Early Stages of Precipitation in Mould-Cast, Cold-Rolled and Heat-Treated Aluminium Alloy AA7075 with Sc,Zr-Addition

Precipitation reactions of the commercial Al-Zn-Mg-Cu(-Sc-Zr) alloy in mould-cast, cold-rolled, and heat-treated states were characterized by electron microscopy, X-ray diffraction, thermal analysis, microhardness testing, and positron annihilation spectroscopy. The distinct changes in microhardness curves as well as in a heat flow of the alloys studied are mainly caused by dissolution of clusters and precipitation of particles from the Al-Zn-Mg-Cu system. An easier diffusion of Zn, Mg, and Cu atoms along dislocations is responsible for the precipitation of Zn,Mg,Cu-containing particles at lower temperatures compared to the mould-cast alloys The mould-cast and cold-rolled alloys contain solute clusters rich in Mg and Zn. Clusters formed in the heat-treated alloys during natural ageing have similar composition but in addition to Mg and Zn contain also Cu. The Cu-concentration increases with increase of period of natural ageing. The mould-cast state after natural ageing contain in addition to solute agglomerates also vacancy clusters formed by agglomeration of thermal vacancies. Addition of Sc and Zr results in a higher hardness above approximate to 270 degrees C due to a strengthening by Al-3(Sc,Zr) particles with a good thermal stability. Sc and Zr have probably no influence on the evolution of solute clusters.