Study On Phase Transformation Characteristics And Superelasticity Of NiTiV Shape Memory Alloys

NiTi-based shape memory alloys have been widely used in medical devices,structural damping and other fields.In recent years,China’s aerospace field（such as deep space exploration）have required Ni Ti-based memory alloys to exhibit high superelastic stress in a wide temperature range,with has the characteristics of low specific gravity at she same time.Ni Ti Nb memory alloy obtained by doping Ni Ti with Nb element was found to shows high superelastic stress in the wide temperature range in our study group,but proportion the of Ni Ti Nb alloy is large due to the high density of Nb element.In order to solve this problem,V element doped Ni Ti alloy with the same main group as Nb element was selected in this thesis.Ni₅₁Ti₄₈V₁ alloy and Ni₅₀Ti_50-xV_x（x=1,2,3）series alloys were designed and prepared.Nanocrystalline Ni Ti V alloy wire was obtained by large deformation cold drawing and annealing.The effects of V element doping and different annealing temperatures on martensitic transformation and superelasticity of Ni₅₀Ti_50-xV_x（x=1,2,3）alloy were investigated,and the effects of aging treatment on martensitic transformation and superelasticity of Ni₅₁Ti₄₈V₁ alloy were investigated.The main research contents and results are as follows.Firstly,the crystallization temperature of amorphous Ni₅₀Ti_50-xV_x（x=1,2,3）alloy was determined.It was found that the crystallization temperature of the alloy increased with the increase of V element content.On this basis,it is found that the annealing temperature affects the type and path of thermal induced phase transition of the alloy.With the increase of annealing temperature,the transformation path changes from B2→R to B2→R→B19’.When the annealing temperature exceeds 550℃（including 550℃）,the alloy undergoes B2→B19’phase transition.Secondly,the martensitic transformation behavior of nanocrystalline Ni₅₀Ti_50-xV_x（x=1,2,3）alloy during heating and cooling was studied.The results show that B2→R phase transformation occurs in nanocrystalline Ni₅₀Ti_50-xV_x（x=1,2,3）alloy annealed at 400°C during heating and cooling.However,this phase transformation is inhibited and the transformation amount decreases with the increase of V element content.This thesis also studied the effect of aging treatment on the phase transformation path of Ni₅₁Ti₄₈V₁ alloy.The results show that the martensitic transformation start temperature M_s of Ni₅₁Ti₄₈V₁ alloy increases with the increase of aging temperature and aging time,and the effect of aging temperature on M_s is more obvious.In addition,it is also found that aging treatment affects the superelastic properties of Ni₅₁Ti₄₈V₁ alloy.With the decrease of aging temperature,the superelastic stress of Ni₅₁Ti₄₈V₁ alloy increases and the stress hysteresis decreases.After aging at 300°C for 8h,the superelastic stress during tensile process at room temperature is 610 MPa,and the residual strain is as small,as 0.3%,which is better than that of Ni Ti V alloy reported in the literature.In addition,the average stress hysteresis is less than115MPa,far less than the reported traditional Ni Ti alloy.In this thesis,the superelasticity of nanocrystalline Ni₅₀Ti_50-xV_x（x=1,2,3）alloys was also investigated.The results show that with the increase of V element content,the hyperelastic temperature range of nanocrystalline Ni₅₀Ti_50-xV_x（x=1,2,3）alloy is significantly broadened and the hyperelastic stress is increased.Among them,the superelastic temperature range of Ni₅₀Ti₄₇V₃ alloy is-100°C～80°C,and the maximum superelastic stress is about 1.3 GPa,which is significantly larger than the reported traditional NiTi alloy.