| Ti-alloys are widely applied in aerospace and biomedical fields because of their high specific strength,low elastic modulus,good biocompatibility and corrosion resistance.Among them,metastableβTi-alloys have received extensive attention in recent years due to their good formability,lower elastic modulus,and heat treatment strengthening.Generally,the strength of solid solution metastableβ-Ti-alloys is low,and large cold deformation is required to improve its strength and elastic allowable strain(σ0.2/E).However,in order to meet the high fatigue performance requirement of biomedical implant materials,the deformed alloys need to be heat treated at 400°C and above for a long time.In addition,when used as the variable wing material for a 3～5 Mach（Ma）hypersonic aircraft,the airfoil temperature can reach 250～550°C,and the alloys also need to be pre-heated to ensure the structure stability.During heat treatment of cold-deformed Ti-alloys,the precipitation ofωandαphases on the one hand increases strength,but also causes a significant increase of elastic modulus.On the basis of clarifying the precipitation law and action mechanism ofωandαphases,how to achieve a reasonable match between high strength and low elastic modulus by adjusting the microstructure of metastableβTi-alloys is the key to solving above application problems.In addition,it has been reported that the coefficient of thermal expansion（CTE）of deformed metastableβTi-alloys is tunable,showing broad application prospects in the fields of high-precision equipment and thermal sensors.However,the study on its negative thermal expansion behavior is still in-depth,and the negative expansion mechanism is still controversial.In this paper,the Ti-（30～40）Nb-（0,0.3）O alloys with a wide range of Nb content are designed,and the evolution law of microstructure and mechanical properties of alloys after solution treatment（ST）and 90%cold rolling（CR）is systematically studied.Subsequently,for the CR-Ti-34Nb alloy,its anomalous thermal expansion behavior during temperature cycling is deeply studied,the related disputes are clarified,and the anomalous thermal expansion mechanism is revealed.Furthermore,the evolution law of microstructure and mechanical properties of a series of CR-Ti-34Nb-（0,0.3,0.6）O and CR-Ti-34Nb-4Zr-（0,2,3,4）Sn（0,0.3）O alloys with differentβ-stability during aging within a wide temperature range is systematically studied,revealing the action mechanism of substitution type（Zr,Sn）and interstitial type（O）elements.The purpose of abovementioned studies is to control the mechanical and thermal expansion properties of metastableβTi-alloys by adjusting its their composition and microstructure,so that it has potential application value in the fields of aerospace and biomedicine.The main conclusions of this paper are as follows:Evolution law of microstructure and mechanical properties of Ti-（30～40）Nb-（0,0.3）O alloys after solution treatment and 90%cold rolling.It is found that both Nb and O have an inhibitory effect on the formation of martensiteα’’,but O is stronger than Nb,about 8 times that of Nb.After 90%cold rolling,[011]β//[010]α’//RD（rolling direction）and[01?1]β//[001]α’//TD（transverse direction）textures are formed,and the texture strength increases with increasing Nb content.The formation of textures is beneficial to reduce elastic modulus,but the elastic modulus of alloys is mainly affected byα’’content(first-principles calculations show that Eα’’（46GPa）>Eβ（35GPa）).The strength of 90%cold-rolled alloys is significantly higher than that of solid solution state due to the massive formation of dislocations and the refinement ofβgrain andα″phase.It is found that excessiveα’’phase seriously deteriorates the plasticity of alloys while increasing strength and elastic modulus.Only when its content is small（β+a small amount/trace ofα’’）,alloys exhibit excellent comprehensive properties with high strength,low elastic modulus and good elongation.Among the solid solution alloys,it is found that the ST-Ti-36Nb-0.3O alloy has better performance（σb=806MPa,E=51.0GPa,δ=19.1%）,which meets the performance requirements of medical implant materials.After 90%cold rolling,all alloys containing 0.3%O except CR-Ti-30Nb-0.3O have higherσ0.2/E（>1.8%）,among them,theσ0.2/E of CR-Ti-34Nb-0.3O alloy is as high as 2.24%,which is very suitable for 1～3Ma supersonic fighter variable wings（airfoil temperature is lower than 250°C,and structure of alloys does not change）.Anomalous thermal expansion behavior of CR-Ti-34Nb alloy and its instantaneous CTE versus temperature.After 90%cold rolling,the TD and RD of CR-Ti-34Nb alloy exhibit anomalous positive and negative thermal expansion,respectively.It is found that the anomalous thermal expansion behavior is attributed to theβandα’’textures formed by cold rolling and theα’?βthermally reversible transformation during thermal cycling.Through designing control experiments,the effects ofα’’anisotropy,strain energy,microstructure defects and deformation texture caused by cold rolling on CTE were excluded,and a cyclic thermal expansion microscopic model is proposed,which can better explain the anomalous thermal expansion.mechanism.In addition,the nonlinear relationship between instantaneous CTE and temperature for TD and RD is found for the first time within the temperature range of anomalous thermal expansion,which is attributed to the increase ofα?→βtransformation rate（temperature increases by 1°C）with increasing temperature and the significant decomposition ofα’’phase.The CTE of CR-Ti-34Nb alloy is controlled byα?content.According to the abovementioned findings,the novel methods to control CTE by alloying and pre-aging are proposed,and the CTE of RD can be adjusted to a large range of-55.0～8.5×10-6 K-1,and a variety of negative/zero CTE Ti-alloys with a wide temperature range have been developed,which have a good application prospect in the fields of high-precision equipment and thermal sensors.Among them,the near-zero expansion Ti-alloys have the characteristics of wide application temperature range,low density and high strength compared with traditional Invar alloy,which have better comprehensive performance.Evolution law of microstructure and mechanical properties of CR-Ti-34Nb-（0,0.3,0.6）O alloys during aging.It is found that the combined effect ofωandαprecipitates during aging makes strength and elastic modulus of alloys reach their maximum values at 350°C and Tα-max,respectively.When aging temperature continues to increase,ωphase disappears,αphase spheroidizes and its volume fraction decreases,resulting in a gradual decrease of strength and elastic modulus of alloys,and a gradual increase of elongation.After O addition,the change rules of strength,elastic modulus and elongation of TN-0.3O and TN-0.6O alloys are similar to those of TN alloys.However,the differences from TN alloy is that,O has an inhibitory effect on the precipitation ofωphase.With the increase of O content,its size and number decrease,resulting in significantly higher plasticity（compression ratio）of both TN-0.3O and TN-0.6O alloys aged at 300 and 350°C than that of TN alloy.In addition,on the one hand,O has a significant solid solution strengthening effect on alloys;on the other hand,O not only has an inhibitory effect on the precipitation ofαphase(when aging temperature is less than or equal to Tα-max),pushing up the Tα-max,but also has a hindering effect on the decomposition ofαphase(when aging temperature is higher than Tα-max),pushing up the（α+β）/βphase transformation temperature(T（α+β）/β).After aging at 400～550°C,the higher O content,the higher strength and the lower elongation of alloys.Theσ0.2/E of 90%cold-rolled TN-0.3O alloy after aging at400°C×8h is as high as 1.80%（σb=1566MPa,E=85.7GPa）,but its elongation is low（δ=3.7%）.Evolution law of microstructure and mechanical properties of CR-Ti-34Nb-4Zr-（0,2,3,4）Sn-（0,0.3）O alloys during aging.As aging temperature increases,the overall variation trend of microstructure and properties of this series of alloys is similar to that of abovementioned three alloys,but there are the following differences:the substitutional elements Zr and Sn significantly reduce the temperature at whichωphase disappears completely,and also have an inhibitory effect on the precipitation and growth ofαphase,pushing up the Tα-max andαphase spheroidization temperature,which results in that the elongation of alloys is significantly improved while maintaining high strength when aged in a wide temperature range,and the elastic modulus is greatly decreased,resulting in excellent elastic properties.For example,CR-Ti-34Nb-4Zr-3Sn-0.3O alloy after aging at 400°C×8h has high strength,low elastic modulus and good plasticity（σb=1432MPa,E=76.4GPa,δ=7.5%）,σ0.2/E up to 1.80%,which is expected to meet the performance requirements of deformable wing material for hypersonic aircraft above 3Ma.In addition,CR-Ti-34Nb-4Zr-4Sn-0.3O alloy after aging at 400°C has low elastic modulus,high strength and good plasticity（E=75.2GPa,σb=1360MPa,δ=9.0%）,which is very suitable for medical implant materials. |
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