Effect of compositional variations on characteristics of coarse intermetallic particles in overaged 7000 aluminium alloys

To provide an understanding of how compositional variations affect the characteristics of coarse intermetallic particles in 7000 aluminium alloys, three Al-Zn-Mg-Cu-Zr alloy plates with different zinc, magnesium, and copper contents have been studied by optical microscopy based image analysis, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and differential scanning calorimetry (DSC). These coarse intermetallic particles are detrimental, especially to the toughness of the alloy. Experimental observations have been interpreted successfully on the basis of the phase diagram and the temperature dependent S phase solvus, derived in turn on the basis of the regular solution model. The temperature dependent S phase solvus indicates that some compositions in the composition windows of 7050 and 7x75 type alloys will give rise to the detrimental S phase, which cannot be dissolved during solution treatment. Also, the T phase has been analysed and conditions for its formation and dissolution are discussed. It is shown that the present results can produce useful information on alloy design and thermomechanical processing via microstructural control.