Microstructure And Mechanical Behavior Of TiNb/NiTi Superelastic Composite

NiTi alloy has been widely adopted for biomedical applications,because of its outstanding superelasticity and shape memory effect,including high critical stress for inducing martensitic transformation(?_M)and large recoverable strain(?).However,so much attention has been paid to a series of health problems,such as Ni-hypersensitivity and carcinogenesis,caused by the release of toxic Ni ions from NiTi alloy.In order to solve this problem,great efforts have been devoted to designing and fabricating Ni-free?-type Ti-based shape memory alloys(SMAs).Nevertheless,although?-type Ti-based SMAs possess excellent biocompatibility,their superelasticity and shape memory effect properties are inferior to those of NiTi alloy,exhibiting low?_M and small?.Overall,neither NiTi alloy nor?-type Ti-based SMAs have good biocompatibility,high?_M and large?.Based on the design concept of composite materials,we designed and fabricated a TiNb/NiTi composite by coating TiNb alloy with NiTi alloy.In this paper,the microstructure,biocompatibility,mechanical properties and martensitic transformation of TiNb/NiTi composite were investigated with the assistance of scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),X-ray diffraction(XRD),transmission electron microscopy(TEM),biocompatibility test,tensile test and in situ synchrotron X-ray diffraction(SXRD).The main results are summarized as follows:A sandwich-like TiNb/NiTi composite with fine metallurgical interfaces were successfully fabricated by a severe deformation hot pack-rolling plus annealing at 485? for 10 min.The phase constitutions of the composite can be indexed as B2-NiTi phase,?-TiNb phase,?-TiNb phase as well as?-Nb phase.Due to the inherent differences of recrystallization temperature between the innermost and outermost alloys of the composite,the recrystallization of B2 parent phase occurs in the innermost NiTi alloy after annealing and finally the average grain size is?64 nm.While in the outermost TiNb alloy,no recrystallization occurs and there are a lot of dislocation tangles and fine?precipitates within?parent matrix.Biocompatibility and mechanical properties of TiNb/NiTi composite have been evaluated.Biocompatibility test results show that the TiNb/NiTi composite has better biocompatibility than the conventional NiTi alloy.This can be mainly ascribed to the outermost TiNb alloy that possesses good biocompatibility and can obstruct the direct contact between NiTi alloy and cells to avoid the release of a large number of toxic Ni ions.Tensile test results show that during loading to 4.0%strain then unloading,TiNb/NiTi composite exhibits a high?_M of?515 MPa,a large?of?4.0%and 0%residual strain.While during uniaxial tensile loading to fracture,the composite exhibits a high?_M of?529 MPa and a fracture elongation of?6.2%.TiNb/NiTi composite possesses excellent biocompatibility,high?_M and large?.The mechanical behaviors of TiNb/NiTi composite during loading-unloading and uniaxial tensile loading to fracture have been studied by in situ SXRD.The SXRD results during the loading-unloading process show that,the excellent superelasticity of the composite is mainly ascribed to two different kinds of fully reversible stress-induced martensitic transformations(SIMTs),involving B2?B19?and???''transformations.In addition to B2?B19?and???''transformations,the deformation mechanism of the composite during loading-unloading also involves the elastic elongation and elastic recovery of?,B2,??and B19?phases.The SXRD results during the uniaxial tensile loading to fracture process show that,when the strains are below 1.2%,the elastic elongation of?,B2,??and B19?phases take place in the composite,and the????and B2?B19?SIMTs occur within the strains of 0.5-4.2%and 0.7-6.2%,respectively.Finally,the TiNb/NiTi composite fractures at a strain of?6.2%.During this deformation process,at the strain of about4.2%,the outermost TiNb alloy of the composite may experience a partial fracture,resulting in the applied stress to be transferred to the innermost NiTi alloy.Meanwhile,the B2?B19?SIMT in the innermost NiTi alloy takes place via a localized manner in the composite.