Effect of deformation on martensitic transformation behavior in Ni50.1Ti46.9Nb3 shape memory alloy

Recently, considerable interest has been generated in the Ni-Ti-Nb ternary alloy because of its wide transformation hysteresis [1, 2]. Parts made of this alloy can be stored and transported at ambient temperature, which is convenient for commercial applications. However, as the melting point of niobium is much higher than that of titanium and nickel, compositional deviation often occurs unavoidably even when considerable care is taken during melting. As a result, lowering the niobium content in Ni-Ti-Nb alloys is an effective way to reduce segregation. Besides, Ni-Ti-Nb alloy with low niobium content is more economic. We found that Ni50.1Ti46.9Nb3 (at%) alloy also has an excellent shape memory effect, and a sufficiently wide transformation temperature hysteresis can be attained after deformation at (Ms +30 ◦C) [3]. The purpose of this work was to investigate deformation dependence of the reverse martensitic transformation behavior in Ni50.1Ti46.9Nb3 alloy. The nominal composition of the experimental alloy was 50.1 at% Ni, 46.9 at% Ti and 3 at% Nb. The alloy was prepared by vacuum induction melting in a calcium oxide crucible. The ingots with a weight of ten kilograms were hot swaged and rolled to rods with a diameter of 8.5 mm, and tensile samples 4 mm in diameter were machined from the rods. The samples were solution treated at 860 ◦C for 2.4 ks, followed by water quenching. X-ray diffraction experiments were carried out on a Rigaku D/max–2500pc diffractometer using Cu Kα radiation at 50 kV and 200 mA to identify the phases presented in the sample. Differential scanning calorimetry (DSC) tests were performed on a PerkinElmer Pyris Diamond DSC. The martensitic start temperature Ms, the martensitic finish temperature Mf , the austenitic start temperature As and the austenitic finish temperature Af were −62, −117, −48 and 18 ◦C, respectively. Tensile tests were carried out on a Schimadzu Autograph DCS-10T type machine at a strain rate of 2.8 × 10−4 s−1 and a temperature of (Ms +30 ◦C). The DSC samples after deformation were carefully cut using a low speed diamond saw cooled by water to avoid any undesired phase transformation prior to the measurement. Fig. 1 shows the DSC curves during heating of the samples after deformation to different strain levels. Prior to the measurement the DSC sample was cooled to −150 ◦C so that the residual austenite would trans-