The Effect of Compositional Variations on the Characteristics of Coarse Intermetallic Particles in Overaged 7 xxx Al Alloys

To provide an understanding of how compositional variations affect the characteristics of coarse intermetallic particles in 7xxx alloys, three Al-Zn-Mg-Cu-Zr aluminium alloy plates with different Zn, Mg and Cu contents were studied by optical microscopy based image analysis, scanning electron microscopy (SEM), energy disperse X-ray spectroscopy (EDS) and differential scanning calorimetry (DSC). These coarse intermetallic particles are detrimental, especially to the toughness of the alloy. The experimental observations were interpreted successfully on the basis of the phase diagram and the temperature dependent S phase solvus which was derived 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 that can not be dissolved during solution treatment. Also the T phase is 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 thermo-mechanical processing via microstructural control.