Investigation On Design, Microstructure And Properties Of Novel ? Type Ti-Ta-Hf-Zr Alloy For Vascular Stent Application

Titanium and some of its alloys are receiving increasing research interest due to their high specific strength,excellent corrosion resistance,superior biocompatibility and lower elastic modulus.Apart from orthopedic and dental biomedical applications,biomedical titanium alloys are also being explored as cardiovascular stent materials.At present,the most commonly used stent materials include metal and polymer materials.Compared with polymer materials,metallic stents exhibit stable performance and therefore can provide preferable supporting strength.316 L SS,which is used as a balloon-expandable stent material,whilst Ni-Ti alloy is widely used as a self-expandable stent material.However,both of them contain Ni,which may trigger a local immune response and inflammatory reactions.Therefore,there is a need for a new material that meets both the mechanical properties required for vascular stents and the biocompatibility.In this stduy,a new series of Ti-Ta-Hf-Zr alloys including Ti-37Ta-26Hf-13 Zr,Ti-40Ta-22Hf-11.7Zr and Ti-45Ta-18.4Hf-10Zr?wt.%?were designed using the d-electron theory combined with electron to atom ratio?e/a?and molybdenum equivalence?Moeq?approaches.The microstructure,mechanical properties and corrosion behavior of Ti-Ta-Hf-Zr alloy under as-cast,heat treatment processes and thermo-mechanical were investigated using OM?optical microscopy?,CLSM?Confocal Laser Scanning Microscope?,XRD?X-ray diffractometry?,SEM?Scanning Electron Microscope?and TEM?transmission electron microscopy?,Vickers micro-indenter,compression and tensile testing machines,electrochemical analysis,XPS?X-ray photoelectron spectroscopy?.And use of heat treatment,and thermo-mechanical to optimize the organization and performance of Ti-Ta-Hf-Zr alloy.The primary results are summarized as follows:1.The Ti-Ta-Hf-Zr alloys possess high strength?compressive yield strength,tensile strength,and tensile yield strength?,elastic admissible strain,micro-hardness,and excellent cytocompatibility.The TTHZ alloys with the highly desirable mechanical properties satisfy the mechanical requirements of self-expandable stents applications.The high yield strength combined with high elastic admissible strain provides flexible design with thinner struts which has a high potential to reduce restenosis rates in metal stents.However,the elongation of the alloy needs to be improved.2.The phases of the Ti-40Ta-22Hf-11.7Zr alloy from as-cast state,solid solution,solid solution plus aging for 24 h followed the transitions: ? + ?ath ? ? ? ? + ? " ? ? + ?" + ? ? ? + ? + ?iso.The strength and hardness of the alloy are improved with the aging time and the plasticity is reduced.In solution treatment states,Ti-40Ta-22Hf-11.7Zr alloy showed excellent compression performance,with a compressive yield strength of approximately 1018 MPa and an excellent compressive strain as no fracturing was observed;and the compression tests were stopped at a compressive strain of ⁷0%.The OCP data show that,in its as-cast state,the corrosion resistance of the Ti-40Ta-22Hf-11.7Zr alloy was superior to those of CP-Ti and Ti6Al4 V,which was due to the Ti O2,Ta2O5,Hf O2,Zr O2 and Zr2O3 oxides formed in the passive film of as-cast Ti-40Ta-22Hf-11.7Zr alloy surface.3.When the deformation is low?20%?60%?,stress-induced martensite ? " was tested in the Ti-40Ta-22Hf-11.7Zr alloy,As the deformation increases to 80% and 90%,the dynamic recrystallization occurred,the grain size was significantly refined,and the grain size decreased from 335.3 ?m in the solid solution state to about 12.4 ?m.Due to dislocation,fine grain strengthening,the mechanical properties of the alloy has a certain increase compare to as cast and solid solution.After hot rolling plus solution,the structure undergoes recrystallization and grain growth.At this time,the strength,hardness and Young's modulus decrease slightly,but the elongation and elastic allowable strain are greatly improved.When the deformation is 90%,the overall performance of the best.The micro-hardness,tensile strength,yield strength,elogation,and Young's modulus of the Ti-40Ta-22Hf-11.7Zr alloys were 347.3 ± 8.0 HV,1272.4 ± 37.6 MPa,1231.8 ± 33.5 MPa,9.69 ± 0.26 %,62.9 ± 2.1 Gpa,respectively.Especially the elastic admissible strain to reach an astonishing 1.96 ± 0.05%,comparable to the performance of bulk metallic glass?BMG?.At this time,the corrosion potential?Ecorr?,corrosion current density?icorr?,and corrosion rate?CR?of those samples were-0.318± 0.013 V,0.32 ± 0.08 ?A/cm2 and 2.79 ± 0.70 ?m / y r,respectively.4.The Ti-Ta-Hf-Zr alloys exhibited excellent cytocompatibility on osteoblast-like cells?SaOS₂?.This study further confirmed that Hf is at the same cytocompatibility level with Ti,Zr,and Ta.