Fig. 1. Optical microstructures of Mg-Zn-Zr-Nd alloy (a) and Mg-Zn-Zr-Y alloy (b).

Fig. 2. Typical stress-strain curves and mechanical performances of Mg-Zn-Zr-Nd and Mg-Zn-Zr-Y alloys, taking pure Mg as a control.

Fig. 3. Tensile fracture morphologies of Mg-Zn-Zr-Nd alloy (a, b) and Mg-Zn-Zr-Y alloy (c, d).

Fig. 4. Electrochemical corrosion behaviors of the experimental materials in α-MEM solution with 10% FBS at 37 °C: (a) Nyquist plots; (b) Bode plots; (c) potentiodynamic polarization curves; (d) electrochemical data. Ecorr represents corrosion potential and icorr represents corrosion current density (n = 3, * p < 0.05).

Fig. 5. (a) Corroded surface photographs (corrosion products were removed); (b) corrosion rate of pure Mg, Mg-Zn-Zr-Nd and Mg-Zn-Zr-Y alloy after immersion in α-MEM with 10% FBS solution at 37 °C 5% CO2 for 3 d and 10 d, respectively (n = 5, * p < 0.05); (c) ion concentration in immersion test of pure Mg, Mg-Zn-Zr-Nd and Mg-Zn-Zr-Y alloy according to ICP-MS test.

Fig. 6. EDS mapping of cross-sections of corrosion products after immersion tests in solution for 3 days: (a) pure Mg; (b) Mg-Zn-Zr-Nd alloy; (c) Mg-Zn-Zr-Y alloy.

Fig. 7. XRD patterns of Mg-Zn-Zr-Nd and Mg-Zn-Zr-Y alloys after 3 d and 10 d immersion in α-MEM with 10% FBS.

Fig. 8. pH values of pure Mg, Mg-Zn-Zr-Nd and Mg-Zn-Zr-Y alloys in α-MEM solution with 10% FBS at 37 °C 5% CO2.

Fig. 9. (a) Optical density of MC3T3-E1 cells in extraction medium of experimental materials measured by CCK8 test and (b) relative growth rate (RGR) and cytotoxicity level at different detection period (n = 5, * p < 0.01, ** p < 0.001).

Fig. 10. Morphologies of MC3T3-E1 cells cultured on pure Mg, Mg-Zn-Zr-Nd and Mg-Zn-Zr-Y alloys for 4 h and 24 h: (a) pure Mg for 4 h; (b)Mg-Zn-Zr-Nd for 4 h; (c) Mg-Zn-Zr-Y for 4 h; (d) pure Mg for 24 h; (e) Mg-Zn-Zr-Nd for 24 h; (f) Mg-Zn-Zr-Y for 24 h.

Fig. 11. ALP staining of MC3T3-E1 cells cultured with pure Mg, Mg-Zn-Zr-Nd alloy and Mg-Zn-Zr-Y alloy extracts for 7 d and 14 d, respectively.

Fig. 12. (a) S-N curves of Mg-Zn-Zr-Nd alloy in air (data points with arrows indicate that the specimens did not fail) and (b) cycles of failure after immersed in Hank’s solution for 7 d, 30 d, 60 d, 90 d, respectively.

Fig. 13. SEM images showing the fatigue fracture morphologies of Mg-Zn-Zr-Nd alloy in air (failure at 1184616 cycles, under the stress amplitude of 115 MPa and test frequency of 15 Hz) (a), immersed in Hank’s solution for 7 d (failure at 5854 cycles, under the stress amplitude of 111.4 MPa and test frequency of 2 Hz) (b), immersed in Hank’s solution for 30 d (failure at 4707 cycles, under the stress amplitude of 111.4 MPa and test frequency of 2 Hz) (c), immersed in Hank’s solution for 60 d (failure at 2105 cycles, under the stress amplitude of 111.4 MPa and test frequency of 2 Hz) (d), immersed in Hank’s solution for 90 d (failure 876 cycles, under the stress amplitude of 111.4 MPa and test frequency of 2 Hz) (e).