Study And Development Of Biodegradable Mg-Sc-based Alloys With Shape Memory Effect

At present,all known biodegradable magnesium alloys do not have shape memory effect and cannot meet the requirement of intelligent medical materials.Therefore,Mg-19Sc?at.%?and Mg-?18.75-x?Sc-xGd?x=0,0.25,0.5,1 at.%?alloys with shape memory effect were prepared and researched for degradable biomaterials in this paper.Firstly,the unique mini-volume induction furnace melting technology was explored in this paper.Then Mg-Sc alloy rolling plates with different content of composed phase were obtained through the combination process of hot extrusion,hot rolling and phase-transition heat-treatment.Finally,the microstructure,martensitic transformation temperature,corrosion property and cell cytotoxicity of Mg-Sc alloy were systematically studied in order to evaluate its feasibility as biodegradable medical material.It was found that the induction melting ingots with less impurities and uniform components could be obtained using magnesium oxide crucible and homogenization treatment at 670? for 8 hours.Mg-19Sc?at.%?alloy with microstructure of single?phase could be obtained by phase transition heat treatment at 690? for 1 hour.Microstructure of single?phase is a necessary prerequisite for the shape memory effect of Mg-Sc base alloy.The martensitic transformation start temperatures of Mg-18.75Sc and Mg-19Sc?at.%?alloys were-128? and-183.4? respectively,and the martensitic transformation start temperature of Mg-19.5Sc?at.%?alloy was below-245?,which means that the reduction of Sc content in the?-type Mg-Sc alloy can significantly raise the martensitic transformation start temperature.However,it was still unable to meet the clinical needs.The result of bending test showed that the superelastic strain of Mg-19.5Sc?at.%?alloy at liquid nitrogen temperature was about 4%.Due to the displacement reaction of Sc and MgO,dense protective films which consist of Sc₂O₃?P-B ratio=1.3>1?formed on the surface of Mg-19Sc?at.%?alloy,so its corrosion resistance was very outstanding.The corrosion rate of?-type Mg-19Sc?at.%?rolling plate was only0.0788±0.0027 mm/year after soaking in Hank's solution for 10 days,which was 49.25%of high purity magnesium?0.16mm/year?.Due to the micro cell corrosion between?and?phase,protective film was damaged,then corrosion rate of some experimental groups began to speed up.The cell activity of Mg-19Sc?at.%?alloy was related to its corrosion rate.The lower the corrosion rate was,the smaller the increment of pH and osmotic pressure was,and the smaller the damage to cells was.The cell activity of the?-type Mg-19Sc?at.%?alloy in 100%extraction was greater than 75%,indicating its cytotoxicity of grade 0 to 1.In order to increase the martensite transformation temperature of Mg-Sc alloy,Gd was selected as the third component to replace Sc.The martensite transformation start temperature of Mg-18.5Sc-0.25Gd?at.%?was-183.6?,which was much lower than Mg-18.75Sc?at.%?alloy?-128??.It showed that replacing Sc with Gd could stabilize the?phase and reduce the martensitic transformation start temperature of Mg-Sc alloy.The result of thermodynamic calculation also showed that the displacement reaction of Gd and MgO wouldn't happen on the surface of Mg-Sc-Gd alloy,so the addition of Gd would destroys dense protective film,which increased the corrosion rate and cytotoxicity of the alloy to some extent.The corrosion rate of the Mg-17.75Sc-1Gd?at.%?with the highest Gd content was 0.6898±0.1006mm/year after soaking in Hank's solution for 10 days,and its cell activity after soaking in 100%extract for5 days was only 73.16±1.80%,indicating the cytotoxicity of grade 2.