| Ti-Nb-based ?-titanium alloy has remarkable biocompatibility,corrosion resistance,excellent formability,low elastic modulus,etc.These alloys can be used as metal materials for human body implantation and become one of the new titanium alloys with the most potential application.In this paper,binary Ti-Nb alloys were analyzed.The effects of high temperature deformation behavior and room temperature deformation and solution treatment on microstructure and properties of single-phase Ti-53 Nb alloy were investigated.And the relationship between microstructure and properties of aging Ti-xNb(x=20,25,45)alloys was discussed.This will provide a theoretical basis for alloy structure control,performance improvement,and design of new beta titanium alloys.The high temperature deformation behavior of Ti-53 Nb alloy was studied by Gleeble thermal simulation experiment.The evolution of the microstructure was discussed.The strain-compensated hyperbolic sinusoidal function Arrhenius constitutive relation and microstructure evolution model were constructed,and the prediction effect was evaluated.The true stress-strain curves and microstructure observations show that the flow curves of Ti-53 Nb alloy at 760 °C and below are governed by dynamic recovery,and recrystallized grains can be observed at 810 °C and a higher temperature,but remain incompletely recrystallized.Large size grains were elongated;The deformation temperature,strain rate and deformation degree have an effect on the microstructure,at low temperature deformation,local instability and uneven deformation occurred in the central deformation region;at a higher temperature and lower strain rate,recrystallization driving force is large and new grains are easier to produce.Based on the stress constitutive relation and dynamic recrystallization model,the database of Ti-53 Nb alloy was established,which provided a model basis for numerical simulation calculation and prediction of stress and microstructure.The Deform 3D finite element simulation method was used to optimize the hot extrusion pipe extrusion conditions(extrusion ratio,extrusion speed and billet preheating temperature),and the suitable thermoforming process range was determined.When the extrusion ratio is increased,the crystal grains are refined;the extrusion speed is increased,and the uniformity of grain distribution is improved.In the actual extrusion test,a tube blank with good inner and outer surface quality was prepared under the process of extrusion speed of 130 mm/s,extrusion ratio of 15 and billet temperature of 950 °C.The dimensional deviation is small and the tissue distribution is relatively uniform.The evolution of alloy structure and the growth behavior of ? grains were studied by means of room temperature composite deformation and solution treatment,and the effects of work hardening and fine grain strengthening were analyzed.Ti-53 Nb alloy rods with relatively uniform microstructure were prepared by room temperature Equal Channel Angular Pressing,cold Rolling and cold Swaging.The tensile strength at room temperature is increased from 380 MPa before deformation to 553 MPa after deformation,which is increased by 45.53%,and the elongation is also above 16%.The dislocation density is increased,the strengthening effect is remarkable and the original grain boundaries are indistinguishable and a large number of criss-cross deformation flow lines appear.When the solid solution temperature increases,the rate of ? grain growth increases,and the grain growth rate decreases with the increase of holding time.The strengthening effect of grain size on the alloy satisfies the Hall-Petch relationship.At 700 °C and 60 min for solution,the microstructure is evenly thin and equiaxed,and good strong plastic matching can be obtained.The Ti-xNb alloy with different Nb content was tested and the dispersion strengthening effect of the second phase was analyzed.It was found that with the increase of Nb content,the phase stability of the alloy is continuously enhanced.The ? phase of Ti-20 Nb alloy precipitated most,the Ti-25 Nb alloy precipitated ? phase decreased and existence of isothermal ? phase was found.The microstructure of Ti-45 Nb alloy consisted of equiaxed ? grains.Different precipitation phases played the role of dispersion strengthening.The ? phase hinders the movement of the grain boundary,causing the secondary ? phase to be small,and hinders the dislocation motion during the deformation process.Therefore,the strengthening effect is the strongest.The Ti-25 Nb alloy has the highest tensile strength,reaching 950 MPa,but the plasticity is poor,and the elongation is only about 13%.During the stretching process,the ductile fracture mainly occurs,and the precipitation of the ? phase will cause the alloy to embrittle and exhibit brittle fracture. |
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