| The key problem faced by medical magnesium alloys is still the difficulty in balancing corrosion resistance and mechanical strength.Although the corrosion rate of high-purity magnesium is relatively low,the mechanical strength is insufficient.Although the mechanical properties have been significantly improved after the addition of alloying elements,due to the existence of alloying elements in the form of intermetallic compounds or precipitated phases,although the mechanical strength will be improved,a large number of corrosion micro-cells will be formed,resulting in decreased corrosion resistance and pitting corrosion.The degradation of the alloy is uncontrollable and difficult to predict.This paper is an enterprise cooperation project,according to the requirements of enterprises to carry out the research and development of clinical bone implant materials.In this paper,on the basis of taking into account the mechanical properties of the material,the microstructure of the magnesium alloy is controlled by means of composition optimization and increasing the degree of deformation,and the corrosion resistance of the magnesium alloy is improved.Finally,a high corrosion resistance magnesium alloy with a composition of Mg-2Zn-0.1Ca magnesium alloy is obtained.The yield strength is 381 MPa,the tensile strength is 415 MPa,the elongation is 27 %,and the annual corrosion rate is below 0.5 mm/y.On this basis,the bone screw instrument was prepared and the related service performance test was carried out.The main conclusions of this paper are as follows :1.The study of Mg-4Zn-0.1Ca and Mg-2Zn-0.1Ca alloys shows that the increase of Zn content causes the increase of strengthening phases in the magnesium matrix,which leads to the deterioration of grain refinement and the distribution of high potential regions between grains.Although the mechanical strength is improved,the corrosion resistance decreases sharply.On the basis of Mg-Zn-Ca alloy,Mn element is added.Mn element precipitates during hot extrusion,resulting in an overall increase in the potential between the grains and inside the grains.Therefore,the addition of Mn element cannot effectively improve the corrosion resistance of the alloy in the small grain state after extrusion.2.Extrusion can effectively improve the compactness and uniformity of the as-cast material.Under two different deformation degrees,the plasticity and strength of the material can be effectively improved.The greater the deformation degree,the more obvious the grain refinement,and the refined alloy tends to uniform corrosion.The mechanical properties of Mg-2Zn-0.1Ca alloy are significantly improved by low-speed and low-temperature controlled extrusion,which is the highest value of the same alloy composition reported so far.3.The corrosion resistance test of magnesium alloy bone screw showed that the weight loss corrosion rate was relatively high in the early stage of immersion due to the large contact area.However,when the immersion entered a relatively stable stage,the corrosion rate of the screw decreased rapidly after the stable passivation film was formed on the surface,and the corrosion rate in the later stage was basically consistent with the corrosion rate of the previous sample.And the average corrosion rate of two months was 0.23 mm/y after 50 % weight loss time point detection.4.The larger the roughness,the larger the contact area between the alloy and the simulated body fluid,and the more difficult the passivation film on the surface is to form under the same conditions,resulting in the worse corrosion resistance.Under the surface modified micro-arc oxidation,the ceramic layer prepared in the constant current mode is thicker than that in the constant voltage mode at the same time,and the bonding is stronger and the corrosion rate is lower. |
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