Influence of zirconium and copper on the early stages of aging in Al-Zn-Mg allays

The effects of zirconium and copper on the early stages of the precipitation processes in an Al-5.5 wt pct Zn-1.2 wt pct Mg alloy have been studied by differential scanning calorimetry (DSC) thermal analysis. Electron diffraction has been used as a complementary technique to aid in the interpretation of the thermal effects observed in the DSC thermograms. The results show that the initial stages of Guinier-Preston zone I (GP(I)) formation at room temperature are not affected by the presence of zirconium, but the rate of Guinier-Preston zone II (GP(II)) precipitation is slowed down significantly. For aging at 100 degreesC, the stability of GP zones is reduced by the addition of zirconium, and this leads to a reduction in the amount of eta' produced during aging. The addition of copper to an Al-5.4 wt pct Zn-1.2 wt pct Mg-0.2 wt pct Zr alloy intensifies the electron diffraction spots from GP(I), suggesting that the strong electron-scatterer copper may be incorporated into GP zones. The rate of growth of GP(I) at room temperature is unaffected by the presence of copper, but the rate of formation of GP(II) at room temperature is retarded. For artificial aging at 100 degreesC, the development of GP(I) and GP(II) is not affected significantly by the presence of copper, but the formation of eta' is stimulated, producing a high number density of very fine eta' precipitates. Preaging at room temperature results in accelerated eta' formation during subsequent aging at 100 degreesC in the zirconium-containing alloy. However, this acceleration of eta' formation is absent when copper is present in the alloy.