Precipitation hardening and phase formation in Mg-Sn-Zn-Al-alloys

In order to increase the structural stability of Mg-Zn-Sn-Al alloy, 0.5%wt MM (50%Ce-25%La-20%Nd-5%Pr) was added. At the as-cast condition, SEM micrographs indicate a very fine micro-structure (DAS below 12 mu m). The study focuses on the precipitation hardening, phase formation and structural stability during aging at elevated temperatures, of solution treated samples. After solution treatment (450 C) and aging at 225 C, Vickers hardness measurements show that this alloy maintains an increase of 30% in hardness for periods of up to 32 days. Very fine homogenously distributed precipitations (less than 500nm) were found. EDS, XRD, TEM and Auger characterization methods were employed in order to identify the phase structure of the alloy and the origin of precipitates. There is no evidence for the presence of the deleterious gamma-Al12Mg17 phase. EDS and Auger spectroscopy study showed that the RE precipitates are Al2Ce1-xLax+yZn2-y.