The Correlation Between Martensitic Transformation And Deformation Behavior In TiNb/Nb/NiTi Composite With Multilayer Structure

Recently,NiTi shape memory alloys are quickly developed and commercially available in the field of biomedical materials due to the shape memory and superelastic properties suitable for human body temperature and good corrosion resistance.However,NiTi alloys are still controversial for biomedical roles with the Ni elements as a risk factor for allergy,mutagenicity and carcinogenicity.In order to obtain the shape memory functionality and biocompatibility required for biomedical materials,considerable research effort has been devoted to the development of Ni-free biocompatible β-type Ti alloys.Despite the native biocompatibility,it is found in previous studies that β-type Ti alloys are poor in terms of the superelastic performances,showing a low shape recovery strain.Therefore,neither NiTi norβ-type Ti alloys can simultaneously perform both the desired shape memory functionality and biocompatibility.To resolve the aforementioned issues,a Ti72.8Nb27.2/Nb/Ni50.3Ti49.7composite with the large linear superelasticity,narrow stress hysteresis and good biocompatibility was prepared with adopting a multi-component composite design concept in this study.Furthermore,with the multilayered Ti72.8Nb27.2/Nb/Ni50.9Ti49.1 composite as the model material,the relationship between microstructure,martensitic transformation characteristics and mechanical behavior of coupling Ti Nb and NiTi systems with two types of thermoelastic martensitic transformation was investigated.In this study,many experiments relating to the detailed microstructural characterization,phase identification,biocompatibility and tensile mechanical properties,together with in situ synchrotron X-ray diffraction(SXRD)and in situ electron backscatter diffraction(EBSD)were carried out.The main contents are as follows:A Ti72.8Nb27.2/Nb/Ni50.3Ti49.7 composite with strong interfacial bonding was prepared by severe deformation hot pack-rolling and cold rolling process.The prepared composite has a shell-core-like structure with intermediate Nb,and mainly consists of B19′-NiTi martensite,β-Ti Nb phase,B2-NiTi parent phase,α-Ti Nb and β-Nb phases.The observation by transmission electron microscope reveals that both Ti72.8Nb27.2 shells and Ni50.3Ti49.7 core exhibit the fine microstructure with microscopic defects(e.g.,dislocations and grain boundaries).In addition,numerous nanometer-scale(001)compound twins can be observed in B19′ martensitic plates.From biocompatibility test,one can see that the biocompatibility of as-prepared composite is superior to that of pristine Ni50.3Ti49.7 alloy,since the biocompatible Ti72.8Nb27.2 shells can play an effective role in blocking Ni50.3Ti49.7 core from implant,thus avoiding the release of cytotoxic Ni ions.There exist multiple deformation mechanisms in Ti72.8Nb27.2/Nb/Ni50.3Ti49.7 composite during the tensile loading-unloading process with pre-straining procedure.At the initial prestraining stage,the elastic elongation,two kinds of homogeneous stress-induced martensitic transformations(i.e.,B2→B19′ and β→α’’)occurred,resulting in the linear deformation behavior of this composite.Afterwards,the martensitic transformations proceeded slightly,accompanied by the plastic deformation.During unloading,the composite concurrently experienced elastic recovery,fully reversible α’’→β and partially reversible B19′→B2transformations.The combined effects of residual B19′ martensite and permanent plastic deformation lead to a residual strain of ～4.6% after unloading.During the loading-unloading process of Ti72.8Nb27.2/Nb/Ni50.3Ti49.7 composite after 8.0%pre-straining procedure,the linear superelasticity with a large recoverable strain of ～3.1%and narrow stress hysteresis(～148 MPa)can be achieved.The intrinsic elastic deformation,two different types of continuous and homogeneous β?α’’ and B2?B19′ transformations,along with the(001)compound twins in B19’ martensitic plates are all responsible for the large linear-superelastic behavior of the composite.These martensitic transformations proceeded homogeneously throughout entire deformation due to the inhibitory effect of microstructural factors(e.g.,dislocations and grain boundaries)on the shearing process of transformation,and the generated phase transformation strain resulted in a large recoverable strain(～3.1%).Additionally,the fine microstructure in this composite leads to a fine-scale martensitic transformation,conducive to a narrow stress hysteresis of composite after loading and unloading.In order to uncover the effect of load transfer coupled with multi-component on the deformation mode and mechanical behavior of composite,a Ti72.8Nb27.2/Nb/Ni50.9Ti49.1multilayer composite was fabricated by severe deformation hot pack-rolling and annealing.The composite shows a highly coupled multi-component laminate structure,and consists ofβ-Ti Nb,B2-NiTi parent phases and β-Nb phase.The EBSD characterization shows that the Ti72.8Nb27.2,Ni50.9Ti49.1 and intermediate Nb layers have fully recrystallized microstructure,exhibiting the equiaxed polycrystalline morphology.The in situ EBSD results suggest that the tensile deformation process for this composite involves the elastic deformation,incompletely reversible B19′?B2 transformation,slipping and deformation twinning.More specifically,the Ni50.9Ti49.1 underwent incompletely reversible B2?B19′transformation,while both the {332}<113>β and {112}<111>β twinning systems were activated in Ti72.8Nb27.2,ascribed to the combined effects of the diffusion of β-stabilizing Nb,the grain size and compressive stress factor.