The Study Of Microstructure,Degradation Behavior And Antibacterial Properties Of Medical Mg-Nd-Ag Alloys

Magnesium alloys as a new kind of biodegradable medical materials have attracted great interest of the researchers both in the field of clinical medicine and material science.Many new alloys with different elements and different functions have been developed for biodegradable implants.The objective of this study was to develop a new biodegradable medical magnesium alloys which can deal with the issue of clinical infection and rapid degradation rate,by using the function of Nd in the improve corrosion resistance and Ag in the antibacterial property.The Mg-2Nd-xAg alloys were prepared by the method of vacuum smelting and in combination with different heat treatment.In order to explore the effect of Ag content and heat-treatment on the microstructure,mechanical properties,corrosion resistance and antibacterial properties of Mg-2Nd-xAg(x=0,1,2wt%)alloys,samples have been systematically investigated by optical microscope,X-ray diffraction(XRD),field emission scanning electron microscopy(FSEM),transmission electron microscopy(TEM),Electro-Probe Microanalyzer(EPMA),electrochemical measurement and antibacterial test.Following results were obtained:Microstructure observation and analysis,show that Mg-2Nd alloy is composed of ?-Mg matrix and Mg₁₂Nd second phase which mainly distributed in grain boundary and dendrite.The second phase in Mg-2Nd-lAg and Mg-2Nd-2Ag alloys is changing from Mg₁₂Nd to(Mg,Ag)₁₂Nd.After solid solution treatment,the second phase of Mg-2Nd-xAg alloys is dissolved in ?-Mg matrix.However,the new phase of NdH2.61 is appeared in matrix and its size is only about 1 ?m.After aging treatment,the phase with the element of Mg,Nd and Ag,is precipitated dispersively on the ?-Mg matrix and this phase is much smaller and distributed more uniformly compared with the second phase of as-cast alloys.Corrosion test shows that the corrosion resistance of Mg-2Nd-xAg alloys in SBF solution are not only associated with the content of Ag elements but also affected by the heat treatment.The alloying element of Ag has a detrimental impact on corrosion resistance of alloys by changing the composition of second phase and reinforcing the effect of galvanic corrosion between second phase and matrix.The second phase in Mg-2Nd-xAg alloys can dissolve into matrix after solid solution and precipitate dispersively on matrix with nanometer size after aging treatment.Both heat treatment can reduce the galvanic corrosion between the second phase with matrix and improve the corrosion resistance of alloys.The result of antibacterial tests shows,the antibacterial properties of Mg-2Nd-xAg(x=0,1,2 wt%)alloys are associated with silver content and heat treatment.The antibacterial rate of Mg-2Nd-1Ag and Mg-2Nd-2Ag alloy are 43.93%and 78.12%under solution treatment and increase to 97.70%and 97.70%in aging state.The antibacterial property of Mg-Nd-Ag alloys was carried out by releasing bactericidal metal ion Ag⁺ during corrosion process.Through the microstructure observation and inspection of leaching solution,the small particles which containing Ag element are the effective carrier of silver ion and has great influence on the antibacterial property.After solution treatment,the second phase of containing Ag elemnet dissolved into ?-Mg matrix,and the Ag⁺ion react with Cl^-ion to produce AgCl precipitate which make the antibacterial effect of Ag elements are suppressed.The silver-rich precipitates would be de-alloying and become the Ag particles or Nd-Ag particles during the alloys degradation.These Ag-rich particles enhanced the transport and release processes and improve antimicrobial property.