| Laser rapid prototyping(LRP)is a process of laser cladding to directly generate physical objects with defined structure and shape on the basis of virtual 3D model data.It can be utilized to design and manufacture the customized medical implants.However,the biomedical titanium alloys used in laser rapid prototyping are mainly traditional Ti-6A1-4V,Ti-6Al-7Nb,etal alloys.The properties such as biocompatibility and Young's modulus of the alloys are not idea.Although the above performances of ?-Ti alloys developed later have been improved,the fluidity is poor.They cannot meet the practical requirements of laser rapid prototyping technology.As medical materials used for laser rapid prototyping,they should not only have excellent biological and mechanical properties,but also meet the requirements of laser rapid prototyping.This requires the alloys have high melt and solid structure compatibility.Therefore,the compositions of eutectic alloys should be selected first.Recent research showed that a binary Ti-Fe eutectic alloy exhibits a novel combination of high mechanical properties,good formability and biocompatibility.However,the Young's modulus of the alloy was high,and the alloy was easily oxidized forming harmful Ti4Fe2O phase,which leads to the decrease of comprehensive properties of the alloy.Ti-Fe binary eutectic clusters are proposed in the light of above-mentioned problems.The non-toxic and low Young's modulus Zr and Sn are used as the elements to reduce the Young's modulus of the alloy,and the non-toxic and high-reactive Y is used as the element to purify the liquid phase.Then a series of Ti-Fe-Zr-Y,Ti-Fe-Sn-Y and Ti-Fe-Zr-Sn-Y medical alloys are designed using the"cluster-plus-glue-atom" model which is applicable to non-equilibrium solidification process based on the characteristics of near-range bonding between atoms.Then the alloys were prepared by laser rapid prototyping.The microstructure,mechanical properties,formability,corrosion resistance and biocompatibility of the alloys were systematically studied.The main research results are as follows:(1)The results of Ti-Fe-Zr-Y alloys show that all alloys are mainly composed of ?-Ti,TiFe,and Zr2Fe phases.No Ti4Fe2O oxide is found in these alloys due to purifying effect of Y element.With the increasing of Zr addition,the microstructure of the alloys changes sequentially from hypereutectic to eutectic to hypoeutectic.When the alloy conforming to the composition formula of Ti22Fe10Zr2,an alloy with the composition of Ti64 52Fe29.32Zr5 86Y0.30 that with the best comprehensive properties is obtained.Compared with the binary Ti70.5Fe29.5 eutectic and Ti-6A1-4V alloys,the studied alloys present higher hardness,lower Young's modulus,better tribological,compression and chemical properties.The Young's modulus of the Ti64.52Fe29.32Zr5.86Y0.30 alloy(102 GPa)is reduced by 32%compared with that of Ti70.5Fe29.5 alloy prepared by laser rapid prototyping.The results suggest that the Young's modulus of the Ti70.5Fe29.5 alloy can be decreased through the alloying with Zr element,but there is a certain limitation in decreasing the modulus.Therefore,Sn(50 GPa)element with lower Young's modulus than Zr(68 GPa)was added to the alloy.(2)Ti-Fe-Sn-Y alloys mainly consist of ?-Ti,TiFe and Ti3Sn phases.With the increasing of Sn addition,the microstructure of the alloys changes sequentially from hypereutectic to near-eutectic to hypoeutectic.The Ti70.38Fe24.92Sn4.40Y0.30 near-eutectic alloy has the best comprehensive performance,and its performance is better than that of Ti70.5Fe29.5 and Ti-6A1-4V alloy.However,the reduction of Young's modulus of the alloy(101 GPa)is not significant compared with Ti64.52Fe29.32Zr5.86Y0.30 alloy.Therefore,it is necessary to further decrease the Young's modulus through multi-alloying by Zr and Sn elements.(3)Ti-Fe-Zr-Sn-Y alloys consist of ?-Ti solid solution,TiFe and Zr2Fe intermetallic compounds.From the general trend of the microstructure evolution,the increasing of Sn addition is beneficial to the microstructure evolution from hypereutectic to eutectic and hypoeutectic.However,the influence of Zr addition is complex and lack of regularity.The range analysis results showed that Ti64.51Fe26.40Zr5.86Sn2.93Y0.30 eutectic alloy has the best comprehensive properties,and the influence of Sn content on properties of the alloys was greater than that of Zr.Compared with the Ti64.52Fe29.32Zr5.86Y0.30 and Ti70.38Fe24.92Sn4.40Y0.30 alloys,the studied alloy has the best compression performance,tribological performance and biocompatibility.In addition,it has the highest hardness and the lowest Young's modulus,which is reduced by 24%compared with that of the two kinds of quaternary eutectic alloys.(4)A series of medical titanium alloys are designed to meet the requirements of laser rapid prototyping using a "cluster-plus-glue-atom" model.Through the analysis of the microstructure and properties of the alloys,the intrinsic mechanism of alloying based on the"cluster-plus-glue-atom" model is summarized.The Young's moduli of the alloys can be effectively reduced by the following reasons:Firstly,the high Young's modulus Fe atoms at the center or Ti atoms at the shell positions of the cluster were replaced by the alloying elements having low Young's modulus.Secondly,the lattice constants,number and size of the corresponding eutectic phase were adjusted by the alloying elements while realizing the microstructure heritability of the alloys.In addition,the alloying elements having low Young's modulus nature plays a positive role in decreasing the Young's moduli of the alloys.The following principles should be followed when designing alloys with low Young's modulus:1)The alloying elements should be non-toxic;2)The alloying elements should have low Young's modulus and large atomic radius difference from matrix elements;3)Alloying elements should not destabilize the eutectic phases so that the alloys have good structural heritability and stability;4)For titanium alloys with high oxygen affinity,the deoxidizer with certain chemical ratio should be appropriately added.On the basis of following the above selection principles,the design criteria are proposed as follows:1)The local structure analysis of eutectic composition of binary alloy was carried out,and the basic structure model of"cluster-plus-glue-atom”was constructed;2)According to the properties and functions of alloying elements to determine which cluster the elements enter into and play a role in making the corresponding phase stable or unstable;3)According to the mixing enthalpy between the alloying elements and matrix elements,cluster close-packing principle and electronic structure of the alloying elements to localize the alloying elements reasonably.The following principles should be followed in positioning:the components with negative mixing enthalpies with matrix elements occupy the center of the cluster,components with positive mixing enthalpies with matrix elements occupy the connection position;elements having similar properties to matrix elements can replace the matrix atoms. |
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