Microstructural Characterization Of Aged Mg-Sn-Al Alloy

The age-hardenable Mg-Sn-based alloys have received extensive attention in recent years due to their potentials to develop low-cost heat-resistant magnesium alloys.It is reported that the precipitation sequence of binary Mg-Sn alloys is:supersaturated solid solution→GP zone→β’（Mg₃Sn）→β（Mg₂Sn）.However,the precipiate in the peak-aged Mg-Sn alloy is mainly the equilibrium phase Mg₂Sn with coarser size and poor number density,hence the age-hardening response of Mg-Sn binary alloy is moderate.In this study,the type,size,morphology,orientation relationship with the matrix of precipitates,and the distribution of Al in aged Mg-9.8Sn-3Al alloy was investated using HAADF-STEM and EDS-STEM,and the mechamism of Al refineing Mg₂Sn precipitates was discussed.Hardness test indicates that Al additions can obviously improve the age-hardening response of Mg-9.8Sn alloy,and decrease the time to reach peak hardness.The peak-hardness value of Mg-9.8Sn alloy containing 3 wt.%Al aged at 200℃ can reach to 78 HV,which is approximately 42%higher than that of Mg-9.8Sn alloy.It was found that the addition of Al to the Mg-9.8Sn alloy can refine Mg₂Sn precipitates and increase the number density of the Mg₂Sn precipitates.From the morphology,the Mg₂Sn precipitates were mainly divided into four categories,such as laths precipitates which were formed in the basal face,prismatic face and pyramidal face and polygonal precipitates.It was observed that there are seven orientation relationships between the Mg₂Sn precipitates and the magnesium matrix.The addition of Al in the Mg-9.8Sn alloy not only forms the Mg₁₇Al₁₂ phase,but also mainly segregates at the interface between the Mg₂Sn phase and the magnesium matrix,and the segregation of the Al element is independent of the orientation relationship of the Mg₂Sn phase and the matrix.The segregation of Al element at the Mg₂Sn phase and the Mg matrix interface can reduce the interfacial energy and increase the nucleation rate of the Mg₂Sn phase.On the other hand,it can inhibit the growth of the Mg₂Sn phase and refine the Mg₂Sn phase,thereby significantly increasing the age hardening effect of the Mg-9.8Sn alloys.