Microstructure And Martensitic Transformation Behavior Of An Ultrafine Grained TiNiNb Shape Memory Alloy

In present work, microstructure, martensitic transformation behavior, mechanical properties and shape memory effect of an ultrafine grained Ti₄₄Ni₄₇Nb₉ shape memory alloy?SMA? processed by equal channel angular pressing ?ECAP? were systematically investigated.The effect of annealing on microstructure of the as-ECAP processed Ti₄₄Ni₄₇Nb₉ alloy was investigated by using transmission electron microscope?TEM?, electron backscattering diffraction ?EBSD?, scanning electron microscope ?SEM? and energy dispersive spectroscopy ?EDS?, X-ray diffraction ?XRD? analysis. Martensitic transformation behavior of the intial coarse-grained and ultrafine grained Ti₄₄Ni₄₇Nb₉ alloy was compared by using differential scanning calorimetry ?DSC?. The effects of annealing, thermal cycling and deformation in martensitic state on martensitic transformation behavior were also investigated.The tensile and bending tests were used to investigate mechanical properties and shape memory effect of the ultrafine grained Ti₄₄Ni₄₇Nb₉ SMA.The results show that the microstructure of Ti₄₄Ni₄₇Nb₉ alloy consists of the matrix phase with B2 structure??-Nb phase and a little ?Ti,Nb?2Ni phase at room temperature. The initial Ti₄₄Ni₄₇Nb₉ alloy has a mean grain size of 2.27?m. After ECAP processing, the microstructure is characterized by refined grain size of about 288nm and a large number of dislocations, and it is inhomogeneous, i.e. the equiaxial grains appear in some regions and the elongated grains appear in other regions.The grain size of as-ECAP processed alloy increases with increasing annealing temperature or duration.A B2???B19' one step martensitic transformation and reverse transformation occurs in the Ti₄₄Ni₄₇Nb₉ alloy during cooling and heating. Compared with the intial alloy, the martensitic transformation temperature of as-ECAP processed Ti₄₄Ni₄₇Nb₉ alloy is sharply reduced and the transformation hysteresis is increased, due to the grain refinement and the dislocations introduced during ECAP. With increasing annealing temperature or duration, the martensitic transformation temperatures and reverse transformation temperatures of the as-ECAP processed alloy increase, however, the transformation hysteresis decreases.The martensitic transformation behavior of as-ECAP processed Ti₄₄Ni₄₇Nb₉ alloy shows a significantly enhanced thermal cycling stability as compared to the initial alloy. After deformation in martensite,Ti₄₄Ni₄₇Nb₉ alloy exhibits a multi-step reverse transformation upon heating and the increase of revser transformation temperature. The transformation hysteresis of the as-ECAP processed alloy is greater than the initial counterpart, which increases with increasing pre-strain and decreases with increasing annealing temperature.After ECAP processing, the yielding strength and tensile strength of Ti₄₄Ni₄₇Nb₉ alloy are obviously increased, but the elongation decreases. This can be attributed to the refine grain size and higher dislocation density in the as-ECAP processed alloy. The shape memory effect and thermal-mechanical cycling stability for as-ECAP processed Ti₄₄Ni₄₇Nb₉ alloy are significantly improved. The shape recovery ratio decreases and shape recovery strain increases with increasing deformation strain. Furthermore, they deteriorate gradually with increasing annealing temperature.