Study On Properties And Surface Modification Of A New Type Degradable Mg-Zn-Zr-Dy Magnesium Alloy For Orthopedics

The ideal biodegradable bone fixation or repair materials need to have high and stable mechanical properties at the beginning of implantation to provide support and fixation to the damaged area.After the new bone tissue has healed,the mechanical properties of the implant materials need to decline rapidly.Magnesium alloys have received a lot of attention because of their good mechanical properties,degradability,and biocompatibility,which can avoid the second surgery required to remove the implant.However,the rapid corrosion rates of magnesium alloys in the human environment lead to a rapid deterioration of their mechanical properties,making it difficult to provide high and stable support and fixation for damaged areas.Therefore,the use of magnesium alloys in clinical is greatly limited.In this paper,a series of magnesium alloys with low rare earth content was designed and prepared by using alloying technology from the viewpoint that the mechanical properties and corrosion resistance of magnesium alloys need to be well matched.The alloy composition was selected by microstructure analysis and property testing.Based on the selected alloy,solid solution and hot extrusion treatments were used to make the alloy with good mechanical properties and corrosion resistance.Then the surface modification process was used to reduce the decay rate of mechanical properties in the corrosive medium and improve the service life of the alloy.The main research results are as follows.The microstructure and properties of the as-cast Mg-2.0Zn-0.5Zr-x Dy（x=0～3.0）alloys are affected by the addition of the Dy element.The second phase in the as-cast alloy is mainly composed of the Mg₇Zn₃ phase,Mg₂₄Dy₅ phase,and Mg₃Zn₃Dy₂ phase.With the increase of Dy content,the grain size decreases,and the volume fraction of the second phase increases.Low content of Dy addition significantly improved the properties of the as-cast alloy,and at 1.5 wt%of Dy content,the alloy has both good mechanical properties and corrosion resistance with tensile strength（UTS）,yield strength（YS）,elongation（EL）,and corrosion rate（CR_w）of 150±14 MPa,89±3 MPa,（9.2±0.6）%,and 0.89±0.12 mm/y,respectively.The corrosion behaviors of the as-cast alloys are mainly influenced by the grain size（d）and volume fraction of second-phase（f）coupling effects.Data analysis of d,f,and CR_w of the investigated alloys and some Mg alloys with rare-earth elements established a quantitative relationship between them:CR_w=0.94-0.38·f+3.05E-3·d+6.38E-2·f²-4.21E-6·d²+4.35E-4·f·d.This relationship can be used as a reference for the rapid assessment of corrosion rates and designing magnesium alloys by alloying.The microstructure and properties of as-cast Mg-2.0Zn-0.5Zr-1.5Dy alloy can be improved by solid solution and hot extrusion treatments.The solid solution treatment dissolves the alloying elements in the second phase into the Mg matrix and improves the alloy’s mechanical properties through solid solution strengthening.The solid solution treatment reduces the second phase content in the alloy,the degree of galvanic corrosion is reduced,and the corrosion resistance is improved.After hot extrusion treatment,the grain of the as-cast and the solid solution treated alloys have been refined,and the mechanical properties of the alloys are significantly improved.While the corrosion resistance of the cast-extruded alloy is poor.The mechanical properties and corrosion resistance of the solid solution-extruded alloy were improved compared with those of the as-cast,solid solution,and cast-extruded alloys,with UTS,YS,EL,and CR_wreaching 312±12 MPa,287±10 MPa,（17.6±0.5）%,and 0.59±0.16 mm/y respectively.The comprehensive performances of solid solution-extruded alloy meet the requirements of biodegradable orthopedic materials.Preparation of micro-arc oxidation（MAO）coating on the surface of the solid solution-extruded alloy can reduce the corrosion rate and avoid the rapid decay of the mechanical properties of the alloy in the corrosive medium.The surface of MAO coating is a porous structure,mainly composed of the Mg O phase,and its structure includes an external porous layer and an internal barrier layer.With the increase of oxidation time from 3 min to 20 min,the average thickness of the coating increases from 10μm to 42μm,while the average pore size gradually increases and the porosity gradually decreases.The dense layer of MAO coating is the main controlling factor to improve the corrosion resistance of the alloy.The barrier layer thickness is thin and vulnerable to damage by the corrosive medium,and its effect on the improvement of alloy corrosion resistance is temporary.The reduction of mechanical properties of the alloy in the corrosive medium（especially EL）is mainly related to the formation of pitting holes on the surface of the alloy.When the oxidation time is 15 min,the thickness of the barrier layer of MAO coating is thicker,about 8μm,and the structure is denser,which has a better hindering effect on the corrosive medium and effectively delays the formation process of pitting holes on the surface of the alloy.MAO coating with an oxidation time of 15 min was sealed with polylactic acid（PLA）to improve the coating protection.The MAO/PLA composite coating was successfully prepared on the surface of the MAO-coated alloy with a thickness of45～50μm after four times dipping in PLA solution.In SBF,the decay rate of mechanical properties of both MAO and MAO/PLA composite coated specimens were linearly correlated with the corrosion time,and the quantitative relationships between the decay rate of mechanical properties and corrosion time were established,which can be used to evaluate the reliable service life of the alloy in the corrosive medium.Compared with MAO coating,MAO/PLA composite coating can increase the alloy’s reliable service life by 2.3 times,which has good application prospects.The corrosion damage process of MAO/PLA composite coating in SBF can be divided into four stages.In the first stage,the amorphous area of PLA is preferentially hydrolyzed to produce oligomer or lactic acid molecules,resulting in concave holes and crack defects in the local area of PLA coating;in the second stage,the number of holes and crack defects in PLA coating increases,and more water molecules enter the interior of PLA coating,accelerating the hydrolysis of PLA coating and causing the p H to decrease,promoting the autocatalytic reaction of PLA,resulting in the complete degradation of PLA coating;the third stage,the corrosive medium penetrates the MAO coating pores and crack defects,and interacts with the barrier layer of MAO coating,causing the local structure of barrier layer damage;the fourth stage,when the barrier layer is damaged,the corrosive medium will react with Mg substrate,and the H₂ release process increases the number of cracks in the MAO coating,and finally leads to the MAO coating structure damage.