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.•The grain boundary Mg3RE and LPSO phases in the casting dissolved after FSP.•Fine grains with LPSO phase distributed only within the grains was achieved.•High mechanical properties were obtained in FSP Mg–Gd–Y–Zn–Zr. 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 ∼2.4μm was developed in the FSP sample. Besides, fine LPSO lamellae with a width of ∼5 to ∼200nm 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 ∼345MPa and an elongation of ∼21.5%. This study provides an effective way for mechanical property optimization in LPSO-contained magnesium alloys.