Study On The Microstructure And Properties Of Mg-Nd-Ca Biomedical Magnesium Alloy

Since its discovery in 1755,magnesium has been closely related to our human life.Of all the elements stored on earth,magnesium ranks eighth.Magnesium is found mainly in minerals and seawater,and if we can make full use of the magnesium found on earth,it will surely greatly advance the development of social science.Due to its poor performance,pure magnesium cannot be used in large areas at present.However,magnesium alloys formed by adding other alloying elements into pure magnesium have the characteristics of good casting property,high specific strength and light weight.Magnesium alloys have been widely used in metallurgy,chemical industry and aerospace.In recent years,the application of magnesium alloys in bio medicine has developed rapidly,which is mainly attributed to the low toxicity and biodegradable characteristics of magnesium alloys.On this basis,a magnesium alloy material with good mechanical and corrosion properties is designed,which is the original intention of this research.It is one of the important directions of magnesium alloy research and development that the implant material can decompose and be absorbed in human body after the specified service life.Mainly used in the use of bone nail bone plate and other bone fixation materials,in the effect of reducing the patient's physical and economic burden,its advantage is that the need for a second operation process.In addition,the elastic modulus and density similar to that of human bone,and the magnesium ions required by human body make magnesium alloy often used in plastic surgery in modern times.However,the rapid degradation of magnesium alloy in human body and the defects of corrosion,such as bulges,are hindering the practical application of magnesium alloy in bio medics.In this project,the elements Nd and Ca with good biocompatibility were selected:basic research was carried out on Mg-2.5Nd matrix alloy,adding 0.5wt%and 1.0wt%Ca elements to optimize the alloying scheme,and the alloy structure was optimized by heat treatment and hot extrusion method.The mechanical properties were taken as the first standard to select magnesium alloy satisfying the conditions of bone fixation materials.At last,the corrosion rate was characterized by soaking hydrogen and electrochemical method.During the actual solidification process,the three as-cast alloys were all cast in non-equilibrium solidification mode.Therefore,there are a large number of Mg₁₂Nd and Mg₂Ca phases in the alloy as divorced eutectic,a large number of dendritic crystals,and the composition segregation is serious.By studying the mechanical tensile and compression properties of the as-cast alloy,it is found that with the increase of Ca content,the grain size of the as-cast alloy is gradually refined from 55?m to about 35?m.As the yield strength of the alloy increases,the tensile strength increases and then decreases,and the elongation decreases.The Mg-2.5Nd-0.5Ca alloy has the most comprehensive strength performance,and its tensile strength,yield strength and elongation are 147MPa,67MPa and 5.7%,respectively.In addition,the fracture mode of the alloy is mainly cleavage fracture.After solid solution treatment,the grain size of Mg-Nd-Ca alloy grew to more than 20?m.Besides supersaturated alloy structure,there were long-spiculate Mg₁₂Nd and granular Mg₂Ca.The mechanical properties of the alloy are slightly improved compared with that of the as-cast alloy.The mechanical properties of aging alloys are greatly improved due to the dispersion strengthening phase.The yield strength of the Mg-2.5 Nd-0.5 Ca alloy in the solid solution state and the aging state is 73MPa and 129Mpa,respectively.The brine ll hardness values were:52.6HBW and 61.3HBW,respectively.After the hot extrusion treatment,the grain size of the as-cast alloy was significantly refined to about 2?m,and the structure was more uniform.The properties of extruded alloy cross section and longitudinal section are completely different.In the same extrusion direction,the compression performance is different from the tensile performance,showing obvious tension asymmetry.With the increase of Ca content,the tensile mechanical properties of the alloy decrease continuously,which weakens the texture in this direction.The yield strength of Mg-2.5Nd and Mg-2.5Nd-0.5Ca alloy is 230Mpa and 203Mpa respectively.Although both alloys meet the mechanical properties requirements of bone fixation materials for medical magnesium alloy implants.However,Mg-2.5Nd-0.5Ca alloy has relatively more uniform comprehensive mechanical properties.The Mg-2.5Nd-0.5Ca alloy with uniform mechanical properties was selected,and the corrosion rates of the transverse section and the longitudinal section of the extruded alloy were studied respectively,and the comparison of the as-cast Mg-2.5Nd-0.5Ca alloy was taken as a reference.The results show that the Mg-2.5Nd-0.5Ca alloy shows the best corrosion resistance in the extruded longitudinal section,and the average corrosion rate is 0.43mm/yr.It can meet the requirements of corrosion performance of bone fixation medical implant materials.