Characterisation of precipitation microstructures in aluminium alloys 7040 and 7050 and their relationship to mechanical behaviour

This paper investigates the precipitation microstructures in aluminium alloys 7040 and 7050 (Al - Zn - Mg - Cu variants) as a function of quench rate and aging treatment, and the associated compromise between yield strength and fracture toughness. The precipitate microstructures are quantitatively characterised by a combination of techniques covering the different scales involved: transmission electron microscopy (TEM), field emission gun scanning electron microscopy (FEG-SEM) and small angle X-ray scattering (SAXS), and fracture toughness is estimated using the Kahn tear test. Yield strength and strain hardening behaviour are determined by conventional tensile tests. It is shown that the composition modification from alloy 7040 to alloy 7050 results in a better compromise between yield strength and toughness. The strength decrease in alloy 7040 following a slow quench is much reduced compared to alloy 7050 because of a lower sensitivity to quench-induced precipitation on dispersoids. Both alloys show a large decrease in toughness upon slow cooling, however this decrease is much more pronounced in the case of alloy 7050 because of the presence of bands of dispersoid nucleated quench-induced precipitates, promoting low energy transgranular fracture. Further improvement of toughness values in the slowly quenched materials would require a decrease in the quench sensitivity of grain boundary precipitation.