Microstructure Evolution of High-Alloyed Al-Zn-Mg-Cu-Zr Alloy Containing Trace Amount of Sc During Homogenization

Microstructure evolution of a new high-alloyed Al-Zn-Mg-Cu-Zr-Sc aluminium alloy during two-stage homogenization process was investigated by use of scanning electron microscope, transition electron microscope and high resolution transition electron microscope. The results indicate that the morphology and chemical composition of Al-3(Sc, Zr) particles formed in the first stage were greatly affected by heating temperature. With the increase of heating temperature, the morphology of Al-3(Sc, Zr) particles transform from cuboidal with evident faceting to spheroidal due to improved Zr diffusivity. More Zr atoms enrich in the interface of precipitate/matrix forming a thin layer. Moreover, the mean diameter of precipitates increases a little bit with the increase of heating temperature, showing very restricted coarsening rate and high thermal stability of Al-3(Sc, Zr) particles. After an appropriate second stage heat treatment (474 degrees C x 48 h), the intermetallic formed in the solidification process could dissolve sufficiently and Al-3(Sc, Zr) particles still keep very good coherency with Al matrix without abnormal growth.