Effects of pre-stretching and ageing on the strength and fracture toughness of aluminum alloy 7050

The effects of the pre-stretching and ageing on the microstructure, strength and fracture toughness of the 7050 aluminum alloy were investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile test and the plane-strain fracture toughness test. The results indicate that the peak aged 7050 aluminum alloy possesses a higher strength, but its fracture toughness is poor. The high temperature and a subsequent low temperature ageing improve the fracture toughness by 8% while the strength is decreased by 3.5%. The fracture toughness of the over aged 7050 aluminum alloy is enhanced by 29% with a decrease in the strength of 7%. The retrogression and reaging provides an optimum combination of the strength and fracture toughness, which improves the fracture toughness by 17% without sacrificing the strength. After the pre-stretching, the precipitates within grains become coarse for all the ageing tempers which results in a lower strength. While the strength difference between the matrix and the grain boundary become lower and the area fraction of the grain boundary precipitates become smaller, leading to the higher fracture toughness.

Research highlights
► The 7xxx series aluminum alloy has been extensively used for manufacturing of aircraft structural wing components where a good compromise between the strength and fracture toughness is required.
► The strength can be improved through the appropriate thermo-mechanical treatments, but it is difficult to keep the fracture toughness at a high level simultaneously.
► In this paper, the effects of pre-stretching, peak ageing, high temperature and a subsequent low temperature ageing, over ageing, and retrogression and reaging on the tensile property and fracture toughness of the 7050 aluminum alloy are studied.
► These results are contributed to get an improved comprehensive property of the 7xxx series aluminum alloy.