| Titanium and titanium alloys have been extensively utilized in biomedical implants due to their low Young's modulus,excellent comprehensive mechanical properties,biocompatibility and corrosion resistance.However,widely used biomedical Ti materials,such as Ti-6Al-4V(~110GPa),even have much higher elastic modulus than that of human bone(~30GPa).The mismatch of modulus between implant and human bone would cause “stress shielding” effect.Additionally,the release of Al and V ions from Ti-6Al-4V can lead to long-term health problems,such as osteomalacia and neuropathy.Therefore,novel ?-type titanium alloys consisting of low Young's modulus,high strength and nontoxic alloying elements have been attracted increasing attention for biomedical implant materials.According to low modulus,as main foundation of composition design,Ti-33Nb-4Sn(wt.%)has been designed and developed.In the present work,the effect of thermo-mechanical treatment on the mechanical behavior of Ti-Nb-Sn alloys was systematically investigated using mechanical testing machine,X-ray diffraction(XRD),transmission electron microscopy(TEM),in-situ synchrotron X-ray diffraction and EshelbyKroner-Kneer elastoplastic self-consistent model,in an attempt to develop metastable ?-type titanium alloys with both low modulus and high strength,elucidate the mechanism of low Young's modulus and deformational behavior,to provide theoretical guide to develop novel titanium alloys with low modulus and high strength.The results are summarized as follows:This alloy is composed of ? and ?? martensite in solution-treated state.Similar phase composition also occurs in cold-rolled state.After severe cold rolling and aging at 425? for 30 min,the alloy consists of dominant ? phase and a small amount of fine ? phase.Interestingly,a good combination of high strength(855 MPa)and low modulus(36 GPa)is obtained.TEM results indicate that high density of dislocation,grain refinement,and nanosized ? precipitates are induced by cold rolling and subsequent aging treatment,leading to high strength.On the other hand,due to the limitation of time during aging treatment,the low modulus of ? phase is kept and only slighter ? phase precipitated after aging,guaranteeing the low Young's modulus.Through cold rolling and the limitation of time during aging treatment,this alloy can obtain with both high strength and low modulus.To explore the mechanism of low Young's modulus,an in-situ synchrotron technique is used to extract the single-crystal elastic constants from polycrystalline sample with the aid of an elastoplastic self-consistent model(EPSC).The results indicate that the Ti-33Nb-4Sn alloy has a comparable C?(12.6 GPa)and a clearly lower C44(22.4 GPa)than that of binary alloy with the same average valence electron number(e/a).It is proposed that the anomalously low C44 as well as the low C? contribute to the low modulus for the metastable ?-type alloy which requires thermo-mechanical.Meanwhile,to research the mechanism of deformational behavior,In situ synchrotron X-ray diffraction study was carried out to focuse on the relationship between deformation behavior and microstructure transition.The “stress plateau” deformation of the solution-treated Ti-33Nb-4Sn alloy can be mainly attributed to the stress-induced ?? martensitic(SIM)transformation and pre-existing ?? variants reorientation.Upon a cold rolling deformation,a mass of dislocations and grain boundaries are introduced into the Ti-33Nb-4Sn specimen.These can be evidenced by their effects on suppressing the martensitic transformation from ? phase to ?? martensite.Therefore,in the cold-rolled alloy,as the extent of SIM transformation is much slighter than that in solution-treated specimen,this slighter SIM transformation gives rise to nonlinear deformation,instead of “stress plateau”.Upon aging,the ? phase survives against SIM transformation due to the suppression effects of high density of dislocations,grain boundaries and nanosized ? precipitates,resulting in a huge elastic deformability. |
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