Transition in fracture micromechanism of SS70 spray cast aluminum alloy

Fracture behavior of spray-deposited SS70 aluminium alloy with Zr, Mn,... as a dispersoid-forming element has been studied as a function of aging temperature. Tensile and notched tensile properties of the alloy at under and overaged heat treatment condition were studied. Scanning electron microscopy fractography of fractured surface of the notched tensile samples showed intergranular mode of fracture with grain boundary facets in the overaged and a dominant transgranular fracture mode in the underaged samples. Transmission electron microscopy examination of an overaged and then dipped in a 330degreesC Pb-Sn bath for a minute specimen showed a remarkable continLIOUS precipitation concurrent with a noticeable precipitate free zone at the grain boundaries. It seems development of these metallurgical changes during artificial aging at 205degreesC are the cause of drastic decrease in tensile and notched tensile fracture toughness of SS70 alloy under overaged heat treatment condition.