| Ti and its alloys are important biomedical materials,which have developed to the third generation.However,there are still some weaknesses:the strength of the first generation pure Ti metal material is not high enough and the biocompatibility is poor;V and Al in the second generation Ti-6Al-4V alloy are easy to lead to neurological disorders and other diseases;The elastic modulus of the third generation Ti-5Al-2.5Fe,Ti-6Al-7Nb and other alloy materials is 4-10 times that of human bone,which is easy to cause stress shielding.Therefore,exploiting new Ti alloys without Al and V,which have good biocompatibility,high strength,low elastic modulus,strong antibacterial and corrosion resistance,is one of the key development directions of biomedical metal materials.In order to reduce the elastic modulus of Ti alloy,binary systems such as Ti-Ta,Ti-Ni and Ti-Zr and multicomponent Ti alloy based on binary system have been gradually developed in recent years.Among them,Ti-Ta alloy not only has excellent elastic modulus,but also has good corrosion resistance.In order to improve the biocompatibility of Ti alloy,adding Ag into Ti alloy is beneficial to improve its corrosion resistance and antibacterial properties,and can control the incorporation amount,precipitation amount and precipitation mode by controlling the content of Ag,while maintaining its excellent antibacterial properties,avoid its cytotoxicity to the maximum extent.Ti-Ta-Ag alloy may become one of the new biomedical Ti alloys with good comprehensive properties.In the preparation method of alloy powder metallurgy,spark plasma sintering(SPS)can effectively control the grain size of alloy and shorten the preparation process.Therefore,Ti-25Ta-x Ag(x=0,1.5,3,4.5 wt%)ternary alloy is taken as the research object in this paper.A series of Ti-25Ta-x Ag ternary alloy materials were prepared by SPS.Combined with the density functional theory calculation,the effect and mechanism of preparation conditions on the crystal structure,microstructure and composition of Ti-25Ta-x Ag ternary alloy were systematically analyzed;The changes and mechanism of mechanical,corrosion resistance and biocompatibility of Ti-25Ta-x Ag ternary alloy were deeply analyzed.Through the above research work,the main results are as follows:(1)Controllable preparation of Ti-25Ta-x Ag alloy material:the mixing method of ball milling Ti-25Ta powder and then adding Ag powder can avoid adhesion and refine the particle size;The density of Ti-25Ta-4.5Ag alloy increases with the increase of SPS sintering temperature(the alloy density reaches 99.8%at 950℃);There is Ag precipitation or residue in Ti-25Ta-4.5Ag alloy during SPS sintering(when the sintering temperature is 950℃,the precipitation or residue of Ag is the most,and the precipitation or residue position is at the grain boundary).(2)Microstructure of Ti-Ta-Ag alloy:Ti-Ta-Ag alloy prepared by SPS consists ofα,martensiteα”α”,β,Ta and Ag.The solid solution ofαphase can be formed between Ti-Ta,and Ag can precipitate or remain at grain boundary in addition to solid solution inαphase.The increase of sintering temperature leads to the decrease ofβphase and the increase of precipitatedαphase.The lamellar region of Ti-Ta-Ag ternary alloy mainly contains martensiteα”α”phase andαphase.The lattice constants of Ti-25Ta-x Ag solid solution show discontinuous variation.When the solid solubility x is less than 0.8,the lattice constant changes according to second order Vegard’s law.(3)Mechanical properties of Ti-Ta-Ag alloy materials:The addition of Ta effectively reduces the elastic modulus of Ti-Ta-Ag alloy,and the change of elastic modulus is more obvious with the increase of Ta content.Due to Ag solution strengthening and grain boundary strengthening,the addition of Ag can significantly enhance the deformation resistance and stiffness of the alloy under shear stress,but also significantly weaken the plasticity and brittleness of the alloy.This is caused by the difference of Ag precipitation or residual amount.Ag precipitation or residual amount is the most at 850℃sintering,which has the most obvious strengthening effect on alloy hardness.The results of first-principles calculation show that the mutation of lattice constants and mechanical properties of Ti-25Ta-x Ag solid solution at the x value of Ag solution is 0.8 is due to the change of 4d10 electronic states of Ag from obviously local electronic states to local and non-local mixed electronic states.(4)Corrosion resistance of Ti-Ta-Ag alloy materials:Compared with Ti-Ta-alloy prepared under the same conditions,the addition of Ag is beneficial to improve the corrosion resistance of Ti-Ta-Ag alloy.However,the precipitation or residual of Ag is not conducive to the improvement of the corrosion resistance of Ti-25Ta binary alloy,which may be because Ag alloying is beneficial to improve the protection ability and corrosion resistance of the passivation layer on the surface of Ti-Ta-Ag alloy,while the precipitation or residual of Ag is not conducive to this process.First-principles calculation results show that Cl~-ions in Hank’s solution bond strongly with surface atoms of Ti-Ta-Ag ternary alloy,which is beneficial to the formation and stability of dense passivation layer.However,on the surface of Ti-Ta-Ag ternary alloy containing Ag trace precipitation,Cl~-ion directly binds closely with the precipitated Ag atom to form an isolated adsorption molecule similar to Ag Cl,which is not conducive to the formation and stability of the passivation layer.Therefore,in order to improve the corrosion resistance of Ti-25Ta-x Ag ternary alloy,it is necessary to avoid trace Ag precipitation or residue as much as possible during the preparation process.(5)Biological compatibility of Ti-Ta-Ag alloy materials:After the addition of Ag element,the antibacterial performance of Ti-Ta alloy is significantly improved,and the antibacterial rate of the alloy increases with the increase of Ag content.This is because the antibacterial mechanism in Ti-Tanium alloy is mainly mediated by the release of silver ion,and the antibacterial mechanism of silver ion is closely related to the interface between silver ion and sulfhydryl groups in enzymes and proteins.The addition of Ag is beneficial to improve the corrosion resistance of Ti-25Ta-x Ag ternary alloy,and its cytotoxicity is also improved.With the increase of Ag content,trace Ag precipitation or residue can reduce the corrosion resistance of Ti-25Ta-x Ag alloy,and its inhibition rate to cells also decreases.This is because the cytotoxicity of metal materials largely depends on their corrosion resistance and chemical stability.In conclusion,the Ti-Ta-Ag alloy prepared by SPS has high density,but due to its fast cooling speed,it changes from high temperature phase in Ti(β-Ti)to low temperature stable phase(α-Ti)during transformation cooling,the solubility of Ag inβ-Ti decreases gradually and increases toα-Ti diffused,and some Ag failed to diffuse in timeβ-Ti precipitates or remains at the grain boundary,and Ta has a high melting point,formingα,α"andβPhase,unmelted Ta and Ag phase.The structure effectively improves the properties of biomedical Ti-Ta alloy.On the one hand,the solid solution strengthening of Ta,Ag and grain boundary strengthening effectively reduce its elastic modulus and significantly improve its hardness;On the other hand,the solid solution strengthening of Ag is conducive to improve its corrosion resistance and cytotoxicity,but the precipitation or residue of Ag not only reduces the corrosion resistance of the alloy,but also reduces its inhibition rate to cells.The first principles calculation results deeply explain and reflect the influence mechanism of structure,mechanical and electrochemical properties of Ti-Ta-Ag alloy.Therefore,in order to improve the comprehensive properties of Ti-25Ta-x Ag alloy and avoid the precipitation or residue of trace Ag in the preparation process,the best Ag content is 1.5-3 wt%.The innovation of this paper is mainly reflected in the following three aspects:(1)By combining the particle size control technology of ball milling powder with the subsequent SPS molding,a series of Ti alloys with different sizes(grain size ranging from micron,submicron to nanometer level)were obtained,and the double strengthening effect of fine grain strengthening and alloy strengthening was further obtained,which can provide ideas for the development of new biomedical metal material combination preparation technology.(2)Trace amounts of Ag(micron scale)were added to Ti alloy powder,and different scales of Ag precipitation or residual Ag were obtained by ball milling and SPS sintering process control.The mechanism of Ag precipitation or residual,the correlation between different sizes and different morphologies of Ag precipitation or residual phase and the corrosion resistance,antibacterial activity and cytotoxicity of Ti alloy were studied.The structural control principle of Ag precipitated or residual phase was obtained,and a new Ti alloy with excellent strength,corrosion resistance,antibacterial property and good biocompatibility was obtained.The research content is novel.(3)The effects of the addition of Ta and Ag elements on the structure,mechanical properties and corrosion resistance of Ti-Ta-Ag were studied using the first-principles calculation system.The influence mechanism of Ta element on the mechanical properties and corrosion resistance of the alloy and the influence mechanism of Ag doping on the mechanical properties and corrosion resistance of the alloy were preliminary-proved.The experimental results were well verified.It provides a theoretical basis for the research of Ti alloy biomedical materials with good synthesis. |
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