Improving precipitation hardening behavior of Mg Zn based alloys with Ce Ca microalloying additions

The precipitation hardening behavior of newly developed Mg--Zn--Ca--Ce alloys, with modified texture and improved ductility, is studied to delineate the microstructural characteristics that lead to effective hardening upon ageing treatments. Advanced electron microscopy and atom probe techniques are used to analyze the structural characteristics in relevance to the hardening potential. It has been found that the formation of a new basal precipitate phase, which evolves from a single atomic layer GP zone, and is finely distributed in both under-aged and peak-aged microstructures, has a significant impact in the improvement of the hardening response compared with the base Mg--Zn alloys. It has also been found that the beta '1 rod precipitates, commonly formed during ageing treatments of Mg--Zn alloys, have their size and distribution significantly refined in the Ca--Ce containing alloys. The role of alloy chemistry in the formation of the fine basal plate GP zones and the refinement in beta '1 precipitation and their relationships to the hardening behavior are discussed. It is proposed that Ca microalloying governs the formation of the GP zones and the enhancement of hardening, particularly in the under-aged conditions, but that this is aided by a beneficial effect from Ce.