Shape memory alloys (SMAs) present some characteristics, which make it unique material to be use in applications that require strength and shape recovery. This alloys was been used to manufacture smart actuators for mechanical industry devices and several other applications in areas as medicine, robotics, aerospace, petroleum and gas industries. However it is important to know the actuators response to external stimulus (heat source, electrical current and/or external stress) in these technological applications. This work investigated the thermomechanical behaviors of helical actuators produced from Ti-Ni alloy commercial wires. Initially, the wire was subjected to some heat treatment and characterized by differential scanning calorimeter (DSC), scanning electron microscopy (SEM), optical microscopy (OM) and Energy dispersive spectroscopy. Then two heat treatments were selected to obtain the helical actuators. The actuators were tested in an apparatus developed to apply an external traction stress in helical actuators during thermal cycles. Two wires were tested in a dynamic mechanical analyzer (DMA). The results were analyzed in comparison with thermoplastic properties obtained in thermomechanical tests. The analysis took into consideration the wiring forming process, the precipitates formation, the stress fields generated by dislocations and reorientation of martensite variants during the actuators training process.

This article studies changes observed on the R-phase thermoelastic behavior in a near-equiatomic Ti-Ni shape memory alloy. Three kinds of procedures have been performed: different treatments, thermomechanical cycling under constant loading in shape memory helical springs and thermal cycling in as-treated and trained samples. Several heat treatments were carried out to investigate evolution of the R-phase by differential scanning calorimetry (DSC). A heat treatment was chosen on which R-phase is absent. Shape memory springs were produced and submitted to a training process in an apparatus by tensioning the springs under constant loading. Thermal cycling in DSC was realized in as-treated and trained samples. Several aspects of one-step (B2→B19') and two-steps (B2→R→B19') martensitic transformations and R-phase formation and their evolution during tests were observed and discussed.

In this work, superelastic behavior of single crystals shape memory alloys has been investigated by the electric resistivity (ER) measure technique coupled simultaneously during traction tests. Samples in austenitic phase were submitted to stress induced martensite (SIM) under several conditions such as: different temperatures, different deformation rate, throughout successive martensitic transformations and different crystallographic orientation axes. ER vs. deformation curves presented a linear relation without hysteresis and independence on temperature. In the case on the increasing of deformation rate, these curves show a little hysteresis due to adiabatic transformations. For samples submitted to successive transformations, ER curves showed changes with induced martensite types. In samples of the same alloy with different crystallographic orientations was observed that single variant martensite nucleation presented distinct values, this characterizes martensite electric resistivity anisotropy.

This paper present a thermomechanical study of actuators in form of helical springs made from shape memory alloy wires that can work as actuator and/or as sensor. These abilities are due to the martensitic transformation. This transformation is a diffusionless phase transition that occurs by a cooperative atomic rearrange mechanism. In this work, helical spring actuators were manufactured from Cu-Zn-Al shape memory alloy wires. The springs were submitted to constant tensile loads and thermal cycles. This procedure allows to determine thermoelastic properties of the shape memory springs. Thermomechanical properties were analyzed during 50 thermal cycles in the temperature range from 20 to 130 °C. Results of variations in critical transformation temperatures, thermoelastic strain and thermal hysteresis are discussed based on defects rearrangement and martensitic transformation theory.