| Magnesium alloys not only have the good mechanical properties and biocompatibilities,but also its elastic modulus is similar to human bone,which can significantly reduce stress shielding effect and promote the formation of new bone.However,corrosion behavior of Mg alloys in the Cl-environment can easily l ead to premature failure of magnesium alloys structure and serious damage to local tissues.Therefore,how to improve the service performance of magnesium alloys in the physiological environment and explore the degradation mechanism is particularly important.In this paper,the composition optimization and performance regulation of Mg-2Zn-0.2Mn alloy were carried out via the human nutritional element Ca and Nd element with the good biocompatibility.The phase characteristics,film formation mechanism and the corrosion behavior of the alloys in the as-cast condition were studied by means of SEM,TEM,EPMA,AFM and XRD.Nextly,the effects of different contents of Ca and Nd on mechanical properties and corrosion behavior of Mg-2Zn-0.2Mn-xCa and Mg-2Zn-0.2Mn-xNd after homogenization were studied,and the cytotoxicity assessment was also evaluated.The microstructure of Mg-2Zn-0.2Mn-1.10Ca and Mg-2Zn-0.2Mn-1.88Nd alloys was controlled by heat treatment at different temperatures,and the coupling effect of grain sizes and the second phase fractions on the mechanical properties and degradation properties of the alloys were analyzed.Finally,the effects of the multipass hot rolling on the microstructure evolution,the film characteristics along the matrix direction,the Cl-promoted film failure mechanism,the 3D corrosion morphology as well as the abnormal anodic phase dissolution behavior of the Mg-2Zn-0.2Mn-xNd alloys were investigated.The main conclusions are as follows:After adding different Ca/Nd element contents in the Mg-2Zn-0.2Mn alloy,the dendritic structure was remarkably refined.The corrosion resistance and mechanical properties of Mg-2Zn-0.2Mn-xCa.and Mg-2Zn-0.2Mn-xNd alloys increased first and then decreased.Mg-2Zn-0.2Mn-0.38%Ca/0.62%Nd alloys exhibited excellent corrosion resistance,with corrosion rates of 6.97 mm/y and 1.15 mm/y,respectively.Magnesium alloys containing Nd is superior in corrosion resistance to Ca.It is due to the formation of insoluble Ca10(PO4)6(OH)2 and CaHPO4 in the surface of Nd-containing magnesium alloy in simulated body fluids.It can also form Nd2O3/Nd(OH)3 by hydrolysis,which can significantly inhibit the adsorption and penetration of corrosion factors.Excessive Ca/Nd addition leads to an increase in the fraction of the second phase,which forms a large number of corrosion galvanic couples and makes the surface film layer become loose and the defects intensified,providing a diffusion channel for the erosive ion penetrating film layer to reach the substrate and a convenient way.At the same time,excessive Ca/Nd accumulates at the grain boundary to produce the segregation,and it is easy to generate the microcracks and expansion channels under the action of external load,resulting in the reduction of mechanical properties of materials.After the homogenization treatment of 380?/24 h,Mg-2Zn-0.2Mn-0.38%Ca has the best corrosion resistance through the electrochemical test and the immersion test.Compared with the as-cast condition,the corrosion rate of Mg-2Zn-0.2Mn-0.38%Ca decreased from 6.97 to 6.27 mm/y.After the 420?/24 treatment,the Mg-2Zn-0.2Mn-0.62%Nd has the lowest corrosion rate of 0.83 mm/y,emerging a uniform degradation morphology.The improvement of corrosion resistance could be attributed to the combined effects of the suitable content of Nd/Ca dissolving into the a-Mg matrix and the modification of Ca/Nd-containing compounds by homogenization treatment.In vitro cytotoxicity tests show that the Mg-2Zn-0.2Mn-xCa/vNd alloys were safe to human bone marrow mesenchymal stem cells and were promising to be utilized as the implant materials in the futureAfter different heat treatments(as-cast,300?/24 h,350?/24 h,420?/24 h,460?/24 h,500?/24 h),the mechanical properties and corrosion resistance of Mg-2Zn-0.2Mn-1.10Ca and Mg-2Zn-0.2Mn-1.88Nd alloys increased firstly and then decreased.The mechanical improvement could be explained by the complete dissolution of precursors and facilitation of structural re-arrangement of' the increased Ca/Nd solute.The weakening of mechanical properties is caused by large grain size and supersaturated lattice distortion caused by high temperature treatment,which promotes stress concentration and promotes crack initiation/nucleation.The difference of Volta potentials between the phase and matrix is the main reason for micro-galvanic corrosion formation.The higher couple densities,the higher the corrosion rate.The best corrosion resistance of Mg-2Zn-0.2Mn-1.10Ca and Mg-2Zn-0.2Mn-1.88Nd alloys must be matched with the moderate grain size and the dispersive distribution of trace precipitates.The surface products of hot-rolled Mg-2Zn-0.2Mn after corrosion immersion were mainly composed of MgO,Mg(OH)2,CaHPO4 and the trace amount of Ca10(PO4)6(OH)2;The surface products of Mg-2Zn-0.2Mn-xNd after corrosion immersion were mainly composed of Mg(OH)2,HA,CaHPO4,Ca3(PO4)2 and the MgCO3?Nd2O3,endowing the larger Rct,|Z| and phase angles.These insoluble product layers can effectively retard the formation of metastable Mg+.The improvement of corrosion resistance can be attributed to three reasons:Firstly,after hot rolling,the micro-segregation is reduced,the grains are refined and the second phase is broken.A closed cathode/anode micro-galvanic is formed between the grain interior and grain boundary,which makes the corrosion mo,re uniform and the corrosion rate lower;Secondly,the fine-grained magnesium alloy has a high proportion of grain boundary density on the activated surface,which can promote the reaction kinetics of film formation,stabilize the film layer and the matrix,and reduce the metastable ion exchange and hydrogen evolution rate;In addition,the hydrolysis/ionization of Nd provides nucleation sites for the deposition of insoluble products,and offers the necessary HPO42-,CO32-and OH-ions for the precipitation of hydroxide and phosphate. |
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