Fatigue behaviors of HP-Mg, Mg–Ca and Mg–Zn–Ca biodegradable metals in air and simulated body fluid

The dynamic loading in human body, along with the corrosive body fluid, presents a great challenge for the practical use of biodegradable magnesium implants. In this study, a high purity magnesium (99.99 wt.%) and two typical promising biodegradable magnesium alloys (binary Mg–1Ca and ternary Mg–2Zn–0.2Ca) were chosen as the experimental materials. Their dynamic mechanical performances were comparatively evaluated by carrying out fatigue tests in air and in simulated body fluid (SBF). The fatigue strengths of HP-Mg, Mg–1Ca and Mg–2Zn–0.2Ca were all around 90 MPa in air, however, they decreased to 52 MPa, 70 MPa and 68 MPa in SBF at 4 × 10⁶ cycles, respectively. The fatigue cracks initiated from the microstructural defects when tested in air, but nucleated from surface corrosion pits when tested in SBF. Cyclic loading significantly increased the corrosion rates of all the experimental materials compared to that in static SBF. Moreover, based on our findings, the fatigue failure processes and interactions between material, corrosion and cyclic loading were systematically discussed. Statement of Significance Fatigue strength and life are vital parameters to the design of metallic implant devices. For the corrosion fatigue of biomedical magnesium alloys, we reported the corrosion fatigue behavior of AZ91D and WE43 in SBF (Acta Biomaterialia, 6 (2010) 4605–4613), and till now there is no other reports to our knowledge. We spent 3 years to finish the fatigue testing and get S-N curves for three more magnesium biomaterials, and our significant finding is that the fatigue strengths of HP-Mg, Mg–1Ca and Mg–2Zn–0.2Ca are all around 90 MPa in air but 52 MPa, 70 MPa and 68 MPa in SBF at 4 × 10⁶ cycles, which will provide the first-hand data for the future magnesium implants design.