Influence of Thermal Cycling on the Phase Transformation Temperatures and Latent Heat of a NiTi Shape Memory Alloy

摘要

Shape memory alloys have been proposed as alternatives to actuators that work through thermal cycling, resulting in temperature-induced phase transformations. Forward transformation occurs between martensite start (M_s) and finish (M_f) temperatures and reverse transformation between austenite start (A_s) and finish (A_f) temperatures. Thermal cycling through the forward and reverse martensitic transformations can imply degradation and defects in the material: structural fatigue failure and functional fatigue, which is a loss in shape memory effect in this case [I], sometimes known as actuation fatigue, as well as changes in material properties such as the phase transformation temperatures [2] and the phase transformation latent heat [3]. In this work, Ni_(55)Ti_(45) wires with a diameter of 0.45 mm were subjected to a certain number of thermal cycles, inducing phase transformations in the material. The alloy was annealed for 20 minutes at a temperature of 450 °C before tests. First, a sample was cycled between -40 and 100 degrees Celsius using a Perkin Elmer Pyris 8500 differential scanning calorimeter (DSC), in order to analyze the variation of phase transformation temperatures and observe changes in the endothermic and exothermic peaks. After that, a wire is cycled 80 times while subjected to axial stress of 95 MPa, monitoring its temperature beyond A_f when heated by joule effect and below M_fwhen naturally cooled with an infrared sensor, and then compared with a non-cycled sample. The results and conclusions are shown below.