Martensitic Transformation And Damping Characteristics Of TiNi-based Shape Memory Alloys

In present work,the influence of composition on microstructure,martensitic transformationbehavior,mechanicalpropertiesanddampingcapacityof(Ti₅₀Ni₅₀)_100-x（Ti₃Sn）_x（x=5,8,15,20at.%）and(Ti₅₀Ni₃₈Cu₁₂)_100-xNb_x（x=0,5,10,15at.%）alloys were investigated by using X-ray diffraction analyzer（XRD）,scanning electron microscopy（SEM）,differential scanning calorimetry（DSC）,tensile testing and dynamic mechanical testing.The influencing mechanism was also analyzed based on the experimental results.The results show that microstructure of TiNiSn alloys consists of B19′martensite and Ti₃Sn phase at room temperature.With the increase of Sn content,the volume fraction of Ti₃Sn phase increases.When the Sn content is 8 at.%,the Ti₃Sn phase distribute in the matrix phase as lamellar structure.A B2?B19′martensitic transformation is observed in TiNiSn alloys upon cooling and heating.With the increase of Sn content,the transformation temperatures decrease due to alloy valence electron concentration increase after addition of Nb.With the increase of thermal cycling number,phase transformation temperatures of TiNiSn alloys decrease gradually.The thermal cycling stability of TiNiSn alloy can be improved by increasing Sn content.The value of compressive stress and compression strain of alloys increases with the increase of Sn content firstly,and then decreases,When the Sn content is 8at.%,The value of compressive stress and compression strain is the maximum,about 1200MPa and 40%respectively.During the continuous cooling,the damping capacity during phase transformation of TiNiSn alloy increases with the increase of Sn content firstly,and then decreases.When the Sn content is 8 at.%,the damping capacity is the maximum,about 0.05.The addition of Nb has significant influence on the microstructure of TiNiCu alloys.Ti₅₀Ni₃₈Cu₁₂ alloy mainly consists of B19 martensite and small amount of Ti₂（Ni,Cu）phase at room temperature.The alloys with the addition of Nb mainly consists of B2 parent phase,β-Nb phase and small amount of Ti₂（Ni,Cu）phase.The B2→B19→B19′martensitic transformation occurs in TiNiCuNb alloys during cooling,and with the increase of Nb content,the phase transformation temperatures decreases gradually,the transformation hysteresis increases gradually.The critical stress to induce martensitic transformation,elongation and tensile strength of TiNiCuNb alloys increases with the Nb content increase.This can be ascribed to improve grain size of alloys after the addition of Nb,and improve the mechanical properties of alloys.The tensile strength and elongation of(Ti₅₀Ni₃₈Cu₁₂)₈₅Nb₁₅ can reach up to 860 MPa and 15%,respectively.After the addition of Nb,the significant superelasticity can be observed,and the recovery strain increases with increasing Nb content.During the continuous cooling,two damping peaks are observed for Ti₅₀Ni₃₈Cu₁₂ alloy,which corresponds to B2→B19 transformation and the overlapped peak of B19→B19′transformation and relaxation.With the increase of Nb content,the peak value of B2→B19damping peak slightly,and the overlapped damping peak becomes no longer clearly.The peak value of B2→B19 damping peak of(Ti₅₀Ni₃₈Cu₁₂)₈₅Nb₁₅ alloy can reach up to 0.098.