Microstructure And Properties Of The NiTi-Nb Eutectic Shape Memory Alloys

NiTiNb alloys have drawn attention thanks to the wide hysteresis,which obtained after deformation under certain conditions.At present,most works of NiTiNb alloys focused on the chemical composition of Ni₄₇Ti₄₄Nb₉,which is the most comprehensively reported.The Nb content affects the microstructure and phase transformation characteristics of NiTiNb alloys,directly.It is found from the pseudo-binary phase diagram of NiTi-Nb alloys that Ni₄₀Ti₄₀Nb₂₀ is a NiTi-Nb eutectic alloy.The previous reports of Ni₄₀Ti₄₀Nb₂₀ focuses on its properties such as hydrogen permeability,eutectic brazing,and liquid sintering.The phase transformation characteristics,mechanical properties and shape memory properties of NiTi-Nb eutectic have seldom been investigated.Therefore,in this project,the microstructure and phase transformation characteristics,mechanical properties and shape memory properties of NiTi-Nb eutectic shape memory alloy have been systematic investigated.It will be a great foundation for further engineering application of the NiTiNb alloys.In present work,a vacuum suspension induction melting furnace is used to melt the Ni_50-x/2Ti_50-x/2Nb_x?x=18,20,22 at.%?hypoeutectic,eutectic and hypereutectic shape memory alloys.The influence of different Nb content and different solution treatment temperature on NiTi-Nb eutectic alloys were investigated.The microstructure and phase transformation characteristics were investigated by scanning electron microscopy coupled with energy dispersive spectrometer?SEM/EDS?,differential scanning calorimetry?DSC?,X-ray diffraction?XRD?.The mechanical properties were investigated by mechanical compression experiment.And the shape memory properties of NiTi-Nb eutectic alloys were investigated by tube-expansion-recovery method.The results show that the microstructure of Ni₄₀Ti₄₀Nb₂₀ eutectic alloy only contains the NiTi-Nb eutectic structure.The Ni₄₁Ti₄₁Nb₁₈ hypoeutectic alloy contain the primary NiTi phase and NiTi-Nb eutectic structure.The Ni₃₉Ti₃₉Nb₂₂ hypereutectic alloy contain primary Nb-rich phase and NiTi-Nb eutectic structure.As the solution treatment temperature increases,the microstructure of the Ni_50-x/2Ti_50-x/2Nb_x?x=18,20,22 at.%?alloys with the same composition does not change significantly,but the Ni/Ti ratio in the NiTi phase decreases.During heating and cooling,the phase transformation of austenite and martensite?B2?B19'?exhibited in the Ni_50-x/2Ti_50-x/2Nb_x?x=18,20,22 at.%?alloy.For the Ni_50-x/2Ti_50-x/2Nb_x?x=18,20,22 at.%?alloy with the same chemical component,as the increment of solution treatment temperature,the martensite start transformation temperature?M_s?decreases.It is very interesting that two peaks occurred in the DSC curves of the Ni₄₁Ti₄₁Nb₁₈ alloy solution treated at 750?and 850?.It has been found that the difference of Ni/Ti ratio between the primary NiTi matrix and NiTi in eutectic structure resulted the appearance of two peaks.With the increment of thermal cyclys,the M_s,martensitic transformation latent heats(?H_B2?B19?)and reverse martensite transformation latent heats(?H_B19??B2)all decrease.Stress-induced martensite transformation yield platform appeared in the true stress-strain curves of as-cast and solution treated Ni_50-x/2Ti_50-x/2Nb_x?x=18,20,22 at.%?alloys during mechanical compression experiment at room temperature.With the increment of solution treatment temperature,the critical stress?_sim and yield stress of stress-induced martensitic transformation of Ni_50-x/2Ti_50-x/2Nb_x alloy decrease.For alloy systems with NiTi-Nb eutectic structure,the content of eutectic structure directly affects the yield strength and plasticity of the alloy.After 16%expansion at-20?,the shape recovery rates of Ni_50-x/2Ti_50-x/2Nb_x?x=18,20,22at.%?alloys were 57.14%,54.76%and 52.38%,respectively.The temperature of the expansion more close to M_s,the smaller diameter inside,and the lower Nb content,lead to the higher shape memory properties of Ni_50-x/2Ti_50-x/2Nb_x alloys.