Effect Of Cold Deformation On Mechanical Properties And Phase Transformation Characteristics Of NiTiV Alloys

In recent years,NiTiNb Shape memory alloy had been widely concerned because of its large hysteresis properties,and the large hysteresis properties could be widely used in pipe joints and connectors.The study found that V element is similar to Nb element,and both can undergo eutectic reaction with NiTi,and V has lower specific gravity and melting point,which can further improve the practical application of NiTi-based shape memory alloy.In order to explore the mechanical properties and phase transformation characteristics of Ti Ni V alloy,the effects of different cold deformation and annealing temperature on mechanical properties and the phase transformation characteristics of hypoeutectic Ti₄₆Ni₄₄V₁₀,eutectic Ti₃₇Ni₃₈V₂₅ and hypereutectic Ti₂₉Ni₃₁V₄₀ alloy were studied.The study found that the V solid solution in the microstructure of the three alloys was linear in the matrix.And as the amount of cold deformation increases,the diameter of the linear V solid solution can reach the nanometer level,which is called V nanowire.The study found that both the V nanowire diameter and the NiTi grain size decreased with increase of cold deformation.The annealing treatment after cold deformation had little effect on the V nanowire diameter of all samples,but the NiTi grain size increased with the increase of annealing temperature.The NiTi grain size increases as the annealing temperature increases.This is because the recrystallization phenomenon of NiTi grains occurs when the annealing temperature is above 400?after cold drawing,the recrystallization of NiTi grain results in the increase of NiTi grain size.The proportion of V element in the hypereutectic samples reached 40%,and due to the formation of the primary phase during the smelting process,the V solid solution in the microstructure exhibited micron and nanometer diameters.The tensile test results showed that the critical phase transformation stress and reversible strain of the three alloys increased with the increase in cold deformation.Under the same cold deformation,the critical phase transformation stress of all samples decreases with the increase of annealing temperature.The hypoeutectic sample had more excellent superelastic properties,and the maximum recoverable strain can reach7.5%.However,when the annealing temperature after cold drawing was higher than400?,the maximum reversible strain decreases,which is due to the recrystallization growth of NiTi,resulting in the decrease of the reversible strain.In addition,under the same conditions,the eutectic sample had a high ultimate transformation stress than the hypoeutectic sample.The hypereutectic sample had highest critical transformation stress,but its plasticity was worse.DSC test was used to analyze the phase transformation process of the sample before and after the tensile.The test results show that the non-annealed samples after cold drawing of hypoeutectic,eutectic and hypereutectic did not generate a phase transformation peak,which is related to a large number of dislocations in deformation.As the amount of cold deformation increases,the B2?R phase transformation temperature of the hypoeutectic sample increases.In the first heating process,the hypoeutectic sample had no phase transformation peak,which was duo to the reverse phase transformation that had occurred during the unloading process of the tensile.However,during the first heating of the eutectic sample,a B19'?B2 phase transformation occurred.Analysis of the phase change temperature difference(?T)before and after the tensile shows that the phase change temperature difference corresponding to hypoeutectic,eutectic and hypereutectic samples was positive,which is related to the elastic strain energy and plastic deformation of martensite.In addition,the R?B19'phase change was more sensitive to dislocations and stress than the R?B19'phase change.