Study On Microstructure,Superelasticity And Deformation Mechanisms Of Nb/NiTi Superelastic Composites

NiTi shape memory alloys?SMAs?have been used in the field of biomedical applications due to their excellent corrosion resistance,shape memory effect and superelasticity.However,when NiTi SMAs are used as implants,the toxic nickel ions can release from them to the human body and thus lead to some deleterious effects.Although the biocompatibility of the?-type Ti-based SMAs composed of non-cytotoxic elements is superior to NiTi-based SMAs,the?-type Ti-based SMAs exhibit lower critical stress for inducing martensitic transformation(?_SIM)and smaller recoverable strain???than those of NiTi-based SMAs.In this study,a Nb/NiTi superelastic composite with a shell-core structure,which consists of Nb outer shells and NiTi inner core,was designed and fabricated through a severe deformation hot pack-rolling plus annealing.With the assistance of X-ray diffraction,scanning electron microscopy,transmission electron microscopy,in-situ synchrotron X-ray diffraction?SXRD?,tensile test and biocompatibility test,the microstructure,superelasticity and biocompatibility of the Nb/NiTi superelastic composite were systematically studied,and the deformation mechanism of the present composite at different strains were revealed.The main researches are summarized as follows:The Nb/NiTi superelastic composite,which possessed a pure metallurgical interface?no visible defects,such as precipitates and microcracks,were found?between the Nb shell and the NiTi SMA core,can be fabricated successfully by the present severe deformation hot pack-rolling plus annealing?at 623 K for 450 s?procedure.The cytotoxicity tests results show that the Nb/NiTi composite displayed better biocompatibility compared with NiTi SMA,which can be reasonably ascribed to that the Nb outer shell of the Nb/NiTi composite with good biocompatibility blocked direct contact between NiTi and cells;Tensile tests results show that the Nb/NiTi composite exhibited good superelasticity,with a high critical stress for inducing martensitic transformation(?_SIM)of about 470 MPa and a large recoverable strain???of near 4.5%.During the tensile loading to 4.5%strain and following unloading process of Nb/NiTi superelastic composite,in-situ SXRD results show that both the Nb shell and the NiTi core first experienced elastic deformation concurrently.Within the strain range from 0.8%to 4.5%,the Nb shell was mainly subjected to tensile plastic deformation,while the NiTi core experienced a stress-induced martensitic?SIM?transformation from B2 to B19?.During unloading,the B19?martensite in the NiTi alloy gradually disappeared,and the B2 parent phase of NiTi alloy returned to its original state before loading,indicating that the NiTi alloy underwent a completely reversible SIM transformation.Due to the recovery forces provided by the NiTi core at the interfaces,the pure Nb layer undergone partial elastic recovery firstly and then accommodated macroscopic strain by compressive deformation during unloading.As a result,a residual compressive lattice strain of⁰.39%was retained in the outer Nb layer after unloading.The above experimental results indicate that the superelasticity of the Nb/NiTi composite was mainly attributed to the B2?B19?SIM transformation in the inner NiTi core,which was completely reversible.During the tensile loading to larger tensile strain?6.9%?and following unloading process of Nb/NiTi superelastic composite,more parent phases were transformed to martensite phase in NiTi alloy.Although transformation volume fractions increased by nearly 70%compared with loading-unloading cycle with 4.5%strain,the parent phase returned to its original state and the transformation was still completely reversible.Because of the larger tensile strain of 6.9%,the outer pure Nb layer may experience a plastic deformation during this loading and unloading process,leading to a residual macroscopic strain of⁰.22%in the Nb/NiTi composite.These results might shed some light on design and development of novel superelastic composites for biomedical applications.