Discontinuous precipitation in Al-50 WT% Zn alloys

Thermal methods are applicable to the discontinuous reaction observed in Al--Zn solid solutions ( > 20%Zn) supersaturated with respect to Zn, either during continuous cooling or during isothermal aging at lower temperatures. Isothermal and DSC cooling rate experiments confirm that the discontinuous precipitation of these solutions is a nucleation and growth process. An Al--50Zn--Si--Fe--Cu alloy was homogenized and solutionized. Isothermal measurements between 30-60 deg C within a differential microcalimeter were made. Continuous cooling experiments at rates between 1/4 and 80 deg C/min were carried out by DSC thermal analysis. In order to correlate microstructural changes with specific stages of the isothermal heat flow curves, SEM-microcharacterization was performed with a microcalimeter after 20 h at 30 deg C. Graphs, Photomicrographs. 10 ref.--A.R.