PRECIPITATION HARDENING OF MG-ZN AND MG-ZN-RE ALLOYS

The age-hardening response of two cast Mg-Zn-RE alloys has been invest igated and compared with that of a binary Mg-Zn alloy. The microstruct ures of the aged specimens were examined by analytical electron micros copy. Formation of a fine dispersion of rodlike beta(1)' precipitates is the main cause for age hardening, while extensive precipitation of disc-shaped beta(2)' coincides with the onset of overaging. Rare earth additions retard the formation of beta(2)' precipitates and thus post pone overaging. Four different orientation relationships between beta( 2)' precipitates and matrix were found and explained in terms of the n ear-CSL model.