Thermal Cycling Stability Of Martensitic Transformation And Mechanical Characteristics Of TiNiCuNb Shape Memory Alloys

In present work,microstructure,martensitic transformation behavior,mechanical properties and damping properties of(Ti₅₄Ni₃₄Cu₁₂)_100-xNb_x and(Ti₅₁Ni₃₆Cu₁₃)_100-xNb_x?x=0,6,10at.%?alloys were studied byscanning electron microscope,X-ray diffractometer,differential scanning calorimeter,electronic universal material testing machine,and dynamic thermomechanical analyzer The influence of thermal cycling on martensitic transformation behaviorwas paied more attention.Results show that at room temperature,microstructure of the alloy without Nb mainly consists of martensite with B19 structure and Ti₂?Ni,Cu?phase.After adding Nb element,room temperature microstructure of the alloy is mainly composed of B19 martensite,Ti₂?Ni,Cu?phase and?-Nb phase.With the increase of Nb content,the volume fraction of?-Nb phase increases,while that of Ti₂?Ni,Cu?phase decreases.During cooling and heating,a one-step martensitic transformation and its reverse transformation of B2?B19 occurs in TiNiCuNb alloy.As the Nb content increases,the transformation temperatures decrease.This is mainly related to changes in the composition of the alloy matrix.Thermal cycling does not affect the type of phase transformation of TiNiCuNb alloy.The martensitic transformation exhibits excellent thermal cycling stability.The addition of Nb element can further improve the thermal cycling stability of martensitic transformation.The(Ti₅₄Ni₃₄Cu₁₂)₉₀Nb₁₀ alloy has the best cycling stability.After 500 thermal cycles,the peak temperature of martensitic transformation changed by only 0.1 ^oC,and the peak temperature of reverse transformation changed by 0.2 ^oC.This can be mainly attributed to the fact that the addition of Nb does not significantly change the lattice parameters,thus the middle eigenvalue of the transformation stretch tensor is close to 1,futhermore,the addition of Nb can suppress the generation and movement of dislocations through improving the yield strength.With the increase of Nb content,the critical stress to induce martensitic transformation,elongation and tensile strength of the alloy increases to some extent.In particular,the tensile strength and elongation of(Ti₅₁Ni₃₆Cu₁₃)₉₀Nb₁₀ alloy reaches 810MPa and 12.7%,respectively.TiNiCuNb alloys exhibit good superelasticity.With the increase of Nb content,the superelastic recovery strain of Ti₅₄Ni₃₄Cu₁₂ base alloy increases little,whilethat of Ti₅₁Ni₃₆Cu₁₃ base alloy increases significantly.(Ti₅₁Ni₃₆Cu₁₃)₉₀Nb₁₀ alloy exhibits the best superelastic recovery characteristics,and its maximum superelastic recovery strain can reach up to 2.79%.During continuous cooling and cooling,TiNiCuNb alloy exhibits a damping peak corresponding to B2?B19 phase transition.With the increase of Nb content,the peak value of B2?B19 phase transformation damping peaks slightly decreases.The phase transformation damping peaks of Ti₅₁Ni₃₆Cu₁₃-based alloys are significantly higher than those of Ti₅₄Ni₃₄Cu₁₂-based alloys.Among them,the damping peak of B2?B19 of(Ti₅₁Ni₃₆Cu₁₃)₉₄Nb₆alloy is about 0.096.