Phase Transformation Characteristics And Mechanical Properties Of NiTi/V In-situ Composites

NiTi-based alloys are the most comprehensive and widely used shape memory alloys,which are mainly attributed to their good shape memory effect and superelasticity.In addition,they are mainly used in aircraft pipe joints.However,NiTi alloys are most sensitive to temperature changes.In order to avoid its reversible martensitic transformation during transportation,they need to be placed in liquid nitrogen,resulting in transportation costs increased.Therefore,it is important to develop a NiTi-based memory alloy with large hysteresis.In order to solve this problem,this paper introduces a low melting point V element to construct a NiTi/V pseudo binary system,and designs three kinds of composite materials according to NiTi/V pseudo binary phase diagram:eutectic Ti₃₇Ni₃₈V₂₅,hypereutectic Ti₂₉Ni₃₁V₄₀ and hypoeutectic Ti₄₆Ni₄₄V₁₀.Secondly,as-cast alloys are prepared by vacuum arc melting furnace and high-frequency magnetic suspension induction melting furnace,and are prepared into wires of different diameters by different drawing processes.Finally,X-ray diffraction?XRD?and scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,differential scanning calorimeter?DSC?and tensile testing methods are used to systematically study the microstructure of alloy ingots and wires and their phase transformation characteristics,mechanical properties after annealing.XRD results show that only the NiTi matrix phase and the V-reinforced phase exist in the as-cast alloy.SEM results show that the microstructure of the as-cast eutectic alloy is lamellar and the structure is more uniform and fine.The single V phase in the as-cast hypereutectic alloy is round and granular,and its whole body like fish bone distributes in the matrix.In the as-cast hypoeutectic alloy,the V phase is distributed in the matrix in the form of a network.TEM results show that after the as-cast alloy is drawn,the V phase undergoes severe plastic deformation and the number of grain boundaries increases.DSC test results of alloys before stretching show that the eutectic alloy only undergoes B2?R phase transformation after annealing at 350⁵00°C,B2?R?B19?phase transformation occurs after annealing at 550°C,and the R phase hysteresis temperature is slightly higher than the other alloys;the hypereutectic alloy only undergoes B2?R phase transformation after the annealing at 350⁶00°C;When the hypoeutectic alloy is annealed at550°C and 600°C,a distinct R?B19'peak is found in the cooling curve.DSC test results of the alloy after stretching show that the B19'?B2 transformation temperature of the hypereutectic alloy is the highest?Tp>100°C?during the first heating process because the severe plastic deformation of the V nanowire release the elastic energy stored in the martensite phase during the stretching process;during the cooling process,the R?B19'transformation temperature of the eutectic and hypoeutectic alloy is higher than before stretching??T>25°C?,due to the residual stress of NiTi and V nanowires promotes the transformation;during the second heating,the B19'?R transformation temperature of the eutectic and hypoeutectic alloy is also higher than before stretching,but the change of temperature??T<10°C?is less than R?B19'transformation.The microhardness test results show that the microhardness of the alloy decreases with temperature increasing after annealing at 350⁶50°C.Tensile test results show that the maximum recoverable strain of the alloy decreases with the increase of annealing temperature;while at the same annealing temperature,the maximum recoverable strain increases with the increase of tensile strain.Among them,the maximum recoverable strain of the hypoeutectic alloy is about 7%,which is higher than the eutectic and hypereutectic alloys.The results of nanoindentation test show that the hypoeutectic alloy has the smallest elastic modulus value when it is treated at the same annealing temperature.When it is the same external force,it is most likely to be elastically deformed and the elastic deformation is the largest,which is consistent with the tensile test results.