Effect of Heat Treatments on Matrix Strength and Precipitation Behavior in Al-Zn-Mg Foams

Al-Zn-Mg foams manufactured by a batch casting process were subjected to aging treatments with and without solution treatment. The effect of the heat treatments on matrix strength (micro-hardness) and precipitation behavior in the foams was investigated by means of transmission electron microscopy (TEM) and X-ray absorption fine structure (XAFS) analysis at a synchrotron radiation facility. Foam aged at 398 K for 8 h after the solution treatment showed the greatest increase in strength. Fine precipitates were dispersed homogeneously and densely, which were speculated to be metastable eta' and equilibrium eta phases transformed from GP zones. It is estimated that the GP zones were formed during water-quenching after the solution treatment. Overaging treatment at 423 K for 48 h after solution treatment softened the foam past its peak strength by approximately 10 in Vickers hardness. During this treatment, eta' and eta phases coarsened significantly and the number density of these precipitates decreased. It is concluded that a significant drop in the strength was suppressed during overaging by the presence of a high density of the GP zones, defined as Zn-Mg ordered phases by the XAFS analysis.