Effect of Sn element on the formation of LPSO phase and mechanical properties of Mg-6Y-2Zn alloy

The long period stacking ordered (LPSO) phase is a key strengthening precipitate phase in the Mg-Y-Zn based alloys. Fully controlled by the chemical ordering and stacking faults, the LPSO phase could be strongly modified by alloying elements. This work explored the effect of trace Sn addition on the Mg-6Y-2Zn alloy, with an emphasis on the LPSO phase and corresponding changes of mechanical properties in the as-cast, as-annealed and as-extruded states. Results show that the 18R LPSO phase forms as needles and distributes at grain boundaries. Meanwhile, its length and density are stimulated dramatically by the Sn addition in the as-cast state. After annealing, the content of the 18R increases accompanied by the change of morphology shapes from needle to block. Additionally, fine 14H LPSO lamellas are precipitated both in grains and between 18R phases. In contrast to the as-annealed Sn-free alloy, the area fraction of 18R phase in the Sn-containing Mg alloy increases from 35% to 58%, the 14H is also promoted to a certain extent. Tensile tests conducted on as-extruded state revealed remarkable mechanical properties improvement in the Sn-containing alloy over its ternary equivalents. The relevant mechanisms are also investigated in detail.