Fig. 1. Schematic of (a) the manufacturing process, (b) samples for tensile tests, and (c) samples for immersion tests. Noted that the specimens were cut from the longitudinal section of the extruded rods.

Fig. 2. (a-c) OM images and (d) the average grain size as well as the volume fraction of the second phase of Mg-Gd-Cu/Ni alloys.

Fig. 3. Bright-field images with (a) low magnification and (b) high magnification of Mg2Cu phases in the Mg-2Gd-0.5Cu alloy.

Fig. 4. (a-d) SEM images of Mg-Gd-Cu/Ni alloys in longitudinal section, where (c) shows the topography in high magnification and corresponding EDS mapping of the Mg-Gd-Ni ternary phase highlighted by the yellow rectangle in (b). (e) HAADF-STEM image of the Mg-Gd-Ni ternary phase and corresponding SAED patterns.

Fig. 5. (a–c) IPF with the reference of the ED and (d) the {0002}, {11$\bar{2}$0}, and {10$\bar{1}$0} PF of Mg-Gd-Cu/Ni alloys.

Fig. 6. Percentage of grains with a φ (the angle between its c-axis and the ED) of 70° to 90° in Mg-Gd-Cu/Ni alloys.

Fig. 7. (a) Brinell hardness, (b) tensile curves, and (c) tensile results of Mg-Gd-Cu/Ni alloys.

Fig. 8. (a) Work-hardening capacity (WHC) and (b) work-hardening exponent (n) of Mg-Gd-Cu/Ni alloys.

Fig. 9. {0001}<11$\bar{2}$0> Schmid Factor of Mg-Gd-Cu/Ni alloys.

Fig. 10. SEM image near the fracture surface of the Mg-2Gd-0.5Ni alloy.

Fig. 11. Polarization curves of Mg-Gd-Cu/Ni alloys.

Fig. 12. (a) Nyquist diagram, (b) Bode diagram, (c) equivalent circuit, and (d) Rsum and 1/Rsum of Mg-Gd-Cu/Ni alloys.

Fig. 13. Macro topographies of Mg-Gd-Cu/Ni alloys with different immersion times.

Fig. 14. Maps show the corrosion rate, EL, and overall alloying elements of different Mg alloys: (a) corrosion rate vs EL and (b) corrosion rate vs content of alloying elements. Mg-Al(-Sm) alloys [64], Mg-Dy alloys [65], Mg-Gd-Zn alloys [9], Mg-Zn-Zr(-La) alloys [66], Mg-Al-Zn alloys [67], Mg-Zn-Mn(-Ca) alloys [68], Mg-Zn-La alloys [69], Mg-Zn-Ca(-Ce/La) alloys [70], Mg-Y-Nd alloys [71], Mg-Li-Al(-Ca) alloys [72], Mg-Cu-Al alloys [23], and Mg-Gd-Cu alloys [31,32] were used to compare with Mg-Gd-(Cu/Ni) alloys prepared in this work.

Fig. 15. (a) Hydrogen gas evolution curves, (b) corrosion rates, and (c) optical topographies and corresponding three-dimensional reconstruction images of Mg-Gd-Cu/Ni alloys. Noted that both macro topographies shown in the insets in (b) and optical topographies shown in (c) were captured after 48 h immersion, and corrosion products were removed before observation.

Fig. 16. SEM images of (a-d) the Mg-2Gd-0.5Ni alloy immersed in 3.5 wt.% NaCl solution for 5 min, (e, f) LPSO phases eroded by picric acid, (g) Mg-2Gd-0.5Cu alloy immersed in 3.5 wt.% NaCl solution for 10 min, (h) α-Mg matrix in the vicinity of Mg2Cu phases eroded by picric acid, (i, j) Mg-2Gd-0.5Ni alloy immersed in 3.5 wt.% NaCl solution for 10 min, (k) Mg-2Gd-0.5Cu alloy immersed in 3.5 wt.% NaCl solution for 60 min, and (l) Mg-2Gd-0.5Ni alloy immersed in 3.5 wt.% NaCl solution for 60 min. Noted that white arrows highlight the uncorroded area, red arrows highlight the filiform corrosion, yellow arrows highlight corroded LPSO phases and blue arrows highlight the corroded α-Mg matrix in the vicinity of Mg2Cu phases.

Fig. 17. Longitudinal section of the Mg-2Gd-0.5Ni alloy after 1 h immersion, where cracks of corrosion product film are highlighted by the yellow triangles.

Fig. 18. Schematic of corrosion progress of Mg-Gd-Cu/Ni alloys. (a1-c1, a3-c3) and (a2-c2, a4-c4) show transverse and longitudinal topographies, respectively. Corrosion occurred preferentially at grain boundaries and expanded into the grain interior (highlighted by the black shadows near grain boundaries in b1, b3, c1, and c3). The α-Mg matrix in the vicinity Mg2Cu phases was corroded prior to the Mg2Cu phases (highlighted by the black polygons in b1), whereas LPSO phases were corroded before the α-Mg matrix (highlighted by black polygons in b3). LPSO phases dispersed along the ED accelerated the longitudinal development of corrosion and grains with the (0001) tilting to the ED showed lower corrosion resistance.