Effect Of Rolling And Annealing Processes On Texture Evolution And Mechanical Properties Of Biomedical Beta-type Titanium Alloy

As the speed of population aging accelerates,the demand for biomedical materials is increasing.Orthopedic products are the main demand in biomedical materials,among which metals and alloys are the main products used.As the third-generation metallic implant materials,biomedical titanium alloys gradually replace stainless steel and the Co-Cr alloys because of good biological compatibility,and non-toxic,etc.As novel ?-type biomedical titanium alloy,Ti-Nb-Ta-Zr has superior wear resistance and biocompatibility as well as lower elastic modulus compared with the traditional ? + ?-type titanium alloy.This relieves effectively stress shielding effect.Consequently,Ti66.23Nb22.53Zr4.59Ta1.65Si5(at.%)with better biocompatibility was selected research object,and the cast alloys were treated by hot-rolling and recrystallization annealling,and then were characterized by various analysis methods,such as SEM,SEM-EDS,EBSD,XRD,and so on.The focus is to systematically study the effects of rolling ratio and recrystallization annealing time on microstructure and mechanical properties of titanium alloy,aiming to fine the effect of thermal processing parameters on microstructure of the alloy,thereby improving its mechanical properties.Firstly,through the study on microstructure and mechanical properties of the cast alloy,it is found that its microstructure is coarse ?-Ti dendritic matrix with spherical and network(Ti,Zr)2Si particles,respectively distributing inside grains of ?-Ti and along grain boundary of?-Ti.By investigateing microstructure,grain orientation and mechanical properties of as-rolled samples,it is found that ?-Ti grains are deformed along the rolling direction and small-angle-orientation subgrains are formed in ?-Ti grains.And the degree of deformation of?-Ti grains increases with the increase of rolling ratio,thereby forming smaller-sized subgrains.Meanwhile,the grains gradually exhibit a preferred orientation and form a strong texture;the continuous network(Ti,Zr)2Si at grain boundary is gradually segmented.Also,with the increase of the rolling ratio,the yield strength and fracture strength of the samples increase gradually,and corresponding fracture mode changes from brittle fracture to plastic fracture.The elastic modulus increases with the increase of the rolling ratio,which is mainly due to the effects of synergistic effect of grain orientation,grain size and grain boundary.Secondly,microstructure,grain orientation and mechanical properties of the rolled specimens with a rolling ratio of 70% at different times of recrystallization annealing were investigated.It is found that the deformed grains after rolling re-nucleate and grow up in a mode of merging subgrains during recrystallization annealing.With the extension of the annealing time,the number of recrystallized grains increases gradually.However,the grain size did not change significantly,due to the hindering growth effect of(Ti,Zr)2Si intermetallics.Also,the preferred orientation induced by the rolling is eliminated after the annealing treatment.Due to the strengthening of(Ti,Zr)2Si phase,the maximum fracture strength and yield strength of the samples after recrystallization annealing are not changed coMPared with those of rolled samples,but the plasticity is significantly improved.And the elastic modulus of the annealed specimen is slightly higher than that of the rolled ones,which is mainly due to the synergistic effect of grain orientation,grain boundary and defect.Finally,the rolled samples with different rolling ratios were annealed at 900°C for 4hours,and the microstructure,grain orientation and mechanical properties of the annealed samples were studied.It is found that due to the lower rolling ratio,the 30%-rolled sample has larger subgrain and lower dislocation density,and dislocations disappear quickly by sliding and climbing when annealed at 900? for 4h.As for the 50%-rolled sample,there is a mass of deformed grains in the initial stage of sbgrain-merging,and it is clear to see the residual subgrain boundary in the deformed grain.Because of suffering a great deformation with the70% rolling ratio,higher energy reserves in deformed grains and thus there is higher degree of recrystallization and uniform grain size in the annealed sample.In terms of the annealed samples with 50% and 70% rolling ratios,the aspect ratios are significantly higher than that of the annealed samples with 30% rolling ratio,due to the preferred orientation growth of the residual deformed grains and the recrystallized grains.In summary,the rolling process effectively reduces the modulus of elasticity of the as-cast specimen while significantly increasing the strength.Under the influence of annealing time,rolling ratio and(Ti,Zr)2Si,the annealed samples still maintain the high strength as the rolled samples,the elastic modulus increase slightly and the plasticity increasesignificantly.The elastic modulus of the obtained titanium alloy samples is 24.4-28.9 GPa,which matches well with the elastic modulus of human bone,and that will play a positive role in the extensive application of titanium alloy in medical field.