Study On Mechanical Properties And Corrosion Behavior Of Biomedical Magnesium Alloy

Aimed at bio-degradable magnesium alloys for medicine application, biocompatable Mn and Zn elements were selected to develop Mg-Mn and Mg-Zn-Mn alloys. Influences rule of Mn and Zn on the as-cast microstructure, mechanical properties and corrosion behaviour of the magnesium alloys were investigated by the use of optical microscope (OM), scanning electronic microscope (SEM), transmission electronic microscope (TEM), XRD, mechanical properties testing, electrochemical measurement and in-vitro evaluations. Influence of extrusion and homogenization on the microstructure, mechanical properties and corrosion behaviour of Mg-Zn-Mn alloys were also investigated. Implantation test was conducted to evaluate the in vivo degradation and mechanical properties reduction of magnesium implant. The results are summarized as follows:Microstructure observation and mechanical properties test showed that Mn can refine magnesium alloy, but also improve the tensile strength, and that significant increase in strength was found at 1.48wt.% Mn. Zn element can further refine the microstructure of the as-cast Mg-Mn alloy by forming Mg7Zn3 phase. The tensile strength, the elongation and the yield strength of the as-cast Mg-Zn-Mn alloys increased with increase in Zn content, and the highest properties was obtained at 3.0 wt% Zn, e.g. the ultimate tensile strength, yield strength and elongation were 218.6 MPa, 66.1MPa and 15.5%, respectively.The grain size of Mg-Zn-Mn alloys was refined remarkably and the tensile strength and the yield strength of the alloy were increased greatly after extrusion. The tensile strength, the yield strength and the elongation of the extruded Mg-1Zn-1Mn were 280.3 MPa, 246.5 MPa and 21.8%, respectively. With the increasing of Zn content, the grain size decreased, e.g. the grain size was 3-5μm at 3wt.% Zn. Sub micron Mg7Zn3 and MgZn phases appeared at grain boundary further increased the ultimate tensile strength and yield strength, but reduced the elongation. Homogenization treatment before the extrusion coarsened the grain size and reduced the tensile strength and yield strength. The corrosion testing showed that Mn could not improve the corrosion resistance. On the contrary, the corrosion resistance was reduced when Mn content exceeded 1.48 wt%. The corrosion resistance of the as-cast Mg-Zn-Mn alloys increased remarkably with Zn addition. However, the corrosion resistance decreased when Zn content was over 3wt.%. Extrusion treatment can significantly reduce the corrosion rate of Mg-Zn-Mn alloy, move the corrosion potential toward more noble and increase the corrosion resistance. With the increasing of Zn content the corrosion mode transformed from pitting corrosion to severe grain boundary corrosion. Homogenization heat treatment before extrusion can reduce the grain boundary corrosion and in turn reduce the corrosion rate of the alloy with high Zn content, such as 3wt.% Zn. The study on the corrosion mechanism showed that the segregation of Zn due to the different casting conditions contributed mainly to the different corrosion modes of the as-cast alloy. Extrusion treatment homogenized the distribution of Zn, therefore, reduced the corrosion rate.In vivo experiment showed the inflamma-tion appeared 3 days postimplantation, and disappeared 2 weeks postimplantation. Blood biochemistry analysis showed that no disorder was tested in the liver function and kidney function within 3 months implantation. Microstructure observation showed that a Ca-and P-containing degradation product layer was found at the interface between magnesium alloy implant and bone tissue. With the degradation of magnesium implant, newborn tissue layer formed on the surface of the magnesium implant and then transformed into newborn bone tissue gradually. The alloy and the newborn bone had good conjunction. The bending strength of the magnesium implant decreased with implantation time. The bending strength after 1 month implantation was 257.7 MPa, reduced by 40% in comparison with the original value, and 113.6 MPa after 3 months implantation, reduced by 70 %. However, the bending strength of the magnesium implant after 3 months postimplantation was still as high as the strength of natural bone.