| Magnesium-based biomaterials have a good biocompatibility and biodegradation in human physiological environment. Many progresses have been made on the development of biodegradable magnesium in cardiovascular and orthopedic applications. However, the extensive applications of magnesium and its alloys are still inhibited mainly by their high degradation rates and consequent loss in mechanical integrity that becomes the biggest obstacle to limit its application in the biomedical area. Improving the mechanical properties as well as the corrosion resistance of magnesium-based biomaterials has become one of the hottest research topics.Alloying and deformation processing is the most effective means to improve the mechanical properties and corrosion resistance of the magnesium-based biomaterials. Corrosion properties of pure magnesium in Hanks’solution were investigated from the content of Fe impurities and the state of deformation. Mg-Zn system alloys and Mg-Mn-RE alloys were prepared by biocompatible Zn and Mn and trace rare earth elements (RE). The conventional extrusion and equal channel angular pressing (ECAP) on the microstructure, mechanical and corrosion properties of pure magnesium were studied. Optical microscope, scanning electronic microscope and X-ray diffraction were used to analyze the magnesium and magnesium alloys microstructure. Electronic universal tensile testing machine and micro-hardness test were used to test the magnesium and magnesium alloys room temperature mechanical properties. The corrosion properties of magnesium and magnesium alloys were studied by potentiodymatic polarization curve and immersion in simulated body fluid. Details are as follows:1. The corrosion investigation of pure magnesium in Hank’s solution shows that the lower the Fe content and the smaller the grain size of pure magnesium, the better the corrosion resistance. The corrosion rate of pure magnesium can be controlled by decreasing Fe content and grain refinement. Alloying can refine grain. Mg-2Zn alloy has more fine and uniform grain than pure magnesium, and improved the corrosion resistance of Mg-2Zn-0.1RE alloy. Adding 0.1wt.% RE in Mg-2Zn alloy, the grain of cast Mg-2Zn-0.1RE alloy was further refined.2. The extrusion temperature can significant influence on corrosion resistance of Mg-2Zn-0.1RE alloy. The cast Mg-2Zn-0.1RE alloy was extruded at 230℃,280℃and 330℃, respectively. The results showed that the grain of as cast alloy is fined by extruding and dynamic recrystallization. The average grain sizes of alloy reached a minimum of sizes about 7±m and the best mechanical properties was attained after conventional extrusion at 330℃. The yield strength, tensile strength, and elongation are 160 MPa,272 MPa, and 12.8%; the corrosion resistance is 0.235 mLcm-2day-1.3. The extrusion ratio has significant influence on mechanical properties and corrosion resistance of Mg-1Mn-O.1RE. When the extrusion ration increased, the microstructure became finer and mechanical properties are improved at 360℃. With extrusion ratio of 32, preferable mechanical properties can be obtained by combining effects of strain hardening and dynamic recrystallization. The grain size could be as small as 3 μm, and correspondingly the yield strength was 185 MPa, tensile strength was 311 MPa, bending strength was 533 MPa, and elongation was 10.5%.The corrosion rate of hydrogen evolution of magnesium alloy is 0.156 mLcm-2day-1.4. In conventional hot extruding process, the grain size of the magnesium was refined gradually with the increase of the extrusion ratio and the decrease of extrusion temperature. Both mechanical and corrosion resistance of magnesium were enhanced. The magnesium was extruded at 230℃,280℃ and 330℃ with an extrusion ratio of 25. The mechanical and corrosion resitance of magnesium deceased gradually with increasing extrusion temperature. At 360℃, the magnesium was extruded with an extrusion ratio of 13 and 32. The higher the extrusion ratio is, the better the mechanical and corrosion properties is. Under the conditions that the extrusion ratio is 25, extrusion temperature is 230 ℃, magnesium has best comprehensive mechanical performance and corrosion resistance after conventional extrusion. The tensile strength and elongation of magnesium are 177 MPa, and 24% respectively. The magnesium processed by extrusion and equal channel angular pressing (ECAP) was investigated. It was found that the magnesium processed by extrusion at 230℃ and ECAP for 1 pass at 280 ℃ obtained grain refinement and exhibited excellent mechanical properties.5. When the extrusion temperature reduced and the extrusion pass increased, the microstructure of became finer and the mechanical properties improved gradually, and the corrosion resistance increased at first and then decreased during ECAP of magnesium. The ECAP of the cast magnesium was performed following route A. After the first pass at 360℃, from the second pass to the fourth pass the temperatures of ECAP are 300℃,250 ℃, and 200℃, respectively. The results show that the mechanical properties of magnesium decrease gradually with increasing extrusion temperature. The sample processed at highest temperature (i.e.,360℃) exhibited the lowest corrosion rate. In the three, the corrosion resistance of 250℃ is the worst, compared with 300℃ and 200℃. The sample at 200℃ exhibited a degradation rate level of approximately 0.222 mLcm-2day-1. After the cast pure magnesium by ECAP following the routes A and C at 200℃, the mechanical properties of magnesium was improved gradually with pass increase. The yield strength, ultimate tensile strength and elongation of pure Mg following route A increase up to 58 MPa,162 MPa and 15%.The result indicated that the comprehensive mechanical properties is the best under the technological condition. The yield stress reached a highest value of 75 MPa following route C. The average hydrogen evolution rate of equal channel angular pressed Mg following route A is much less than that of route C.6. Equal channel angular pressing was performed on the extruded Mg-2Zn magnesium alloy at the temperature from 175℃ to 330℃ for different passes. The corrosion resistance of alloy was decreased with extrusion pass increasing. When ECAP deformation temperature is higher than that conventional extrusion state, the ECAP-processed alloy exhibited lower tensile strength, yield strength, and increased elongation after one ECAP pass using route A. The mechanical properties of the ECAP-processed alloy were improved gradually after two passes. The corrosion rate of the extruded sample was at approximately 0.035 mL cm-2 d-1 However, additional ECAP processing does not decelerate corrosion further. Increasing ECAP passes induces lower corrosion resistance. |
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