| Metastable ? titanium alloy Ti-B12(Ti-10Mo-6Zr-4Sn-3Nb)is a novel type of medical material with lower elastic modulus and higher strength.It is avoided for the alloy to use Al,V,Ni,Cr,Cu and other elements that pose a hazard to biological tissues and cells.The elastic modulus of the material can effectively be reduced by the cold deformation process,as well as the precipitation phase refinement method can enhance the strength of titanium alloy materials,including inhibiting the growth of a-layers and increasing the nucleation particles of ?-phase.In this paper,by studying the solid solution aging heat treatment of Ti-B12 alloy,the effect of a precipitation phase were analyzed,and the phase transition conditions and the laws of microstructure evolution were obtained.The effects of cold deformation on the microstructure,texture and mechanical properties of the alloy have been discussed.The effect of lowtemperature pre-aging on a precipitated phase and mechanical properties was studied through a two-stage aging heat treatment system.And the effects of defects such as dislocation slip zones on the ?-precipitation phase during the aging treatments were studied.Finally,the results of research on phase transformation conditions,microstructure evolution and properties control of Ti-B 12 alloy materials were obtained,which provide a solid theoretical basis for the application of the alloy.Ti-B12 alloy after solution at 790?/1 h then water-cooled or air-cooled,the microstructure obtained by aging is basically the same,and the nucleation of needle-like a phase occurs in the grain boundaries and inside of the grains at 510? aging.After the solution cooling by FC,the morphology of the primary ? phase gradually grows from ellipsoid to lamellar with the aging temperature increasing.Heating rates during aging treatment have less effect on the microstructures.The secondary a phase transition kinetics of Ti-B12 alloy during isothermal aging at 510? can be described by JMAK equation:?=1-exp(-1.625 t0.477).The aging response process of Ti-B12 titanium alloy can be divided into three stages;The aging response process of Ti-B12 titanium alloy can be divided into three stages;the first stage is the a phase nucleation incubation stage within 5 min of the holding time;the second stage is the rapid precipitation stage from 5 min to 3 h;the third stage is a phase transition stable stage from 3 h to 8 h.Ti-B12 alloy undergoes stress-induced ?" phase transition,formation of dislocation substructure,deformation zone evolution and grain rotation and elongation in the axial direction during cold deformation.After cold deformation,the alloy turns from {112}<111>and{110}<112>texture to y-fiber{111}<110>and ?-fiber{112}<110>,?-fiber decreases first and then increases with the increase of deformation.The strength of Ti-B12 alloy increases with the increase of deformation and plasticity remains at a certain level at the beginning of deformation due to the uncompleted fracture of the original grains,therefore the alloy has good cold deformation ability.There is no ? phase precipitation during the low temperature pre-aging process at 350?,and the low temperature pre-aging without ? phase transformation does not change the precipitation temperature of the a phase,which is still around 450?.However,the pre-aging alloy is aged at 510?,the length of the a phase is reduced,and the mechanical properties of alloys with single aging and double aging have little difference.The secondary a phase preferentially nucleates on the deformation zone when the pre-deformationed alloy is aged at 430?.The dislocation slip zones inside of grains reduce the a phase nucleation barrier and promotes the a phase nucleation.With the increase of aging temperature,the thickness of ?layer gradually increases.With the increase of deformation degree,the volume fraction of ?precipitate phase increases and Vickers hardness increases accordingly. |
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