Fig. 1 Schematic illustration of experiments: a friction stir welding, b Vickers microhardness measurement, c geometry and size of tensile specimens in mm

Fig. 2 a Optical macrograph of the Mg95.8Zn3.6Gd0.6/Mg97Cu1Y2 FSW joint; BM stands for base materials. b Magnified optical macrograph of the FSW SZ, showing the position of tensile specimensFull size image

Fig. 3 BSE-SEM micrographs for a Mg95.8Zn3.6Gd0.6, b Mg97Cu1Y2. Insets in a, b show SAED patterns recorded along the fivefold axis of IQCs and the [1120] zone axis of LPSO plates

Fig. 4 a, b BSE-SEM micrographs for TMAZ at AS and RS, respectively. c, d Magnified images for regions indicated by black rectangles in a, b, respectively

Fig. 5 Microstructure of TMAZ near the TMAZ/SZ interface at the advancing side: a a low-magnification image showing breakup of IQCs, inset SAED patterns for particles indicated by black arrows along the fivefold axis, b W-phase and IQC particles with random orientation. c, d A TEM image and corresponding composite SAED pattern showing IQC and W particles with a specific orientation relationship

Fig. 6 Microstructure of TMAZ near the TMAZ/SZ interface at the retreating side: a a low-magnification image showing bending and fracture of a thick LPSO plate, and an inset SAED pattern recorded along the [11$\overline{2}$0] zone axis, b a magnified image showing small lamellae within Mg matrix adjacent to one thick18R-LPSO plate. c, d High-resolution TEM images for regions containing 14H-LPSO lamellae and surrounding α-Mg matrix. Inset in d is a corresponding composite SAED pattern recorded along the [11$\overline{2}$0] zone axis of the 14H-LPSO lamella, and diffraction spots from adjacent Mg matrix are indicated by dotted circlesFull size image

Fig. 7 a A low-magnification SEM micrograph of the SZ, b a BSE-SEM micrograph of the center region. c, d Magnified BSE-SEM micrographs for the center region and onion rings, respectively

Fig. 8 SEM GB maps of a center region, b layers of onion rings at advancing side, c layers of onion rings at retreating side. d-f Statistical results of grain sizes in the center region, particle-rich and lamella-rich layers in the onion-ring region, respectively

Fig. 9 Low-magnification STEM images for a particle-rich center region and b onion rings. c-e SAED patterns of W-phase particles along the [001], [011] and [$\overline{1}$11] zone axis, respectively. f SAED pattern of lamella phases in the lamella-rich layer along the [11 $\overline{2}$ 0] zone axis

Fig. 10 a A low-magnification HAADF-STEM image for nanoprecipitates in Mg matrix, b an atomic resolution HAADF-STEM image recorded along the [0001]Mg zone axis for a region with a precipitate

Fig. 11 TEM images for a granular particles, b LPSO lamellae in the SZ

Fig. 12 a A low-magnification STEM image for the onion-ring region, b, c EDS mappings for the white and black rectangular regions indicated in a

Fig. 13 Tensile properties of SZ and as-cast samples