Research On Microstructure And Mechanical Properties Of New Type Of Ti-32.5Nb-6.8Zr-2.7Sn-（0，0.3，0.6）O Alloys With High Strength And Low Elastic Modulus After Cold Rolling And Heat Treatments

As a kind of bio-implant material,the new type of low elastic modulus and high strength metastableβtype titanium（Ti）alloys has been extensively studied in recent years.In addition,βTi alloys also possess high elastic admissible strain(σ_0.2/E),which show important application prospects in aerospace elastic components and variable wing materials.Based on“Three Electronic Numbers（e/a,?Bo?and?Md?）”alloy design theory,a kind of low elastic modulus metastableβtype Ti-32.5Nb-6.8Zr-2.7Sn-xO（TNZS-xO,wt.%）based alloys is designed by author’s group.Subsequently,the effects of O content on the microstructures and mechanical properties of TNZS-xO（x=0,0.3,0.6）alloys during cold deformation and heat treatment are systematically studied,and the relevant effects and mechanisms of O in the above processes are also revealed.The purpose of this study is to control the mechanical properties of alloys by means of microstructures,so as to make the alloys have potential applications value in both biomedical and aerospace fields.After cold rolling,the equiaxed grains of TNZS alloy are elongated along the rolling direction,and transitional grain boundaries are formed.With the increase of deformation reductions,the number of grain boundaries and dislocation density increase.It is also found that the intensity(I_Texture)of{001}<110>_α-fiber texture increases,the I_Texture of{111}<112>_γ-fiber texture weakens,and the other three textures remain unchanged.After 50%deformation,{112}<111>twins are observed.After 90% deformation,slip bands and dominant{001}<110>_α-fiber texture are formed.During the deformation process,the deformed specimens are still composed of singleβphase,and dislocation slip runs through the whole cold working process.With the increase of deformation reductions,the strength of the alloy increases owing to the increase of dislocation density and grain refinement,and the elastic modulus obviously decreases mainly owing to the formation of an enhanced I_Textureof{001}<110>_α-fiber texture.The microstructures and properties of the TNZS-0.3O and TNZS-0.6O alloys are similar to those of TNZS alloy in general during deformation,but there are some differences as follows:（1）With the increase of O content,the lamellar thickness of{112}<111>twins decreases（O suppresses the long range development of twins）.With the increase of deformation reductions,the I_Texture of the{111}<110>_α-fiber texture of TNZS-0.3O alloy as well as{112}<110>_α-fiber and{111}<110>_α-fibertextures of TNZS-0.6O alloy increase in varying degrees,but the dominant mechanism for the decrease of elastic modulus is still attributed to the enhancement of{001}<110>_α-fiber texture.（2）The tensile strength of the TNZS-xO alloys increases by about 80 MPa with each increase of 0.1wt.%O,and the elastic modulus increases by about 2.6 GPa.It is also found that the 90%deformed TNZS-0.3O alloy(σ_b=1093 MPa,σ_0.2/E=1.81%)and the 50-90%deformed TNZS-0.6O alloys(σ_b=1280-1322 MPa,σ_0.2/E=1.88-2.03%)possess highσ_0.2/E,which are superior to the traditionally used Ti alloys for aviation(σ_0.2/E=1.0-1.35%),and can meet the requirements of 1-3 Mach（Ma）supersonic aircraft variable wing materials（the wing surface temperature below 300℃）.After aging at 350℃for 24h,ωandαdual-phases are simultaneously precipitated in the deformed TNZS alloy.Increasing the aging temperature,the content ofωphase gradually decreases,while the content ofαphase increases and the maximum volume fraction temperature(T_αmax)is about 400℃.And then,theαphase starts to decomposition,and the complete decomposition temperature(T_{（α+β）/β})is about 575℃.With the further increase of temperature,dislocation climbing occurs in the deformed structure,and the recrystallization temperature（T_r）is about 600℃.When heat treatment at T_αmax,deformation textures are basically unchanged,and the{001}<110>_α-fibertexture is still dominant.After heat treatment at T_r,the{001}<110>_α-fiber texture weakens significantly,while the other textures remains unchanged.With the further increase of temperature（>T_r）,the recrystallization is completed and the texture returns to the state before deformation.Between 350℃and T_{（α+β）/β},the strength and elastic modulus of the alloy are mainly dominated by the precipitation ofωphase and/orαphase,reaching the maximum value at T_αmax.The strength tends to be stable near T_{（α+β）/β},while the elastic modulus possesses the minimum value.Between T_{（α+β）/β}and T_r,dislocation climbing,and the elastic modulus remains unchanged.After heat treatment at T_ror above,recrystallization and the weakening of{001}<110>_α-fiber texture result in the slightly increase of elastic modulus.Finally,the texture and mechanical properties of the alloy basically return to the state before deformation.In addition,the precipitation of strengthening phases leads to the decrease of elongation,while the increase in the volume fraction of theβphase contributes to the increase in elongation.The microstructures and properties of the 90%deformed TNZS-0.3O and TNZS-0.6O alloys are similar to those of TNZS alloy in general during heat treatment,but there are some differences as follows:（1）the addition of O significantly increases the T_αmax、T_{（α+β）/β}and T_r.（2）the addition of O suppresses the formation ofωphase during aging.（3）the contribution of O to the strength improvement is greater than the elastic modulus,so it can improve theσ_0.2/E of the O content alloys after heat treatment at 450℃or above.It is found that the 90%deformed TNZS-0.3O alloy（E=68.7-72.1 GPa,σ_b=1028-1327 MPa）can meet the mechanical properties requirements of bio-implant materials after heat treatment at 400-500℃.After heat treatment at 500℃,theσ_0.2/E of the90%deformed TNZS-0.3O and TNZS-0.6O alloys is between 1.67% and 1.86%(theσ_0.2/E of the TNZS-0.6O alloy is still 1.32% after heat treatment at 650℃),which is expected to meet theσ_0.2/E requirements of the variable wing of 3-5 Ma supersonic aircraft（the wing surface temperature no more than 600℃）.In brief,the comprehensive mechanical properties of the TNZS-xO alloys can be controlled over a wide range by adjusting the O content,cold working and heat treatment processes,which is expected to meet the above application requirements.