The thermo-mechanical behavior of NiTi-X shape memory alloys

Abstract

A commercial NiTi alloy with the chemical composition of 54.5 Ni-45.5 Ti (mass %) was used as a standard material to be compared with Ti50Ni45Cu5, 48 Ni-37 Ti-15 Nb (mass. %) and 48 Ni- 37 Ti-15 Ta (mass. %). The experimental alloys were cast, annealed and hot rolled in order to render the material more compact and to obtain 0.5 mm-thick lamellas. Two types of specimens were then obtained. The former were fragments with a mass of less than 50 mg that were cut and cleaned in alcohol, in an ultrasonic bath, before being subjected to differential scanning calorimetry (DSC) analysis. The latter were 0.5 × 4 × 25 mm rectangular specimens that were prepared to be subjected to dynamic mechanical analyses (DMA) and optical analysis (OM). The DSC investigation program comprised a heating–cooling cycle with a rate of 10 K/min and was used in order to obtain the critical transformation temperatures of direct and reverse martensitic transformations. DMA measurements consisting in a heating cycle with a heating rate of 5 K/min, a frequency of 1 Hz, a maximum deformation of 20 µm and a maximum of 9 N dynamical applied force, were used in order to observe the mechanical behavior under dynamic conditions. DSC tests suggested that Cu and Ta alloying led to an increase of critical temperatures of reverse martensitic transformation while Nb alloying dropped these critical temperatures as low as 0 °C. In addition, Ta alloying significantly increased the transformation temperatures. All the DMA thermograms emphasized a maximum of internal friction that could be associated with the reverse martensitic transformation. Storage modulus exhibited either continuous or step-like increase during heating, suggesting the presence of a thermoelastic transformation.