| Compared with traditional bone implant materials,the elastic modulus and density of magnesium and magnesium alloy are much more similar to human bone.When using as bone implant materials,the stress shielding effect would not be produced in human body.In addition,magnesium and magnesium alloys also have the advantages of biodegradability in vivo,good biocompatibility and improved bone growth,which give them great potential to be used as bone implants,such as bone nails and bone plates.Compared with magnesium alloys,pure magnesium,especially high-purity magnesium（wt>99.98%）has very few impurities and no second phase,which could significantly reduce the probability of galvanic corrosion in magnesium matrix.However,high purity magnesium has low strength and poor plasticity,which can not meet the service requirements for bone implant materials in human body.Therefore,it is of great significance to produce high-purity magnesium with improved strength,plasticity and good corrosion resistance through reasonable processing techniques.In this paper,high purity magnesium（99.99 wt.%）were used as raw material,which were processed by caliber rolling（CR）and double extrusion（DE）technology,respectively.By these means,fine-grain structured high-purity magnesium could be obtained as bone implant material.The main results and conclusions are as follows:Both CR and DE can be used for the grain refinement of the high purity Mg,however,with different extent and mechanism of grain refinement.Through the CR process,the average grain size of high-purity magnesium was decreased from 30.1μm to 4.3μm and the grain size distribution was uniform.The grain refinement of high-purity magnesium to 9.2μm processed by DE process was significantly lower than that of CR counterpart.It is supposed that the different dynamic recrystallization modes of high-purity magnesium in the two processing methods lead to the different extent of grain refinement.The occurrence of discontinuous dynamic recrystallization in CR and continuous dynamic recrystallization is in DE samples was confirmed.Both kinds of high-purity magnesium samples mainly consisted（0001）<0 0>basal texture.The texture distribution of the CR samples were relatively random,and the texture types were（0001）<0 0>and（0001）<0>.The lowest texture strength was7.9;the texture distribution of the DE samples were relatively concentrated,the texture component only contained（0001）<0 0>,and the minimum texture strength was 9.4.The main reason for the texture randomization of CR samples is the continuous alternation of main axes during processing,resulting in the deflection of grain c-axis from ND to TD.The tensile and corrosion properties of both CR and DE samples were studied.It was showed that the elongation and tensile strength of the two samples have been improved.The tensile strength of DE samples was 190.6±3.5 MPa,and the maximum elongation of the samples was 21.1±0.5%.The tensile strength of CR samples was reached to 213.8±7.1 MPa and the elongation was 23.5±2.1%.The main contribution to the strength improvement of the two samples is fine grain strengthening.However,the work hardening of DE sample was more obvious than that of CR samples due to its high texture strength.The corrosion resistance of all the CR samples were improved after different processing parameters due to the grain refinement,uniformly grain distribution and reduced density of crystalline defects,the corrosion resistance of samples caliber rolled at 275°C was the best and the corrosion rate decreased from 1.71 mg cm-2 h-1 of the initial sample to 0.29mg cm-2 h-1.The corrosion resistance of the DE samples was higher than that of the CR samples because of the reduced amount of non-equilibrium grain boundaries and strong basal texture.The corrosion rate of the sample is 0.24 mg cm-2 h-1 when extruded at 260°C. |
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