| Titanium and its alloys have become very promising biomedical materials for their high specific strength and biocompatibility.However,their relatively high elastic modulus(110 GPa for Ti-6Al-4V)which would cause “stress shielding” effect and their lack of bioactivity are the two outstanding problems that hold back their application as load-bearing biomedical materials.And magnesium and its alloys are also very promising biomedical materials for their biodegradability,bioactivity and biocompatibility.However,the poor corrosion resistance of magnesium alloys has become a bottleneck limiting their development.In order to select and combine their essences,a bicontinuous titanium-magnesium composite has been developed,using low modulus porous titanium preform as scaffold,and then infiltrate magnesium into the continuous pores of the porous titanium,using ultrasonic wave assisted infiltration method.In order to avoid the toxic elements in the porous titanium,high purity spherical titanium powder is selected as starting material of preparation of porous titanium using sintering method.Powder size of titanium,sintering pressure,sintering temperature,sintering time and sintering path are the five factors that determine the morphology and properties of porous titanium.Sintering experiments were carried out accordingly to reveal their influence.And a final sintering process is obtained,first the titanium powder was sintered at 900℃ for 50 min into pre sintered compact,and then was cleaned to get rid of the impurities,finally the pre sintered compact was sintered at 1200℃ for 120 min to get the porous titanium with continuous pores high surface quality and high strength.In the process of ultrasonic assisted infiltration,the wettability and mass diffusion between titanium and magnesium was improved by the "cavitation" effect of ultrasonic wave,and the bicontinuous titanium-magnesium composites were obtained with dense structure was obtained.The SEM and XRD results indicate that there is no other phase or compound besides α-Ti and Mg.A ~1μm think diffusion layer is observed between titanium and magnesium which could improve the interface bonding strength.The mechanical properties were characterized by compression and three-points bending tests.The results indicate that the composite inherits the mechanical properties of the porous titanium only strengthened and stabilized by magnesium.The elastic modulus is relatively low,which are 23.39 GPa,33.55 GPa and 37.57 GPa for the composite with the titanium particle size of 230μm,130μm and 100μm respectively.The compression yield strength and bending strength are 230.20 MPa,262.89 MPa,300.09 MPa,and 631.51 MPa,468.80 MPa,375.23 MPa respectively.The mechanical properties are very suitable for load bearing biomedical materials for its low elastic modulus and high specific strength.The magnesium part of the composite,which could induce the growth of bone tissue,is selectively corroded in Ringer’s solution,leaving the original pores of the porous titanium,which could provide the space of tissue ingrowth and substance transportation.Because of the low purity of the commercial pure magnesium and the its directly contact with titanium,the Ti-Mg composite’s corrosion resistance is poor comparing with other biomedical materials.Therefore,we can choose other magnesium alloys with better corrosion resistance instead of low purity commercial pure magnesium and place a diffusion barrier layer between titanium and magnesium to avoid the galvanic corrosion.We tried the direct oxidization method to place a titanium oxide layer between the two metals.However,it turned out that the quality of the oxide layer is compromised,and the diffusion of between titanium and magnesium is not completely eliminated,so the corrosion resistance is not improved,and the ductility of Ti-Mg composite is lower,but the high strength remains.Therefore,this method is feasible if we could find the appropriate coating method coating material. |
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