Shape memory alloys (SMAs) have two particular well- known effects: the shape memory effect and pseudoelasticity (or superelasticity). These special effects originate from martensitic phase transformation between martensite and austenite. The shape memory effect describes restoration of the original shape of a deformed material upon heating, and it results from the phase transformation from martensite to austenite. The pseudoelastic behavior is a material response in which the original shape restores itself upon unloading after a large deformation. Pseudoelastic SMAs can be considerably deformed without being plastically distorted. Employing these two features SMA's have been used in various forms of actuators. In particular, helical spring are the most widely used SMA actuators. To fabricate SMA elements with a desirable shape from elementary products, specific treatments called "shape setting" are required. Shape setting is accomplished by deforming an SMA specimen to a specified shape, constraining the configuration, and conducting appropriate heat treatments. In this work, shape memory alloy helical springs are produced by shape setting two types of wires, one with shape memory effect (SME) and another with pseudoelastic (PE) at the ambient temperature. To achieve this goal, once the wire has been wounded and locked on a screw, shape setting is performed. Then, to reduce the transformation temperatures, especially temperature , samples are heat-treated. Annealing is performed with the use of a tube furnace under inert atmospheric (Ar). Suitable duration and temperature for heat treatment of the shape set springs must be chosen so that transformation temperatures, especially temperature fall below the ambient temperature. Since many factors affect the behavior of shape memory alloys, several conditions such cooling rate, the temperature and time of heat treatment, and also nickel- rich or titanium- rich compositions give rise to different behaviors. Phase transformation temperatures of the products are measured by Differential Scanning Calorimetry (DSC). The method of cooling or the cooling rate has the greatest impact, and time interval of the heat treatment has the less impact on the transformation temperatures. Increase in cooling rate causes reduction in transformation temperatures and increase in stiffness of springs. Mechanical properties of springs are determined by tension and compression tests at different temperatures. Numerically-predicted force-displacement response of the fabricated pseudoelastic spring are shown to be in good agreement with experimental results. Key Words:Shape Memory Alloys, Transformation Temperature, Heat Treatment, Quasi-Elastic Spring
