Enhancement of hardening and thermal resistance of Mg-Sn-based alloys by addition of Cu and Al

Mg-Sn-based alloys are considered as a promising precipitation-hardening system for applications at elevated temperatures, but the hardening effect is not satisfactory owing to sluggish nucleation and rapid coarsening of the major Mg2Sn lath precipitates. In this study, Cu and Al are added to a Mg-6Sn-1Mn base alloy. The age-hardening response and the microstructures of these modified alloys have been investigated and are compared to that of the base alloy. The additional elements are found to bring several beneficial effects to the alloys for applications at elevated temperatures. Firstly, a eutectic structure consisting of strong intermetallic phases, i.e. Mg2Cu in the Mg-6Sn-1Mn-2Cu alloy and Al0.93Cu1.07Mg in the Mg-6Sn-1Mn-2Cu-2Al alloy, remains stable along the grain boundaries after solution and ageing heat treatments. Secondly, the precipitate density has been increased significantly and the precipitate size has been refined remarkably during ageing at 200 degrees C. Moreover, the growth of the precipitates is inhibited remarkably during the over-ageing period. Therefore, the age-hardening response and over-ageing resistance are notably improved.