Effects Of Zn,Mn And Ca On Microstructures And Degradation Properties Of Bio-magnesium Aiioys

In order to better utilize the excellent biocompatibility and biodegradation property of magnesium alloys,and avoid the secondary damage that caused by the removal of implantable materials,the effects of elements on the microstructrues and degradation properties of biomagnesium alloys were investigated in this paper.First of all,12 groups of magnesium alloys with different contents of Mn,Zn and Ca were prepared by melting-casting method and hot extrusion treatment based on uniform design experiment,and then their degradation ability was tested by using electrochemical corrosion and static immersion method.Then,regression analysis was used to study the effect of alloy elements on the degradation ability and then the composition of bio-magnesium alloy with maximum degradation rate was acquired.Subsequently,the relationship among alloying elements,microstructure and degradation properties were discussed from the viewpoint of microstructure analysis,especially grain size of a-Mg and the composition and distribution characteristics of second phase.Meanwhile,the mechanical properties of the alloy were checked as well.Finally,Mg-4Zn-xCa alloys were prepared with the highest degradation rate by changing the Ca content,and its solidification process and degradation behavior were studied in detail.Based on above expements and theory analysis,corrosion sequence of the alloy was defined and the degradation mechanism of magnesium alloys was revealed in this paper.12 groups of as-cast magnesium alloys with different chemical compositions present equiaxed grain morphology except the differences in grain size and phase composition of the second phase.When Ca is added,the second phase is mainly a-Mn.All magnesium alloys contain Ca₂Mg₆Zn₃ when Ca is present.When Zn/Ca atomic ratio is less than or equal to 1.18,the second phase contains Ca₂Mg₆Zn₃ and Mg₂Ca phase.Among the three alloying elements,Ca element has the most significant effect on the grain size of magnesium alloy matrix,since it has the greatest inhibition growth ability.Under the as-cast condition,the polarization curves of 12 groups Mg alloys do not show inflection points,indicating that the alloy is not easy to be deactivated in the simulated body solution of SBF and the degradation is sustainable.Electrochemical AC impedance and static corrosion rate are used for multiple regression analysis of the degradation capacity,and the composition of the alloy with the highest degradation velocity is determined as Mg-4Zn-1Ca.After extrusion processing,there is not inflection point in the polarization curves of 12 groups of magnesium alloys,indicating that as-extruded alloy shows continuous degradation process in SBF solution as well.Multiple regression analysis results show that as-extruded Mg-4Zn-1Ca alloy has the most degradation rate in this paper.The degradation rate of as-extruded alloy is slightly lower than the as-cast magnesium alloy,which can be attributed to the Ca₂Mg₆Zn₃ phase in the presence of a large network structure.In this case,the corrosion of the matrix will be concentrated in certain areas,resulting in large corrosion current and non-uniform corrosion.After broken of such network structure,the corrosion current of the alloy decreases and the corrosion is homogeneous.The mechanical properties of magnesium alloys could be significantly improve by the Ca₂Mg₆Zn₃ phase.The Ca₂Mg₆Zn₃ phase of extruded alloys is dispersed homogeneous and has a stronger strengthening effect than that of the as-cast alloys.The minimum hardness of 12 as-cast alloys and extruded alloys are 41.62HV and 56.18HV respectively.The minimum tensile strength and compressive strength of as-extruded alloys are 256.71MPa and 381.21MPa respectively.All of them are greater than the maximum mechanical properties of human bones,indicating that all 12 groups of magnesium alloy designed in this paper can meet the requirements on the mechanical properties of the implanted materials.The solidification behaviors and microstructures of Mg-4Zn-xCa are significantly affected by the addition and content of Ca element.When Ca contents are higher than 2wt%,their solidification sequence are L??-Mg+L',L"??-Mg+ Ca₂Mg₆Zn₃,L"??-Mg+ Ca₂Mg₆Zn₃,and the microstructure is a-Mg+Ca₂Mg₆Zn₃+Mg₂Ca.When Ca content is lwt%,the phase transition corresponding to MgCa2 is absent,and the microstructure of the alloy is a-Mg+Ca₂Mg₆Zn₃.The results of microstructure analysis and dynamic corrosion of Mg-4Zn-xCa alloys show that the corrosion sequence of the three phases is Mg₂Ca>a-Mg>Ca₂Mg₆Zn₃,which can be attributed to the self-corrosion potential of different phases.During the corrosion process,the presence of Mg₂Ca phase can significantly affect the degradation rate of alloy in the early stage,but the degradation rate of alloy in the later stage is mainly affected by both the content and distribution of Ca₂Mg₆Zn₃ phase.