Effects of Exposure Temperature on Tensile and Charpy Impact Properties of the 7xxx Aluminum Alloy for Aerospace Applications

Abstract

Aluminum alloys have been widely used for aerospace applications due to their high strength and lightweight. New demand is growing for aluminum alloys with high temperature mechanical properties that can sustain plastic deformation at high temperature. In this study, correlations among microstructure, tensile properties and Charpy impact properties were analyzed at high temperature in the 7xxx aluminum alloy. Microstructures were analyzed using optical and scanning electron microscopes at varying exposure temperatures and time. Tensile and Charpy impact tests were conducted after exposure at 93, 149, 204, and 260 °C (200, 300, 400, and 500 °F) for 0.5 and 10 hours, and the results were compared to those at 25 °C (77 °F). Yield strength decreased with increasing exposure temperature, while total elongation and Charpy absorbed energy increased. Above 149 °C (300 °F), yield strength sharply dropped due to coarsening of the η' or η phase, and Charpy absorbed energy dramatically increased due to thermal softening of the matrix. The effect of exposure temperature and time on the mechanical properties of the 7xxx aluminum alloy was further investigated by fractographic analysis. The fracture toughness at high temperature was predicted on the basis of yield strength and Charpy absorbed energy suggesting a relationship between them. Key words: 7xxx aluminum alloy, elevated temperature strength, Charpy impact properties, Fracture toughness