Microstructure And Mechanical Behavior Of TiNiSn Shape Memory Alloys

In the present work, microstructure, martensitic transformation behavior, mechanicalbehavior and damping characteristic of Ti50+xNi50-2xSnx(x=1,2,4,6,8at.%) were studied. Theeffect of Sn content on microstructure of TiNiSn alloys were studied by using the X-raydiffraction analysis, optical microscope, scanning electron microscope and transmissionelectron microscope. The martensitic transformation behavior of TiNiSn alloys was studiedas a function of Sn content and thermal cycling number by using differential scanningcalorimetry The tensile test was used to investigate the effect of Sn content and deformationtemperature on mechanical behaviors of TiNiSn alloys. The damping capacity of TiNiSnalloys were investigated by the dynamic mechanical testing.The results show that Sn addition significantly influences the microstructure of TiNiSnalloys. TiNiSn alloys consist of two phases at room temperature, the matrix phase is B19′martensite phase andthe second phase is Ti3Sn. When the Sn content is lower than4at.%, thesecond phase with a shape of small block or long strip shows a grid distribution in the matrix.When the Sn content is higher than4at.%, most of the second phase distribut in the matrixphase as lamellar eutectic structure. With the increase of Sn content, the volume fraction ofTi3Sn phase increases.During cooling and heating, a B2B19′one step martensitic transformation and reversetransformation occurs in the TiNiSn alloys. The increase of Sn content has no significanteffect on the martensitic transformation temperature. However, with the increase the contentof Sn, latent heat of phase transformation decreases gradually. This is mainly because on theone hand, the alloy valence electron concentration does not change significantly with theincrease of Sn content. On the other hand, there is no martensitic phase transformation forTi3Sn during heating and cooling. The transformaiton temperatures drop continuously withthe increase of thermal cycling number, when the thermal cycling number is more than30times, the transformation temperatures keep almost constant. With increasing Sn content, thethermal cycling stability becomes better.The content of Sn has significant influence on the mechanical behavior of TiNiSn alloy.When the Sn content is lower than2%, the stress-strain curves exhibita stress platform. When the Sn content iss higher than4%, the stress-strain curves are mainly characterized by acontinuous work-hardening. With the increase of Sn content, the fracture strength andelongation decreases at room temperature. Fracture morphology observation shows that, thefracture of TiNi phase in the alloy is dominant by a ductile manner, and the Ti3Sn phase isbrittle. The effect of Sn content on the shape recovery ratio is related to the deformation strain.when the strain is less than3.8%, the shape recovery ratio decreases with the Sn contentincrease. When the deformation strain is larger than5.8%, the shape recovery ratio increaseswith the increase of Sn content firstly, and then decreases. With the increase of Sn content,the damping capacity almost linearly decreases. This can be ascribed to the change of volumefraction and distribution of Ti3Sn phase..