Study On Microstructure And Properties Of Biodegradable Mg-2Zn-xCa-ySr Alloy

Magnesium alloys have excellent biocompatibility and biodegradability,making them a preferred candidate material for bone fixation,stents,and dental implants.The density and Young’s modulus of magnesium alloy are compatible with human bone.More importantly,magnesium alloy will completely degrade after completing its mission in the human body,avoiding the economic burden and physical and mental pain caused by secondary surgery for patients,and has great clinical application value.However,the rapid loss of mechanical integrity of magnesium alloy implants and the high degradation rate of magnesium alloys under human physiological conditions remain the main challenges for their use as orthopedic implant materials.Therefore,the research and development of new biomedical magnesium alloys has become an urgent problem to be solved in expanding the clinical application of magnesium alloys.By designing alloying and plastic processing processes to refine grain size while generating precipitation phase strengthening effects,the comprehensive mechanical and corrosion resistance of magnesium alloys can be greatly improved,effectively solving the bottleneck that restricts their clinical application.This study prepared Mg-Zn-Ca-Sr quaternary deformed magnesium alloys through vacuum melting and hot extrusion processes;The effects of Ca,Sr and heat treatment processes on the microstructure evolution,mechanical properties,corrosion behavior,and biocompatibility of Mg-2Zn alloy were studied and compared.The main research conclusions are summarized as follows:(1)After the addition of Ca and Sr,the grains of the cast Mg-Zn-Ca-Sr alloys are refined.With the increase of alloying element content,the second phase tends to form a network along the grain boundaries.After hot extrusion,the grains are further refined,and the second phase is fragmented and arranged along the extrusion direction.The phase composition of Mg-Zn-CaSr alloy is α-Mg,Ca2Mg6Zn3 and Mg17Sr2.When Ca is added to 1.0 wt.%,the corresponding diffraction peaks of Mg2Ca can be observed in the XRD spectrum.Texture analysis shows that after hot extrusion,a<10-10>+<11-20>fiber texture is formed inside the alloy,and its strength decreases with the increase of Ca and Sr addition.(2)Mg-2Zn-0.5Ca-0.5Sr alloy has excellent combination of strength and plasticity,UTS=340 MPa,δ=19%,but the tensile-compression yield asymmetry of Mg-Zn-Ca-Sr alloy is more significant.The analysis shows that the existence of various strengthening mechanisms such as fine grain strengthening,solid solution strengthening,second phase strengthening,dislocation and texture is the reason for the high tensile and compressive strength of the alloy.The corrosion resistance of Mg-Zn-Ca-Sr alloy is also improved after adding appropriate amount of Ca and Sr.It is found that there is corrosion anisotropy in the longitudinal section and cross section of the alloy bar.The fitted self-corrosion current density of the longitudinal section is about one order of magnitude lower than that of the cross section.The longitudinal section of Mg-2Zn-0.5Ca-0.5Sr alloy has the best corrosion resistance.The self-corrosion current density and annual average corrosion rate are Icorr=7.50E-5 A/cm2 and K=3.07 mm/a,respectively.(3)The optimization of the heat treatment process of the selected alloys shows that the Mg-2Zn-0.5Ca and Mg-2Zn-0.5Ca-0.5Sr alloys after 350℃×4 h solution treatment have a better combination of strength and plasticity,and the corrosion resistance is improved.The optimal heat treatment process of Mg-2Zn-0.5Sr alloy is determined to be solid solution+aging process.The UTS and δ of the alloy after solid solution and aging are 286 MPa and 22%,respectively.The comparative analysis of the effects of Ca and Sr elements on the comprehensive properties of Mg-Zn alloys shows that the grain refinement effect of Ca is much greater than that of Sr.The addition of excessive Ca/Sr leads to a significant decrease in the mechanical and corrosion resistance of the ternary alloy,but the tension-compression asymmetry is significantly reduced.The quaternary alloy with higher Sr content has better microstructure uniformity and better cell compatibility,but the existence of large-size Ca2Mg6Zn3 and Mg17Sr2 eutectic structures damages the corrosion resistance of the alloy.