Strength and toughness of 1.1wt% Li-containing Al-Zn-Mg-Cu alloys

The effect of ageing processes on the toughness and the yield strength of the Li-containing Al-Zn-Mg-Cu alloys (Al-5.6%Zn-1.9%Mg-1.6%Cu-1.1%Li-0.24Cr) was investigated, The microstructure was observed by transmission electron microscopy, tensile test was performed at a rate of 1mm.s(-1), fracture toughness was experimentally determined by the impact test, and the fracture modes had been assessed by image analysis of scanning electron microscopy micrographs. The strength of the Li-containing Al-Zn-Mg-Cu alloys treated at 120 degrees C is not more than 430MPa, which is very lower than that of 7075 alloys. The strength is comparable to that of 7075-T6 alloys after double-ageing or multi-ageing, however its ductility is lower than that of 7075-T6 alloys. The single-aged Li-containing alloys have high toughness. The multi-aged alloys become brittle. The fracture surface for the Li-containing Al-Zn-Mg-Cu alloys is intergranular, since the intergranular precipitation weakness the grain boundaries.