Study On Microstructure And Properties Of Mg-3Zn-1Ca-0.5Sr Alloy(Wt%)for Bone Implantation

In this paper,aiming at bone implant materials,Mg-3Zn-1Ca-0.5Sr(wt%)alloy was prepared,and the microstructure,mechanical properties,corrosion properties,corrosion mechanism,stress corrosion and biocompatibility of as-cast and extruded alloys were studied.The corrosion dynamic model of the alloy in multi-environment was established,and the biosafety of the alloy was evaluated.First of all,the as-cast alloys solidified at different solidification cooling rates were prepared by changing the cooling conditions.it was found that with the increase of solidification cooling rate,the microstructure of as-cast alloys became denser,the number of pitting pits increased after corrosion,and the number of corrosion microcouples increased.Corrosion rate increased.When the area of single pitting pit decreases,after the surface ?-Mg is corroded,the protective effect of residual structure on deep ?-Mg is enhanced,and the corrosion rate decreases,which alternately plays a leading role,so that the corrosion rate of the alloy increases at first and then decreases.In order to ensure the solidification of as-cast alloy at as fast cooling rate as possible,the micro structure of as-cast alloy prepared by multi-stage cooling system is composed of ?-Mg,skeletal Ca2Mg6Zn3 phase and massive Mg17Sr2 phase.The microstructure of the alloy is coarse,the strength is very low,and it is difficult to deform.After optimizing the homogenization system of the alloy,the hot deformation behavior of the alloy was studied by hot compression experiment and the reasonable hot deformation window of the alloy was determined.The results show that the second phase in the alloy can promote the dynamic recrystallization nucleation and restrain the dynamic recrystallization grain growth,and the reasonable processing range of the alloy is T=330-360?,?=0.02-0.03s-1.After hot extrusion deformation in the processing zone,the mechanical properties of the alloy are significantly improved,the yield strength reaches 190 MPA,the tensile strength reaches 250 MPA,and the elongation after fracture reaches 16.5%.According to the difference of the potential difference of each structure of the alloy,three corrosion primary cells are constructed.The two primary corrosion primary cells with ?-Mg as anode have large potential difference and start first,while the secondary corrosion primary cell with Mg17Sr2 phase as anode has a small difference and will start only after the nearby primary corrosion cell disappears.The alloy releases Mg2+ion through the corrosion of ?-Mg and Sr2+ ion through the corrosion of Mg17Sr2 phase.Apatite and apatite containing Sr are formed on the surface of the alloy.Compared with the as-cast alloy,due to the effect of homogenization heat treatment,the segregation of extruded alloy elements is weakened,and the potential difference between phases is reduced,so that the release time of Sr ions is earlier,and apatite containing Sr is formed earlier on the surface of the alloy.The corrosion is organized deeply along the streamline composed of the second phase,forming a corrosion pit at the bottom consistent with the direction of the streamline and peeling off along the streamline,realizing layer-by-layer corrosion.Under the coupling action of stress and corrosion,the crack is easy to initiate in the corroded position,and the appearance of the crack accelerates the corrosion process.As a barrier,the streamline structure hinders the expansion of the crack and corrosion to the center,forming a stress corrosion step.The lower the service stress is,the more obvious the hindering effect of streamline structure on crack expansion is,and the crack is easy to expand transversely along the streamline;the higher the service stress is,the weaker the hindrance effect of streamline structure on crack expansion is,and the easier it is for the crack to expand to the center,which makes the alloy fail quickly.The results of biocompatibility test in vitro and in vivo showed that the extruded alloy had good biocompatibility in vitro.The extract of different concentrations showed grade 0-1 toxicity to rat preembryonic osteoblasts for a long time,and there was no hemolysis.There was no inflammation and emergency reaction after long-term implantation into the thigh femur of SD rats,indicating that the alloy had no side effects on SD rats.The implant material can induce the bone tissue to grow to the damaged position,and the surrounding bone tissue density is higher than that in the TC4 control group,which effectively alleviates the stress shielding effect.Two weeks after implantation,the peripheral bone tissue grows into the implant site under the induction of the material,which realizes the matching of alloy degradation and bone tissue healing,and has a good application prospect.