Effects Of Accumulative Roll-bonding On The Microstructure And Superelasticty Of A Beta-type Ti-7.5Nb-4Mo-2Sn Shape Memory Alloy

Due to their excellent properties such as low elastic modulus, excellent biocompatibility and good corrosion resistance, the new type beta titanium based shape memory alloy has broad application prospects in the field of biological transplantation materials.However, compared to the traditional Ni Ti shape memory alloy, Ti-based SMA exhibit a low critical stress for thestress induced martensitic transformation,low super elastic recovery rate and poor super elastic stability,which limit their application in practice.It is well documented that grain refinement is a effective way to improve the mechanical properties of beta Ti-based shape memory alloys.Therefore,in the present thesis,a ?-Ti based shape memory alloy,Ti-7.5Nb-4Mo-2Sn was taken as the research object.The alloy was firstly heavily deformed by using Accumulative Roll-Bonding(ARB) method,and then annealed to produce recrystallization to refine the grain size of alloy.The effects of ARB and subsequent annealing treatment on microstructure and mechanical properties were investigated.The main result are as follows.1. The Ti-7.5Nb-4Mo-2Sn alloy was prepared by using vacuum arc melting method prepared, and the as-cast alloy was rolled at room temperature. The effects of the rolling deformation on the recrystallization temperature and the microstructure of the alloy were investigated. The results show that the alloy with v the deformation of 50% exhibits the recrystallization temperature of about 740 ?, and the recrystallization temperature decreases with the increase of the deformation. After annealed at 800 ?for 5 min, the alloy with the deformation of 87.5% was completely recrystallized, and its grain size is obviously refined, which reaches 50 ?m.2. The Ti-7.5Nb-4Mo-2Sn alloy was rolled at room temperature by using the ARB method, and then annealed at 700 ?for 5 min. The microstructures of the alloy were characterized by using optical microscope(OM), X-ray diffraction(XRD) and scanning electron microscope(SEM), and the mechanical properties of the alloy were evaluated. It is found that the ARB process can effectively refine the grain of the alloy. After ARB rolling for 8 times, and annealing at 700 ? for 5 min, the microstructure containing full b phase with grain size of 1 ?m can be obtained in the alloy, and the alloy exhibit an improved superelasticity. Under of condition of the pre-strain of 6%, the alloy exhibits a stable superelasticity with a the strain recoveryof 5.94%, and recovery rate of 99%.3. The effects of the annealing temperature and time on the superelasticity of Ti-7.5Nb-4Mo-2Sn alloy after 8 cycles of ARB process were studied. It is found that the temperature and time of the heat treatment have a remarkable influence on the superelasticity of the alloy, The heat treatment at 700 ? for 5 min is helpful to the improvement of the superelasticity of the Ti-7.5Nb-4Mo-2Sn alloy after 8 cycles of the ARB process, and the alloy exhibits the highest recovery rate, which reaches86%.