The Effect Of Second Phase And Hot Extrusion On The Microstructure And Corrosion Behavior Of Biomedical Mg-Zn-Ca Alloys

Mg-Zn-Ca alloy has good mechanical properties and corrosion resistance,the added Zn and Ca are biologically safe and can be completely degraded in the human body,and the degradation products are non-toxic and harmless,which making it a good material for orthopedics implants.However,there are still some problems with the Mg-Zn-Ca alloy in practical applications,such as too fast degradation and insufficient mechanical properties.After researching a large amount of papers,this study firstly designed four Mg-Zn-Ca alloys containing different types of second phases,and initially investigated the effect of the second phase on the microstructure and corrosion behavior of Mg-Zn-Ca alloys.On the basis of screening suitable second phases and keeping the Zn/Ca ratio constant,the effect of the content of the second phase on the microstructure and corrosion behavior of Mg-Zn-Ca alloys was investigated.Finally,through hot extrusion,the effect of hot extrusion on the microstructure and corrosion behavior of the Mg-Zn-Ca alloys was investigated.The specific results of the study are as follows:（1）The as-cast Mg-Zn-Ca alloys containing different types of second phase show significant differences in microstructure,mechanical properties and corrosion resistance.When1 wt.%Ca was added to Mg-5Zn,the second phases changed from Mg Zn phase to Ca₂Mg₆Zn₃ phase,and the second phase changes from a diffuse distribution to a continuous network distribution.In vitro degradation experiments show that this transformation reduced the corrosion resistance.The addition of 1 wt.%Zn to the Mg-5Ca alloy transformed the second phases from a separate Mg₂Ca phase to a state where the Mg₂Ca（Zn）phase coexisted with the Ca₂Mg₆Zn₃,and reduced the potential difference between the second phase and the Mg matrix improved the corrosion resistance of the alloys.Combining the cross-sectional corrosion morphology and quasi-in-situ corrosion behavior of the four alloys,the Mg-Zn-Ca alloy with the predominant Mg₂Ca（Zn）phase and a small amount of Ca₂Mg₆Zn₃ phase has the best corrosion resistance.Room temperature compression experiments show that the addition of Zn increases the strength and plasticity of the alloy,while the addition of higher levels of Ca substantially reduces the plasticity of the alloy.（2）On the basis of determining the second phase to be mainly Mg₂Ca（Zn）,doped with a small amount of Ca₂Mg₆Zn₃,and containing the Zn/Ca=0.1,we designed Mg-1Ca-0.2Zn,Mg-1.7Ca-0.4Zn and Mg-2.8Ca-0.6Zn alloys to explored the effect of the content of the second phase on the microstructure and corrosion behavior.The experimental results show with a reduction in the content of the second phase,the strength of the as-cast alloy decreased,but a significant increase in plasticity happened.The in vitro immersion results show that the Mg-1Ca-0.2Zn alloy has the best corrosion resistance.And the weight loss rate increases with the increase of the second phase content in the early stage of corrosion,and the higher content of the second phase in the late stage of corrosion generates a dense corrosion product film which reduces the corrosion rate of the alloy.（3）After hot extrusion,the grain size of the extruded alloys decreased significantly compared to the as-cast alloys and the second phase changed from a continuous net-like distribution to a strip-like and block-like distribution along the extrusion direction.Secondly,the strength of the alloy after extrusion was significantly increased,which was attributed to grain boundary strengthening and second phase strengthening.Finally,the corrosion behavior of the alloy changes after extrusion.The continuous reticulation of the extruded alloy is disrupted and loses its barrier effect on corrosion,resulting in accelerated corrosion of the extruded alloy.（4）Comprehensive analysis of the experimental results shows that the second phase and grain size have a greater influence on the microstructure and corrosion behavior of Mg alloys.And for the results of this paper,the addition of Ca to Mg alloys should not exceed 1.7 wt.%.Taking into consideration,the mechanical properties and corrosion resistance of the cast Mg-1.7Ca-0.4Zn alloy are better and are expected to be suitable for orthopedic implant materials.