Study Of Superelasticity Of NiTi Shape Memory Alloy And Biomedical Applications

A combination of shape memory effect,superelasticity and good biocompatibility makes NiTi alloys a desirable candidate material for biomedical research field.NiTi alloys have been used as interventional medical devices basing on its superelasticity.During the manufacturing process of these medical devices,tube drawing,plate rolling and subsequent heat treatments are performed to adjust the microstructure,phase transformation temperature and mechanical property.To satisfy the medical application's demand,the NiTi devices should own best superelasticity at body temperature.Aiming at the aspect metioned above,the effects of texture characters of cold rolling and recrystallization on NiTi alloy superelastic phase transformation strain are investigated.The high magnetic field effect mechanism of Ti₃Ni₄ precipitation orientation distribution and corresponding mechanical property are also studied.Ageing treatments are performed on the stent units,which are knitted by NiTi wires.And the shape-setting techniques are optimized. The mechanical response curves of stent units are obtained by finite element method.The primary conclusions are listed as follows:After 30%reduction,the NiTi alloy is recovered at 400℃annealing and complete recrystallized at 600℃annealing.High density dislocations suppress the martensite transformation,two-stage phase transformation of B2←→R←→B19' can be observed until 400 and 500℃annealing.One-stage phase transformation of B2←→B19' happened after 600℃complete recrystallized.During the process of cold-rolling,recovering and recrystallization,the texture components of NiTi alloy change remarkable.As cold-rolled NiTi samples,the dominating texture components are {223}<110> and(322)deformation texture.And the texture change into near {111} and(132)[023]during recovering.When the recrystallization finished the texture componets change into {122}<223>.The mechanical property of NiTi alloy is anisotropy after annealing,and the transformation strains are effect by the texture character and grain size.The Ti₃Ni₄ paticles form preferentially around grain boundary after the sample aged at 500℃for 2.5 h in Ti-50.6at.%Ni.There are three-stage phase transformations exist after different magnetic-field intensity ageing,including B2←→R,R←→B 19' transformations around the grain boundary and B2←→B19' transformation at grain interior.The magnetic force changes the local strain of precipitation variants,causing the Ti₃Ni₄ phase precipitate orientation distribution,i.e.promote the growth along the composite force direction and suppress the growth along the other directions.The Ti₃Ni₄ paticles are paralled distribution and neighboring precipitations are linked up into one piece.Heat treatment temperature is the most crucial factor to superelastic NiTi stent units' shape-setting.The proper shape-setting temperature should be chosen between 450-550℃. And the shape-setting is stable when the NiTi wires are constrain-treated at 500 and 550℃, the differences of angles between the treated stent units and the expected angles are not more than 3°.Phase transformation temperature A_f change remarkable with heat treatments,those are increase with ageing time and the peak values are obtained after ageing at 400℃.The plateau stresses decrease with ageing time at body temperature.Most of the treated NiTi alloy exhibite good recovery ability at body temperature with little permanent sets except ageing at 500 and 550℃for 120 and 180 min.The optimal ageing temperature should be 500℃,and the ageing time should be limited to not more than 60 min to retain maximal recoverability and excellent non-linear superelasticity.The proper shape-setted stent units are simulated by finite element method and the mechanical response curves of stent units are obtained.Comparison between experiment and simulation shows that whenαis set as 0.16 the theoretical and experimental curves of force-displacement are matched up.The stiffness of stent units decrease with ageing time,so the maximal force during crush and the elastic energy storage efficiency of the stent unit are both decrease.That means the flexibility of NiTi stent can be increased by ageing time prolong at proper ageing temperature.