Effects Of Low Temperatue And Two-step Aging On Phase Transformation Behavior And Mechanical Properties Of Ni-rich TiNi-based Alloys

This thesis aims to study the effect of low temperature aging and two-step aging on the phase transformation behavior and mechanical properties of Ni-rich NiTi-based alloy.Four Ni-rich NiTi-based shape memory alloys,Ni_51.5Ti_48.5,Ni_51.3Ti_48.2Nb_0.5,Ni_51.3Ti_47.7Nb₁and Ni_51.3Ti_46.7Nb₂were fabricated.The phase transformation behavior,mechanical properties and evolution of microstructures were investigated by using differential scanning calorimetry,resistance test,transmission electron microscope,X-ray diffraction and tensile test.After low temperature aging,it is found that annealing at 600℃and then aging at250℃has the most significant effect on the transfommation behavior of the sample.The most prominent change of phase transformation behavior after low temperature aging came from Ni_51.3Ti_48.2Nb_0.5sample.It is found that the early stage of low temperature aging can inhibit the martensitic phase transformation,and lowered the martensitic phase transformation temperature significantly.Prolonging the aging time can increase the phase transformation temperature of M and R.In addition,the thermal hysteresis of the alloy increases first and then decreases.The possible mechanism is that there are some Ni clusters formed during early stage of aging due to insufficient energy for diffusion.These clusters were evenly dispersed in the matrix and hindered the occurrence of phase transformation.In the later stage,the aging time is prolonged,there are sufficient conditions to form large Ti₃Ni₄precipitation,and the inhibition effect on phase transformation is weaken.The phenomenon that the enthalpy first decreases and then increases with the extension of aging time also proves this point.In the two-step aging experiment,the solution samples undergo the first step aging of high temperature and short time,and the second step of low temperature and long time.It is found that the aging at lower temperature in the second step can obviously increase the phase transformation temperature and affect the mechanical properties of the samples.This is due to the precipitation behavior caused by the difference in the solid solubility of Ni atoms at different temperatures.In the first step,Ni atoms can rapidly diffuse at high temperature aging and then precipitate,while in the second step,when the temperature is lowered,the solid solubility of Ni atoms decreases and Ti₃Ni₄was precipitated further.The Ni content in the matrix was decreased,which promotes the phase transformation and increases the phase transformation temperature.