Phenomenological Study of the Shape Setting of NiTi WiresThursday (17.05.2018) 12:00 - 12:25 Part of:
In order to use NiTi-based Shape Memory Alloys (SMA) for shape recovery, actuation, and superelastic applications, NiTi elements are prepared through thermomechanical heat treatments that bring them into a desired initial shape. This process is conventionally called “shape setting”, and is equally applicable to cold-worked and annealed elements (shape memory and superelastic NiTi). The main shape setting procedure consists in constraining the shape of a NiTi element on a fixture of the desired shape, then heating it up for times and temperatures that vary according to the alloy composition and its heat treatment history. For instance, the temperatures needed for shape setting cold-worked NiTi elements are usually higher than those for shape setting superelastic and shape memory NiTi elements. Moreover, shape setting of cold-worked NiTi, apart from shape changes, also gives the material its functional properties. In a contrast, shape memory and superelastic NiTi elements, although they can be shape set at lower temperatures, may exhibit worsening of their functional properties if the shape setting is not performed properly. This complicates seriously shape setting of annealed NiTi elements for NiTi-based devices. Fundamentally, the shape setting of NiTi is very interesting, since it involves reverse martensitic transformation into plastically deformed austenite. Since the interaction of phase transformation and dislocation slip in NiTi remains largely unclear in spite of recent efforts in the literature, and shape setting of NiTi belongs among key procedures in NiTi technologies, it deserves further research attention.
In this work, we investigated the shape setting of cold-worked and annealed NiTi wires by series of thermomechanical tensile experiments in a standard tensile deformation rig equipped with a dedicated thermal chamber. A constrained shape was assured via a DIC extensometer, the signal of which was used as feedback for the PID controller of the deformation rig. In addition, the electrical resistivity of the wire was evaluated and dynamic mechanical analysis (DMA) was performed during controlled heating-cooling cycles. The combination of electrical resistivity, stress, and stored modulus data allowed for better understanding of the thermomechanical processes occurring during the shape setting of cold-worked and annealed NiTi wires.