Fabrication And Study On Microstructure And Properties Of Degradable Mg-Zn-Zr-Nd Series Alloys

Due to good biocompatibility,degradability,low density and elastic modulus close to human bone,Mg alloys have great potential as degradable implant materials.However,the rapid degradation rate and local corrosion in body fluids can lead to the accumulation of subcutaneous bubbles and premature loss of mechanical integrity.Therefore,Mg-Zn-Zr-Nd alloys with excellent mechanical and corrosion properties were fabricated through composition design,melting,casting and extrusion.The influence of microstructure evolution on mechanical and corrosion properties was discussed by using advanced characterization techniques and analytical test methods.In addition,the degradation behavior and deposition mechanism of corrosion products were investigated.The influence of Nd content on microstructure and properties of as-cast Mg-2Zn-0.6Zr-xNd(x=0,0.2,0.6 and 1.0 wt.%)alloy showed that the ternary Mg-Zn-Nd(T)phase formed with 0.2-1.0 wt.%Nd addition.With the increase of Nd content,the grain size first increased and then decreased,and the volume fraction of second phases increased continuously.In addition,the type of T phase changed from T2(Mg60Zn32Nd8)to T3(Mg35Zn40Nd25).The tensile yield strengths of as-cast alloys were about 55-80 MPa,which increased with the increase of Nd content,but there was no significant difference in tensile strength,which were all around 200 MPa.The compressive yield strengths were about 70-90 MPa,and the compressive strengths were about 410-440 MPa,both of which increased with the increase of Nd content.By optimizing the extrusion parameters,the Mg-2Zn-0.6Zr-0.6Nd alloy was prepared by three different extrusion processes,which met different mechanical properties,and its degradation rate was slow and its corrosion behavior was uniform.The degradation rate was slow and the corrosion behavior was uniform.When adopting forward extrusion,the extrusion temperature,extrusion speed and extrusion ratio were 350℃,3 mm/s and 16:1,the yield strength,tensile strength,elongation and corrosion rate were 190 MPa,254 MPa,16.7%and 0.21 mm/year,respectively.When reverse extrusion was adopted,the extrusion temperature and extrusion ratio were maintained at 350℃ and 16:1,and the extrusion speed was reduced to 0.5 mm/s,the yield strength,tensile strength and elongation were 269 MPa,298 MPa and 25.6%,respectively.It exhibited uniform corrosion characteristics in DMEM or Hank’s solution,and the corrosion rates were about 0.03 mm/year and 0.07 mm/year,respectively.When reverse extrusion was adopted,the extrusion speed and extrusion ratio were maintained at 0.5 mm/s and 16:1,and the extrusion temperature was reduced to 300℃,the yield strength,tensile strength,elongation,coefficient of friction and wear rate were 353 MPa,362 MPa,15.3%,0.15 and 3.9×10-3 mm3·N-1·m-1,respectively.Taking the Mg-Zn-Zr-Nd alloy in different states as the research objects,the effect of second phase content,second phase morphology and grain size on tensile fracture behavior was clarified.In addition,through pre-immersed tensile tests,the effect of corrosion induced surface integrity on tensile fracture behavior was studied.The elastic modulus and hardness of T phase were 57.5 GPa and 1.47 GPa,respectively.Its hard and brittle properties not only cause itself to be easily broken during deformation,but also induced microcracks at the interface with theα-Mg matrix.The fracture mode of as-cast,homogenized,solution treated and extruded+aged alloys in conventional tensile tests changed from intergranular fracture to quasi-cleavage fracture,transgranular fracture and ductile fracture.When the pre-immersed tensile tests were performed,the effect of reduced surface integrity on crack sensitivity is arranged in the following order:as-cast>homogenized>solution treated>extruded+aged.Corrosion studies with microscopic resolution showed that the relative volt potential of the T phase was about-400 mV.The pore-like defects on grain boundary indicated that the anodic dissolution process is highly regionally distributed,and the T phases were corroded first.Mg(OH)2 generated in the early stage could not effectively protect the substrate.While the dense and insoluble CaHPO4·2H2O precipitated outside or in the Mg(OH)2 layer,which effectively reduced the corrosion rate.In the inorganic salt solution,the corrosion products layer showed a dynamic reshaping process.When the solution contained multiple organic substances,the corrosion products were continuously deposited,and the protection was continuously strengthened.