Cooling Evolution Behavior Of Nano-domain NiTiNb(Low Nb Content) Superelastic Alloy

Shape memory alloys are super-elastic by stress-induced reversible martensite transformation,and they have been widely used in aerospace,medical and other industrial fields.In recent years,the continuously expanding application scenarios have put forward new requirements for the super-elastic shape memory alloy’s superelasticity,temperature range and other indicators.Previous research by the research group has shown that Ni Ti Nb(high Nb)alloy has excellent characteristics of high super-elasticity and wide temperature range,and preliminary judgment is made that super-elasticity,temperature range and Nb content are related.Reducing the Nb content is expected to obtain an alloy system with moderate super-elasticity but wider temperature range.In this paper,Ni Ti Nb(low-Nb)shape memory alloy wires with different compositions are prepared by smelting,forging,wire drawing,annealing and other processes.The mechanical properties,physical properties,and phase transition characteristics of the wire are initially explored,and the in-situ transmission electron microscope technology focus on the evolution of the microstructure during the cooling process.The results show that Ni Ti Nb(low Nb)alloy wire annealed at different temperatures exhibits good superelastic properties in the temperature range of-150 °C to 150 °C.The high-temperature superelastic stress can reach 1500 MPa,and the lowtemperature superelastic stress is still 600 MPa,which is higher than any of the alloys reported in the entire test temperature range.The slope dσ/d T of the super-elastic stress changing with the stretching temperature is between 3-4 MPa/K,the lower slope makes the alloy still have high super-elastic stress at low temperature,and when the stretching temperature is lower than-100 ℃,the relationship between the superelastic stress and the stretching temperature no longer satisfies the C-C equation,making the relationship between the superelastic stress and the stretching temperature deviate from linearity.Further research found that Ni Ti Nb alloy wire exhibits abnormal resistance increase,modulus softening,and parent phase structure remain unchanged during the cooling process,but high-energy synchrotron radiation shows slight changes in the crystal plane spacing and half-height width of the parent phase during the cooling process.There may be some kind of micro-zone evolution behavior.The in-situ cooling TEM was used to study the microscopic evolution behavior of the cooling process.The mother phase structure remained unchanged during the cooling process,but additional diffraction spots appeared near the diffraction spots of the mother phase,and gradually increased with the decrease of temperature,and the size is basically kept around 10 nm.The martensite structure appeared in the pre-stretched sample.During the subsequent cooling process,the main phase was still the main phase in the sample,and it was not completely transformed into the martensite structure.In the Ni Ti Fe alloy with obvious R transformation behavior,the inconsistency of the phase change occurs in the microregions of the material,some areas change phase first,and some areas have no phase change from beginning to end.These phenomena are similar to the previously reported pre-martensitic transformation and strained glass transition phenomena.The initial guess is related to the obstruction of long-range martensitic transformation.