Microstructure And Mechanical Properties Of Gradient Structural ? Titanium Alloy By Friction Stir Processing

New novel ? titanium has attracted lots of attention in the field of impants due to low Young's modulus and good biocompatibility.However,? titanium has lots of problems,like low wear resistance,low tensile strength and low fatigue property.To improve alloy's mechanical properties,preparing gradient structural alloy is a very effective way.Recently,some researchers has studied the surface modification of ?titanium processed by friction stir processing.But few research about constructing gradient structural ? titanium has been reported.In this paper,we used friction stir processing(FSP)to build gradient structure of Ti-35Nb-2Ta-3Zr ? titanium alloy and studied how different processing ways affect the microstructure and mechanical properties of the gradient structure.In this work,we used two different ways to prepare gradient structural ? titanium alloy.The first way was to add Hf and Zn powder and we investigated how Hf and Zn affect the microstructure and mechanical properties.The second was to process the upper and lower surface to construct composite gradient structure and we investigated the microstructure and mechanical properties.The conclusions are as follows:We got micro-nanoscale gradient structural(Hf+Zn)/Ti-35Nb-3Zr-2Ta alloy by adding Hf and Zn during FSP.Band agglomeration of Hf and Zn was observed in the stirring zone(SZ)under a metallographic microscope.Grains from SZ to heat affected zone(HAZ)increased from submicron to several micrometers.Fine acicular martensite and dispersed nano-scale ?phase existed in SZ.The ? phase contained Hf element and Hf increased the size of ? phase.The TZ was affected by plastic and shear deformation and lath-shaped martensite existed in TZ.The HAZ underwent shear deformation under a large axial pressure and we found a large number of parallel twin martensite and dispersed ? phase around the martensite.The hard ? particles increased the critical slip stress,conducive to improving the superelastic properties and played role in a second phase strengthen.Therefore,the superelastic recovery and average hardness of the(Hf+Zn)/Ti-35Nb-3Zr-2Ta alloy in SZ were higher than those of alloy processed without adding alloying elements.Cell reproduction results showed the(Hf+Zn)/Ti-35Nb-3Zr-2Ta alloy had good biocompatibility.We also built of composite gradients by double-sided friction stir processing.We found large axial pressure during double-sided processing,increasing the dislocation density and shear deformation of the material.Therefore,we found the acicular martensite was further refined in SZ;the length of martensite in TZ reached nearly 10 ?m.The mechanical property test of the double-sided composite gradient structure showed its yield strength was nearly doubled and the elongation could reach 37%.The cyclic tensile test showed that the superelastic properties of the alloy samples were significantly improved after friction stir processing.The micropillar compression test showed that the yield strength in SZ was higher and the plasticity in HAZ was better.Therefore,the composite gradient structure had high strength and good plasticity.