In Vitro Degradation of High Pressure Torsion ZXJ310 Magnesium Alloy After Annealing Treatment
In recent years,the degradable metals represented by magnesium and magnesium alloys have gradually become a research hotspot for fracture internal fixation and bone defect repair materials due to their good biocompatibility,suitable elastic modulus and degradable characteristics,and are a type of medical metal materials with great application potential.ZXJ310 magnesium alloy has the characteristics of full nutrient composition,but it is found that the alloy still has the problems of non-uniform corrosion,high corrosion rate and serious gas production.The use of severe plastic deformation(SPD)process can refine the grains,thereby increasing the strength of the alloy and improving the corrosion resistance,and the application of the large plastic deformation process to magnesium and magnesium alloy processing has improved its potential for biomedical applications.Commonly used methods include equal channel angular pressing(ECAP)and high pressure torsion(HPT).HPT is more suitable for the metal materials with close-packed hexagonal structure that are difficult to deform,and HPT can significantly reduce the grain size of magnesium alloys,and some scientists have shown that the distribution and number of the second phase will be improved during the high-pressure torsional process,which can help improve the corrosion resistance of the alloy.In the early stage,a bulk ZXJ310 alloy with nanocrystalline characteristics was suc-cessfully prepared by exploring HPT parameters,with a good second phase dispersion distribution state.HPT was carried out at room temperature using the HPT-4 high-pressure torsion equipment manufactured by TRANSMST,Austria in this study,and the specimen size was Φ10 mm×1.0 mm.According to the preliminary process study,the torsion cycle was set to 15 cycles,the test pressure was 7.85 GPa,and the rotational speed was set to 1 r·min-1.There were high residual stresses and crystal defects in the processing process of HPT alloy,such as high-density dislocations,and it was necessary to combine the low-temperature annealing process to improve the residual stress and crystal defects and other factors that were not conducive to corrosion of HPT alloy.The effects of annealing treat-ment on the microstructure and in vitro degradation performance of ZXJ310 magnesium alloy after high pressure torsion were studied using scanning electron microscopy(SEM),transmission electron microscopy(TEM),energy dispersive spectroscopy(EDS),weight loss measurement and electrochemical test.The results showed that when HPT state ZXJ310 magnesium alloy annealed at 150 ℃,with the extension of the annealing time,the microstructure was observed and the grain size of the alloy gradually increased,and the average grain size increased from 98.3 nm in HPT state to 169.8,217.6,220.2 and 259.5 nm after 1,2,3 and 4 h annealing,respec-tively.At the same time,TEM images showed that the dislocation density in HPT state ZXJ310 magnesium alloy was significantly re-duced.The electrochemical test showed that the corrosion potential of the high-pressure torsion alloy increased significantly with the annealing time from 1 to 4 h,and the capacitive arc radius in Nyquist spectrum of the alloy gradually increased,and the electrochemi-cal corrosion rate of the alloy could be reduced to 0.32 mm·y-1 after annealing for 4 h.The results of immersion weight loss of alloy in simulated body fluids also showed the same trend,and the corrosion resistance of high-pressure torsion alloy after 3～4 h annealing was significantly improved.The annealing treatment significantly improved the immersion corrosion behavior of the alloy in simulated body fluids(SBF)and increased the compactness and stability of the corrosion product film,but did not affect the type of alloy corrosion products.There was no difference in the types of corrosion products produced by alloys under the same immersion conditions under dif-ferent states.When the immersion time was extended to 120 h,the proportion of Ca and P atoms in the corrosion products on the sur-face of the alloy continued to increase,and the ratio of Ca/P(atomic ratio)increased to 1.68,indicating that Ca-P compounds contain-ing hydroxyapatite(HA)had been formed in the alloy,which was the main component of inorganic components of human bone.After 24 h of immersion of HPT state ZXJ310 magnesium alloy after annealing at 150 ℃ for 4 h,the corrosion product film formed on the surface of the alloy was relatively smooth.When the corrosion time was extended to 72 h,the compactness of the corrosion film layer was further improved,and the dense protective layer formed by the deposition of the corrosion products could effectively protect the al-loy matrix.When the corrosion time was extended to 120 h,a large number of corrosion products accumulated on the surface of the al-loy.By observing the morphology of the corrosion products after removing them,the roughness of the alloy was significantly reduced compared with HPT state alloy,which further indicated that the corrosion resistance of the alloy could be significantly improved by eliminating the microscopic defects generated during the deformation process by annealing.In summary,annealing at 150 ℃ for 3～4 h could obtain uniform fine structure and lower corrosion weight loss rate.
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