Revealing the mechanism of Sm element on the long period stacking ordered structures of Mg-Zn-Gd alloy

In the current work, the mechanism of the Sm-modified Mg-Zn-Gd alloy and its effect on the microstructure and mechanical properties of as-cast and as-aged alloy are systematically investigated. Experimental results reveal that adding Sm promotes the formation of long-period stacking ordered (LPSO) phase in as-cast Mg-Zn-Gd alloy. The reason is that the Sm element can reduce the stacking fault energy and introduce a large number of stacking faults (SFs) in the magnesium, which promotes the formation of the LPSO phase. During the heat treatment process, SFs and Mg3RE phases can promote the nucleation of beta ' precipitates owing to the Mg3RE phase being coherent with the alpha-Mg matrix and LPSO phase. The Sm addition not only effectively refines the grain structure of as-cast alloy but also promotes the formation of stacking faults and the LPSO phase. The mechanical properties of Sm-containing alloys in as-cast and as-aged states are superior to Sm-free alloys, ascribed to the relatively refined grains, solid-solution strengthening, and sufficient fine beta' aging precipitates.