Solidification and Age Hardening Behaviour of Mg-Zn-Gd Alloys

The solidification and age hardening behaviour of an Mg-6Zn-2Gd (wt.%) alloy has been investigated. It was found that the microstructure of the as-cast samples was composed of equiaxed alpha-Mg grains surrounded by some eutectic compound both at triple points and along grain boundary. The eutectic compound was composed of MgZn2, Mg5Gd phases and alpha-Mg matrix. After a solution treatment at 500 degrees C for 18 h, the eutectic compound almost dissolved, but some discontinuous second phase particles still survived at grain boundaries. The Mg3Gd2Zn3 phase also formed during subsequent ageing. The addition of Gd not only improved the thermal stability of the second phase formed during solidification, but also postponed the overaging during ageing at 200 degrees C up to 100 h. The precipitates with three different morphologies: [0001] (alpha) rods/ lath, (0001) (alpha) plates and blocky particles, were observed in the alpha-Mg matrix. The solute elements of Zn and Gd were found to significantly partition into the precipitates, especially for blocky particles. This significant partition can be correlated directly to improved mechanical properties at elevated temperature.