Study On Microstructure And Mechanical Behavior Of Metastable ?-type Titanium Alloy With Ultra Low Elastic Modulus

Titanium and titanium alloys have been widely used in biological implants due to their good biocompatibility,excellent corrosion resistance and low elastic modulus.Ti-6Al-4V alloy is the most widely used biomedical titanium alloy.Its elastic modulus is about 110GPa,which is significantly lower than that of medical stainless steel(210GPa)and medical cobalt alloy(240GPa).However,there is still a big gap in modulus between Ti-6Al-4V alloy and human bone(10?30GPa).It is worth noting that the Al and V elements in Ti-6Al-4V alloy are biologically toxic,and their long-term presence in the human body can cause harm to human health.Therefore,?-type titanium alloy composed of completely non-biotoxic elements has become a research hotspot in the field of biomedical materials due to its better biocompatibility and lower modulus.By changing the content of?-stabilizing elements and thermomechanical treatment,our team prepared a metastable?-type Ti-33Nb-4Sn alloy with ultra-low modulus and high strength.However,the physical mechanism of ultra-low elastic modulus and the influence of martensitic transformation on mechanical behavior are unclear.In this paper,the physical mechanism of ultra-low elastic modulus and the influence of martensitic transformation on mechanical behavior of Ti-Nb-Sn alloys was systematically investigated by combining results from X-ray diffraction,optical microscope,transmission electron microscopy,tensile test and synchrotron X-ray diffraction.A large amount of??martensite was formed after solution quenching in Ti-33Nb-4Sn alloys due to the low?-stabilizing element content.After cold rolling treatment and short-time annealing at 400?for 20 minutes,the grains in Ti-33Nb-4Sn alloy were significantly refined and the??martensite phase disappeared.In such a case,a large number of dislocations and nano-scale?precipitates distribute on the?-matrix and the interaction between dislocations and?precipitates has significantly improved the strength of the alloy.The cold-rolled annealed Ti-33Nb-4Sn alloy with the highest strength and the lowest modulus(E=36GPa,?_s=850MPa)was selected to discuss its physical mechanism of ultra-low elastic modulus.According to the Hill approximation,the polycrystalline Young's modulus of a material depends on its single crystal elastic constant.However,the Ti-33Nb-4Sn alloy studied in this paper cannot obtain?-phase single crystals due to its low?-phase stability.Therefore,elastoplastic self-consistent model combined with synchrotron radiation technology were used to measure the single crystal elastic constant of the cold-rolled annealed Ti-33Nb-4Sn alloy,which makes it possible to further study the correlation between elastic modulus and elastic constant.The results show that the shear modulus C?and C₄₄ of the cold-rolled annealed Ti-33Nb-4Sn alloy are significantly lower than those of the binary titanium alloy,which means that the Ti–33Nb–4Sn alloy exhibits lower?-phase stability with respect to{110}<11 0>shear and{001}<100>shear.The abnormally low shear modulus C?and C₄₄give the cold rolled annealed Ti-33Nb-4Sn alloy an ultra-low elastic modulus.The stability of the?-phase in Ti-33Nb-4Sn alloy not only changes the elastic modulus of the alloy,but also has influence on the mechanical behavior of the alloy during the tensile process.The solution treated Ti-33Nb-4Sn alloy exhibits a"double yielding"deformation behavior characterized by a"stress plateau"during the deforming process.The"stress plateau"was replaced by the non-linear elastic deformation in the cold rolled Ti-33Nb-4Sn alloy.This is mainly because the volume fraction of the initial?parent phase is significantly reduced after the cold rolling treatment,and the dislocations and grain boundaries introduced by the cold rolling treatment can effectively suppress the occurrence of martensitic transformation.The cold rolled annealed Ti-33Nb-4Sn alloy can exhibit a large elastic strain(?3%),which is significantly higher than the titanium alloys currently reported publicly,and also higher than most currently available elastic metal materials.This large elastic deformation ability is mainly attributed to the moderate?-phase stability and inhibition of martensite transformation by structural factors such as dislocation grain boundaries in the cold-rolled annealed Ti-33Nb-4Sn alloy.Both the solution treated and the cold-rolled Ti-33Nb-4Sn alloy exhibited almost completely linear elastic deformation behavior after pre-deformation treatment.This is mainly due to the fact that an intense stress-induced martensite transformation occurred in the alloy during the pre-deformation process,resulting in the stress-induced martensite transformation during the subsequent stretching process is low and reversible and occurring continuously in a wide range of stress.The above-mentioned research results show that through appropriate thermomechanical treatment supplemented and pre-deformation treatment,the martensitic transformation characteristics in?+??dual-phase titanium alloy can be adjusted,so that it has good mechanical properties(low modulus and high strength)and linear elastic deformation behavior,which changes the current situation that traditional dual-phase titanium alloys cannot have good application prospects due to the existence of"stress plateau".To this end,we had a try in binary Ti-Nb alloys which are convenient for industrial production.After pre-deformation treatment,the cold-drawn Ti-36Nb alloy has an elastic modulus of 30 GPa,a tensile strength of 670 MPa,and an elastic strain of 2.14%.This result is expected to promote the design and development of large-line elastically deformed beta titanium alloys.