Grain Refinement Mechanism of the As-Cast and As-Extruded Mg-14Li Alloys with Al or Sn Addition

The microstructures of the as-cast and as-extruded Mg-14 wt. % Li-1 wt. % Al (LA141) and Mg-14 wt. % Li-2 wt. % Sn (LT142) were observed by optical and scanning electron microscope (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The effects of Al and Sn on the grain refinement on the Mg-14Li alloy were investigated. In addition, the mechanism of grain refinement on the as-cast and as-extruded alloys was discussed from the view of the solute effect and heterogeneous nucleation effect via edge-to-edge matching model. The results showed that the average grain sizes of the as-cast LA141 and LT142 alloys were similar due to the close solute effect of 1.1wt. % Al and 1.8 wt. % Sn, while, in the as-extruded alloys, the average grain size of LT142 was over two times finer than that of LA141. This was attributed to the reason that Li2MgSn particles can serve as heterogeneous nucleation sites for the beta-Li matrix during the process of dynamic recrystallization (DRX), but LiMgAl2 cannot serve the same way. Therefore, Sn can act as a more effective grain refiner for the Mg-14Li alloy compared to Al.