatigue properties.