| New β Ti based shape memory alloys show an important potential as the implantmaterials due to their excellent corrosion resistance, low elastic modulus, goodbiocompatibility and non-toxicity to human body. However, in comparison with Ti-Nialloy, the non-toxic Ti alloys exhibit relatively poor shape memory and superelasticbehavior, such as small recovery strain, low functional stability and low critical stressfor slip, which limit their application in practice. Thus, it is essential to improve thesuperelasticity of β Ti based shape memory alloys to meet the demand of theirapplication. In this thesis, the effects of the third alloying elements on the shapememory and superelastic behaviors of the Ti-Nb binary shape memory alloys weresystematically studied. On the basis of the result, a new quaternary Ti based alloy,Ti-Nb-Mo-Sn, which exhibits good shape memory and superelastic behavior, wasdesigned by the d-electron orbit theory. Moreover, the effects of the alloying ele ments,and microstructure, including the second phase strengthening, the texture and grain size,on superelasticity of the Ti-Nb-Mo-Sn alloys were investigated. By adjusting thecomposition and controlling the microstructure, the superelasticity of the Ti-Nb-Mo-Snalloys was significantly improved. Finally, the superelasticity of Ti-7.5Nb-4Mo-2Snalloy under various loading conditions in service was evaluated. The main conclusionsare as follow,1. The effects of the third alloying elements （Ta, Fe, Zr, Mo, Si and Sn） on the stressinduced martensitic transformation and superelasticity of Ti-22Nb alloy wereinvestigated systematically. It is found that the number of valence electrons and theatomic size of the alloying elements have an important influence on the stress forinducing martensitic transformation（σSIM）. The addition of the third element with a highnumber of valence electrons and a small atomic size can significantly promote σSIMofthe Ti-22Nb alloy.2. A new quaternary Ti based alloy, Ti-Nb-Mo-Sn, which exhibits good shapememory and superelastic behavior, was developed by using the d-electron orbit theory,and the effects of the Sn content on microstructure, deformation mode andsuperelasticity of Ti-Nb-Mo-Sn alloy were investigated. It is found that, with theincrease of Sn content, the deformation mode of the alloys changes from twining to "transformation and then to slip. Ti-7.5Nb-4Mo-1Sn and Ti-7.5Nb-4Mo-3Sn alloys exhibit a good superelasticity with relatively high SIMat room temperature due to theathermal phases containing and/or the solid-solution hardening effect of Sn.3. The Ti-7.5Nb-4Mo-2Sn alloys were processed by different methods and theeffects of the different processing treatments on the superelasticity of the alloy wereinvestigated. The rolled alloy annealed at700oC for0.5h exhibits an excellentsuperelasticity with a high recovered strain （recoverable） and a strain recovery rate （η）.The superelasicity of the solution treated alloy can be further improved due to thereinforcement of and ω phases induced by aging treatment. The alloy with strongcomplex multiple textures exhibits high σSIMbut poor superelasticity, because complexmultiple textures are prejudicial to the stress induced martensitic transformation andreverse martensitic transformation.4. The Ti-7.5Nb-4Mo-2Sn alloy with ultra fine grained β phase was preparedthrough heavy deformation and then recovery and recrystallization. The influence of thegrain refinement on the superelastic behavior of the alloy was studied. It is found thatthe fine grain size induced by the recrystallization processing can effectively promotethe critical stress for the martensitic transformation. The relationship between thecritical stress for the martensitic transformation and grain size follows Hall-Petchequation with a slop of about0.27MN/m3/2.5. The superelasticity of the Ti-7.5Nb-4Mo-2Sn alloy under various loadingconditions in service was evaluated. It is found that the superelasticity of the alloy isstongly dependent on the thermomechanical conditions, including the plasticdeformation, temperature, strain rate and number of cyclic loading. By cyclic loadingand unloading training, the superelasticity of the alloy can be improved. |
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