Room Temperature Creep Behavior Of Ti-Nb-Mo-Sn Shape Memory Alloy And It's Effects On Related Performance

The?-titanium shape memory alloys containing non-toxic?phase stabilizers have the excellent mechanical compatibility and biocompatibility,which show a wide application prospect as the implanting materials.As the hard tissue materials,?titanium alloys may bear a variety of loads,such as constant load and fluctuating load,for a long time in the course of service in human body.Therefore,room temperature creep of?titanium alloy is an unavoidable issue in the clinical application.Thus,deeply understanding the creep behavior at room temperature and deformation mechanisms and its effects the mechanical properties and biocompatibility of the?type shape memory alloy is conducive to the improvement of the safety and life span of the implants.In the present work,a?type shape memory alloy,Ti-7.5Nb-4Mo-2Sn alloy,was taken as the research object,its creep behavior at room temperature and mechanism were studied,and the effects of creep deformation on the mechanical properties?modulus of elasticity,superelasticity?and biocompatibility of the alloy were discussed.Moreover,the microstructure of the alloy was optimized by rapid aging heat treatment to improve the creep resistance at room temperature.The main research contents and conclusions are as follows:?1?The creep properties and deformation mechanism of Ti-7.5Nb-4Mo-2Sn alloy at different stress states were investigated.It is found that the loading stress level has an important influence on the room temperature creep of the alloy system.As the applied stress is lower than the stress-induced critical stress of martensite(?_SIM),the room creep deformation is hard to be detected in the alloy,while,as the applied stress is slightly higher than?_SIM,the alloy exhibits a relatively high the creep strain rate and large creep strain.However,as the applied stress further increases,the creep deformation tends to decrease.Cyclic loading mode has a impact on the early stage of the creep deformation,rather than on the creep life of the alloy.The observations of the microstructures of the alloy after creep reveal the room temperature creep of the alloy is caused by the synergistic effect of domino detwinning of the twin martensite and dislocation slip.The creep stain and the strain deformation rate is strongly dependent on the number the twin martensites induced by the applied stress.?2?The effects of the creep deformation at room temperature on the mechanical properties?superelasticity,elastic modulus?and the biocompatibility of the alloy were investigated.It is found that room temperature creep may lead to the decrease of the superelastic properties of the alloy,and of the relationship between the strain recovery rate and the creep time obey a logical function.Room temperature creep deformation may also increase the elastic modulus of the alloy,which is consistent with the law of creep deformation.Furthermore,the biocompatibility of the alloy after creep is evaluated.It is found that the solid solution Ti-7.5Nb-4Mo-2Sn is better than pure titanium,and the creep deformation has no obvious impact on the biocompatibility of the alloy.?3?The effect of rapid aging treatment on the creep properties of Ti-7.5Nb-4Mo-2Sn alloy at room temperature was investigated.After rapid aging treatment,many fine and dispersed?phases can be observed in the solid solution treated alloy The fine precipitates effectively promote the critical stress for the stress-induced martensite by can hindering the martensitic transformation induced by the stress,,which may be responsible for the improvement of the creep resistance of the alloy.