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Microstructural evolution during severe plastic deformation and mixing of Mg95.8Zn3.6Gd0.6 and Mg97Cu1Y2(at％)alloys upon friction stir welding was studied.A laminated onion-ring structure composed of alternative distribution of layers with significantly refined microstructures from different alloys was formed in the stirred zone.Coarse quasicrystals were broken up and dispersed with most of them being transformed into cubic W-phase particles,and thick 18R long-period stacking ordered plates were fractured and transformed into fine 14H-LPSO lamellae in the stirred zone(SZ)experiencing complex material flow under high strain rate.Fine W-phase particles and 14H-LPSO lamellae formed during dissimilar friction stir welding(FSW)usually have no specific orientation relationship with surrounding Mg matrix.Chemical measurements demonstrated occurrence of interdiffusion between dissimilar layers in the SZ.Phase transformation was observed for some particles of quasicrystals and long-period stacking ordered(LPSO)in regions slightly outside the SZ.An ultimate tensile strength of～415 MPa and an elongation to failure of～27.8％,both exceeding those of base materials,were obtained in the SZ,due to microstructural refinement and formation of a laminated structure.