The Study On The New Type Orthopedic Mg-Zn-Zr-Sr Alloy And Its Corrosion Behavior In Vitro

To investigate a new kind of biodegradable magnesium（Mg） alloy with good bone compatibility. The Mg-Zn-Zr-Sr alloy was designed and compared with Mg-Zn-Zr alloy to find out the effect of zirconium（Zr） and strontium（Sr） on the overall performance of the Mg alloys. After that, the Mg-Zn-Zr-Sr alloy was treated by T6 heat treatment to enhance the mechanical properties and corrosion resisitance of the Mg-Zn-Zr-Sr alloy. What’s more, Mg-Zn-Sr alloy, Mg-Zn-Zr alloy and Mg-Zn-Zr-Sr alloy were immersed in SBF in 37℃ to reveal the formation mechanism of the corrosion production layer on these three alloys. The results were as follows:（1） By contrasting the microstructure, mechanical properties and electrochemical properties of the alloys with different Zr content, the Mg-3.2wt%Zn-0.8wt%Zr-0.3wt%Sr alloy was proved to have the best overall performance. After the addition of trace Sr, the average grain size of as-cast Mg-3.2wt%Zn-0.8wt%Zr-0.3wt%Sr alloy was decreased and a new strip-shaped Mg₁₇Sr₂ phase appeared along the grain boundary. Meanwhile, there were lots of rod-shaped Mg Zn second phase appeared in the as-extruded Mg-3.2wt%Zn-0.8wt%Zr-0.3wt%Sr alloy. Yield Tensile Strength（YTS） of 337 Mpa and Ultimate Tensile Strength（UTS） of 380 Mpa were obtained in the Mg-3.2Zn-0.8Zr-0.3Sr alloy meanwhile the elongation was 16%, much higher than that of the Mg-3.2Zn-0.8Zr alloy.（2） The best heat treatment method of Mg-3.2Zn-0.8Zr-0.3Sr alloy was homogenize 300℃ for 20 h, and then treated by solution treatment at 420℃ for 2h, at last, the alloy was treated by aging treatment at 170℃ for 40 h. Yield Tensile Strength（YTS） of 330.347 Mpa and elongation of 17.2% were obtained in Mg-3.2Zn-0.8Zr-0.3Sr alloy after heat treatment, higher than that of the alloy without heat treatment.（3） The order of degradation rate of the three alloys in SBF with a temperature of 37℃ was CR_{Mg-3.2wt%Zn-0.3wt%Sr}>CR_{Mg-3.2wt%Zn-0.8wt%Zr-0.3wt%Sr}>CR_{Mg-3.2wt%Zn-0.8wt%Zr}. What’s more, the changing process of the three alloys was different; the detail process of the corrosion production was as follows: Mg-3.2wt%Zn-0.3wt%Sr alloy: Compact CaHPO₄ granule → globular shaped CaHPO₄ → cotton like Mg（OH）2 → flake-shaped Mg（OH）2. Mg-3.2wt%Zn-0.8wt%Zr-0.3wt%Sr alloy: Compact CaHPO₄ granule→needle-like HA→granulate CaPO₄. Mg-3.2wt%Zn-0.8wt%Zr alloy: Compact CaHPO₄ granule all the time or changed from compact CaHPO₄ granule to granulate CaPO₄.