Study On The Design,preparation And Properties Of Zinc-based Magnesium Core-shell Material For Bone Implantation

Biodegradable zinc（Zn）and zinc alloys are considered as the most attractive new generation of biodegradable metals because of their low degradation rate and good biocompatibility.However,zinc has poor mechanical properties and cannot meet the requirements of mechanical support after implantation.As the main alloying element of Zn alloy,magnesium（Mg）can improve the strength and biocompatibility of the alloy,but also produce a large number of complex metallic phases,which affect the corrosion rate of Zn alloy.Therefore,it is urgent to develop medical degradable implant materials with single phase,good mechanical and corrosion properties.Based on this,the first-principles calculation method was firstly used to explore the physical and chemical properties of Zn-Mg alloy phases(Mg₂Zn₁₁,Mg Zn₂,Mg₄Zn₇)to guide the design and development of materials.Then,Zn-Mg composite materials with core-shell structure（CS）were prepared by using spark plasma sintering technology.The formation mechanism of core-shell structure was analyzed by ABAQUS finite element simulation,and the microstructure,mechanical properties and corrosion properties of CS-x（x=0,5,10,15 w.%）Zn-Mg samples with different Mg contents were studied.Finally,the optimal Mg content ratio was optimized,and the properties of the material were further explored by changing the SPS holding time T-x（x=10,15,20,25 min）.The results show that the Mg Zn₂ phase is the easiest to form and the most stable in the Zn-Mg alloy system by comprehensive analysis of the formation heat and binding energy of the three metallic phases.Mg₂Zn₁₁ metal phase has the highest hardness and stiffness,and plays an obvious role in the strength improvement of Zn-Mg alloy.However,Mg Zn₂ phase with good ductility has the lowest stiffness and hardness,and the best ductility,and the relative improvement of the strength and ductility of Zn-Mg alloy has a certain promoting effect.Finite element simulation analysis found that heterogeneous metal particles in the hot coupling field temperature will be significantly higher than other local contact position,and different size than the zinc,the Mg particles contact temperature difference is not big,in addition,due to the increase of local heat in the process of sintering metal powder expansion,will happen under the action of applied stress,surrounding the Mg particles of zinc particles can spread to the internal formation of core-shell structure;Under the pretreatment of heterogeneous metal powders by high-energy ball milling,Zn powders will be subjected to severe plastic deformation and bond to large Mg spheres,which will also increase the Gibbs free energy of powder interface and contribute to the formation of core-shell structure.Among the core-shell Zn-Mg alloys with different Mg contents,CS-10 has the most suitable comprehensive mechanical properties（214 MPa,5.7%）,and CS-15 has the second best mechanical properties（207 MPa,5.2%）.The results showed that Mg₂Zn₁₁ and Mg Zn₂dominated the second phase in CS-5,while Mg Zn₂ dominated the second phase in CS-10 and CS-15.Pure Zn has the highest corrosion potential and the lowest corrosion current,while the corrosion potential decreases and the corrosion current increases with the addition of Mg.In the immersion test,hydrogen production and p H value increased with the increase of magnesium content,and all the samples except CS-15 could meet the requirements of degradation rate.The microstructure and phase analysis of CS-10 samples with different holding time showed that the thickness of the core-shell structure increased with the increase of holding time,and the thickness of the shell increased obviously after holding for 15 min,reaching the maximum thickness of 70μm.The lowest hardness of T-10 sample is 52.7 HV,and the highest hardness of T-25 sample is 61 HV.T-20 has the best comprehensive mechanical properties,and its compressive（226MPa,6.5%）and bending（103 MPa,7.1 GPa）mechanical properties can meet the requirements of implant mechanics.T-25 corrosion potential and corrosion current up to-1.17V,121×10^-6A·cm^-2,and calculate the polarization resistance(5.687 KΩ·cm^-2),followed by T-20.In the soaking test,both the hydrogen evolution amount and p H value of the samples decreased with the increase of holding time.The hydrogen evolution amount of T-20 and T-25was not significantly different（～4.5 m L/cm²）,and the average degradation rate of T-25 in one month was the lowest 0.084 mm/y.In general,the zinc-based magnesium core-shell material prepared by CS-10 holding for 20 min has the best mechanical and corrosion properties,and is expected to be a new type of bone implant material.