Age hardening and precipitation in a cast magnesium rare-earth alloy

The precipitation sequence responsible for the age-hardening behaviour of a cast Mg-1.3 wt% rare-earth alloy has been investigated by analytical electron microscopy. Very fine intermediate precipitates formed at an early stage of ageing. Plate-shaped Mg(3)MM beta' precipitates (MM = misch metal) and hexagonal prism-shaped Mg(12)MM beta precipitates were primarily responsible for age hardening. Precipitate morphologies, crystal structures and crystallographic orientation relationship were determined for the various types of precipitates that formed during ageing at different temperatures.