Martensitic Transformation Characteristics Of Ni₅₀Ti_50-XNb_X Memory Alloy Characteristics And Functional Characteristics Research

Superelastic Ni Ti-based shape memory alloys are widely used due to their large recovery strain,energy absorption and work output.In recent years,the development of aerospace（such as deep space exploration）and other fields requires superelastic Ni Ti-based shape memory alloys to exhibit high-stress superelasticity at low temperature and wide temperature range.However,commercial Ni Ti-based memory alloys cannot meet it.In this thesis,the thermal/mechanical martensitic transformation behavior of Ni₅₀Ti_50-XNb _X is regulated by doping Ni Ti alloy with Nb element and controlling the grain size（adjusting the annealing temperature）,so that the alloy exhibits high transformation stress at low temperature and wide temperature range.elasticity.The main research contents and results are as follows:In this thesis,the wire composed of amorphous phase is obtained by cold drawing deformation of Ni₅₀Ti₅₀,Ni₅₀Ti₄₇Nb₃,Ni₅₀Ti₄₄Nb₆ alloys,and then the amorphous phase is crystallized into nanocrystals by annealing,and the alloy wire composed of nanocrystals is obtained.The research results show that the doping of Nb element increases the crystallization temperature of the amorphous phase of the cold drawn wire.The content inhibits the grain size growth.For example,when the annealing temperature is 500℃,if the doping amount of Nb element is increased from 3 at.%to 6at.%,the average grain size of the alloy decreases from 75 nm to 30 nm.The effects of annealing temperature and Nb element doping amount on the thermally induced martensitic transformation behavior of Ni₅₀Ti_50-XNb _X alloy were also studied.For Ni₅₀Ti₅₀(or Ni_50.2Ti_49.8)alloy,the annealing temperature changes the phase transformation path:as the annealing temperature decreases,the phase transformation path changes from B2→B19?to B2→R→B19?,and then from B2→R→B19?to B2→R,which may originate from the decrease in grain size with decreasing annealing temperature,which leads to an increase in grain boundary density,which inhibits the B2→B19?phase transition,and the B2→R phase transition occurs.When the grain boundary density is further increased,the B2→B19?phase transition is completely suppressed,and only the B2→R phase transition occurs.In addition,it was also found that Nb element doping has a great influence on the phase transition temperature and phase transition path of the alloy.When the Nb doping amount is 3 at.%,the B2→B19?phase transition starting temperature M_s decreases significantly,and with the decrease of the annealing temperature,the annealing temperature decreases.The transition from parent phase to nanodomain occurred during the cooling process;when the Nb doping amount was 6 at.%,the B2→B19?phase transition was completely suppressed,and only the transition from parent phase to nanodomain occurred.In this thesis,the tensile superelastic behavior of nanocrystalline Ni₅₀Ti₅₀,Ni₅₀Ti₄₇Nb₃,Ni₅₀Ti₄₄Nb₆ wire samples at different temperatures was also studied.The results show that with the increase of Nb element doping,the lower limit temperature of the superelastic temperature range of the alloy decreases,specifically:20°C(Ni₅₀Ti₅₀nanocrystalline),-80°C(Ni₅₀Ti₄₇Nb₃nanocrystalline),-196°C(Ni₅₀Ti₄₄Nb₆ nano crystalline);at the same time,Nb element doping increases the superelastic stress of the alloy,from 580 MPa(Ni₅₀Ti₅₀)to 800 MPa(Ni₅₀Ti₄₇Nb₃)and 1500 MPa(Ni₅₀Ti₄₄Nb₆).In addition,the basic problem of the force-induced martensitic transformation of Ni Ti-based memory alloys（a problem debated by the international cooperation team of this research group）is experimentally confirmed.The results show that:when the tensile cycle temperature is constant,the increase,the equilibrium stress of thermoelastic martensite forward and reverse transformation decreases（increase the hysteresis of forward and reverse transformation）;when the amount of tensile strain is the same,with the increase of tensile temperature,the equilibrium stress of forward and reverse transformation of thermoelastic martensite decreases,which proves the existing literature on the problem of forward and reverse phase transition lag symmetry（equal）.In addition,by comparing the reverse transformation temperature of the parent Ni Ti pre-strained and non-pre-strained samples,it was found that pre-strained（the grains were elongated）increased the reverse transformation temperature,confirming the elasticity stored in the martensite phase of the pre-strained samples may not be ignored.