Formation of long-period stacking ordered phase only within grains in Mg-Gd-Y-Zn-Zr casting by friction stir processing

Friction stir processing (FSP) was applied to Mg-Gd-Y-Zn-Zr casting, resulting in significant dissolution of large grain boundary eutectic β-Mg3RE and long-period stacking ordered (LPSO) phases. A fine-grained structure with a grain size of ∼2.4 μm was developed in the FSP sample. Besides, fine LPSO lamellae with a width of ∼5 to ∼200 nm were distributed only within the grains in the FSP sample, which is different from that observed in both solution-treated and other plastically deformed counterparts. Formation of such special structure is attributed to the high temperature severe plastic deformation generated by FSP. The FSP Mg-Gd-Y-Zn-Zr exhibited high mechanical properties with a yield strength of ∼345 MPa and an elongation of ∼21.5%. This study provides an effective way for mechanical property optimization in LPSO-contained magnesium alloys.