| At present,the Ti6Al4V alloy clinically used for orthopedic implants has biocompatibility problems caused by the mismatch with the elastic modulus（10-30GPa）of human bone and the release of harmful metal ions（Al,V）in service.To meet the needs of orthopedic implants,the third-generationβ-type titanium alloys with low elastic modulus and good biocompatibility have been developed rapidly,but the low strength of this type of alloy limits their application.Therefore,there is an urgent need to develop medical materials with both low elastic modulus and high strength.On the other hand,medium/high-entropy alloys（M/HEAs）proposed in recent years have attracted much attention due to their significant advantages such as high strength,high hardness,high corrosion resistance,and high wear resistance.The alloy concept provides a compositional space for the design of new medical meta l materials,especially its excellent mechanical properties caused by high solid solution strengthening and elastic modulus with a wide control range,which is highly compatible with the performance requirements and characteristic requirements of bone implant materials.In this work,the equiatomic ratio TiZrNb medium entropy alloy was used as the initial composition,and the microstructure,phase structure and mechanical properties of the as-cast,cold-rolled and cold-rolled annealed were studied.The TiZrNb medium entropy alloys in all states were of BCC structure and the structure was stable at 400-1000℃.The yield strength of the as-cast alloy was about 830±21 MPa,and it performed a lower degree of work hardening.The elastic modulus of the alloy was72±3 GPa,lower than that of Ti6Al4V alloy and other Ti contained medium/high entropy alloys.In addition,the as-cast alloy exhibited the good plasticity,including excellent tensile plasticity（13±2%）,ultra-high compressive plasticity（>55%）and it could be cold rolled 70%directly at the room temperature.After annealing,fine-grain strengthening increased both the strength and plasticity of the alloy and the work hardening effect was enhanced.The microstructure evolution,deformation mechanism and mechanical strengthening of TiZrNb alloy during equal channel angular pressing（ECAP）deformation were investigated.Before and after ECAP deformation,the alloy maintained the BCC structure.As the deformation progressed,the grain morphology evolved from dendritic grains,equiaxed grains,and finally to elongated grains rich in shear bands and dislocations.Dislocations also undergone different configurations,long straight dislocations and dislocation loops arranged along the specific directions,a large number of dislocations got together into high dense dislocation walls（HDDWs）,and finally groups of HDDWs formed shear bands.Shear bands coordinated the deformation continuity and grain refinement.Compared with the as-cast alloy,the yield strength of the 6-pass deformed alloy was increased by 340 MPa.In order to improve the adaptability of TiZrNb alloy to human bone,five kinds of Ti-Zr-Nb alloys with non-equiatomic ratio were designed by adjusting its atomic percentages of elements.The yield strength of Ti-Zr-Nb alloy ranged from 675±18 MPa to 830±21 MPa,and the elastic modulus ranged from 52±2 GPa t o 73±3 GPa.Compared withβand nearβTi alloys,the alloy acquired both low elastic modulus and excellent strength.At the same time,the solid solution strengthening theory and the interatomic bonding force model of transition group elements have been u sed to reveal the regulation mechanism of the constituent elements on the mechanical strength and elastic modulus of the alloy.In addition,the wear resistance of the alloy was lower than that of the Ti6Al4V alloy.For the biological corrosion resistance of the non-equiatomic ratio alloys,the electrochemical corrosion behavior and the formation of surface passivation films in PBS solution were investigated,compared with those of the constituent elements.Potentiodynamic polarization curves show that the corrosion resistance of non-equiatomic ratio alloys was better than that of pure Ti.The contribution of the constituent elements in the alloy to the corrosion resistance was Zr>Ti>Nb,which was consistent with the corrosion resistance of each element.A s table passivation film composed of Ti O2,Zr O2 and Nb2O5 were spontaneously formed on the surface of the alloy identified by XPS,in which Ti and Nb were more enriched in the passivation film.Furthermore,the electrochemical equivalent circuit（EEC）of the alloy corrosion and the element distribution in the passivation film with different thicknesses indicate d that the passivation film was consisted of inner and outer layers.To further optimize the performance of non-equiatomic ratio alloys,（Ti Zr）x Nby（Ta Mo）z alloys were designed by adding trace amounts of Ta and Mo elements into them.The yield strength of（Ti Zr）x Nby（Ta Mo）z alloys ranged from678±17 MPa to 821±14 MPa,the elastic modulus ranged from 52±2 to 59±2 GPa,and the corrosion current density ranged from 3.277×10-7 A/cm2 to 5.105×10-7 A/cm2.The alloy got both low elastic modulus and medium and high strength,and their corrosion resistance was still better than that of pure Ti.In addition,this series of alloys exhibited comparable in vitro biocompatibility to pure metals Ti.Finally,after the optimal composition alloy was implanted into the middle femur of SD rats for 12 weeks,no diseased tissue was found in the main organs and new bone was formed around the alloy.In summary,the TiZrNb based medium entropy alloys developed in this work performed the characteristics of bone-adapted mechanics,good plastic processing ability,high corrosion resistance and well biocompatibility,and they could be used as bone nails,intramedullary nails and bone pl ates.Simultaneously,control mechanism of constituent elements on the mechanical strength,elastic modulus and corrosion behavior of TiZrNb based solid solution alloys revealed in this work can provide certain guiding significance for optimization and des ign of new biomedical alloys. |
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