The Effects Of Thermo Mechanical Treatment On The Texture And Superelasticity Of Ti-7.5Nb-4Mo-2Sn Alloys

? type Ti-based shape memory alloys show important application prospects as the implant materials due to their excellent properties,such as,low density,high specific strength,low elastic modulus,good biocompatibility and good superelasticity,etc.However,in comparison with Ti-Ni alloy,the alloys exhibit a small superelastic recovery strain.Thus,how to improve the superelasticity of ? Ti-based shape memory alloys is a key issue to be solved to meet the demand of their application in practice.It has been well documented that,thermal mechanical treatment(cold deformation and annealing)can produce a number of dislocations and refine the grain size of the alloys,which may promote the critical stress for inducing martensitic transformation,and thus improve the superelasticity of alloys.It is well known that the cold deformation may produce the deformation textures in the alloys,and subsequent annealing treatment may also introduce crystallization textures in the alloys.The martensitic phase transformation and superelasticity of a ? type Ti-based shape memory alloy are sensitive to the presence of textures in the alloys.Thus,the study on the effects of the rolling reduction and the annealing temperature on the microstructures,the evolution of the textures and the superelasticity are important for the improvement of the superelasticity of the alloys.In the thesis,a ?type Ti-based shape memory alloy,Ti-7.5Nb-4Mo-2Sn,was taken as a research object.The alloy was cold rolled at room temperature with different rolling reductions(40%,75%and 95%),and then annealed at the temperatures ranging from 650 ? to 850 ?.The effects of the rolling reductions and annealing temperature on the microstructures,texture and the superelasticity of the alloy were investigated.The main research content and results are as follows.1.The effects of the rolling reduction on the recrystallization temperature and the effects of the annealing temperature on the microstructures of the Ti-7.5Nb-4Mo-2Sn alloy were investigated.The results indicate that with the rolling reduction increasing,the recrystallization temperature of the alloy decreases.The recrystallization temperatures are about 755?,700 ? and 660 ? for the alloy samples with the rolling reductions of 40%,75%and 95%,respectively.The microstructure of the Ti-7.5Nb-4Mo-2Sn alloy after rolled with the rolling reductions of 40%,75%and 95%mainly consists of ? phases.After annealed at the temperatures ranging from 650 ?to 850 ?,recrystallization occur in the alloy,leading to the formation of fine equiaxed grains.With the rolling reduction increasing,the grain size of the alloy after annealed at same temperature decreases.2.The effects of the rolling reduction on the deformation textures of the Ti-7.5Nb-4Mo-2Sn alloy were investigated.It is found that,after cold rolled with the different rolling reductions,the {111}<110>,{111}<112>and {001}<110>types textures are formed in the alloy.With the rolling reduction increasing,the intensity of the textures increasing,and eventually the textures of {111}<110>and {111}<112>become the strongest.3.The effects of the annealing temperature on the recrystallization textures of the Ti-7.5Nb-4Mo-2Sn alloy with different rolling reductions were investigated.The results indicate that with the rolling reduction increasing,the intensity of the deformation texture and the recrystallization texture increases.After annealed at 650 ?,the alloy exhibits the strongest recrystallization texture in y direction.With the annealing temperature further increasing to 750 ? and 850 ?,the recrystallization texture in y direction becomes weaker.Thus,the cold rolled alloy after annealed at 650 ? exhibits the strong recrystallization textures with the strongest texture of {111}<112>.4.The effects of the rolling reduction and annealing temperature on the the superelasticity of the Ti-7.5Nb-4Mo-2Sn alloy were investigated.It is found that the rolling reduction and annealing temperature have an important influence on the superelasticity of the alloy.The alloy sample with the rolling reduction of 95%and the annealing treatment at 650 ? exhibits a superior elasticity,and its strain recovery rate reaches 71.5%.The fine grain size and the {111}<112>texture are responsible for the improvement of the superelasticity of the alloy.