Shape memory alloys (SMAs) show a particular behavior that is the ability to recuperate the original shape while heating above specific critical temperatures (shape memory effect) or to withstand high deformations recoverable while unloading (pseudoelasticity). In many cases the SMAs play the actuator's role. Starting from the origin of the shape memory effect, the mechanical properties of these alloys are illustrated. This paper presents a review of SMAs applications in the aerospace field with particular emphasis on morphing wings (experimental and modeling), tailoring of the orientation and inlet geometry of many propulsion system, variable geometry chevron for thrust and noise optimization, and more in general reduction of power consumption. Space applications are described too: to isolate the micro-vibrations, for low-shock release devices and self-deployable solar sails. Novel configurations and devices are highlighted too.Materials 2020, 13, 1856 2 of 16 and novelty of the proposals. Ni-Ti alloys are the most employed ones, but the subject is not limited to them. Origin of the Shape Memory EffectFor the comprehension of the basic principles regarding the shape memory effect and pseudoelasticity, many contributions are useful. The crystallography of martensite, the transformation temperatures, and rate of martensite formation are discussed in details by Nishiyama [6].Materials 2020, 13, x FOR PEER REVIEW 2 of 16 relevance, number of articles, and novelty of the proposals. Ni-Ti alloys are the most employed ones, but the subject is not limited to them. Origin of the Shape Memory EffectFor the comprehension of the basic principles regarding the shape memory effect and pseudoelasticity, many contributions are useful. The crystallography of martensite, the transformation temperatures, and rate of martensite formation are discussed in details by Nishiyama [6]. Figure 1. Binary phase diagram of Ti-Ni alloy [7]. Figure 1

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Keywords:One-way shape memory effect NiTi alloy Heat treatments High-cycle behavior Linear actuator design a b s t r a c t One-way shape memory effect (OWSME) in NiTi springs has been investigated in this work. The main goal is the definition of a guide-line for the design of a linear actuator for high cycles duty. Some SMA and steel springs with various geometrical features have been produced from wires with different diameters. SMA spring's behavior has been analyzed measuring the maximum length (austenitic condition, T > A Faustenite finish) under different applied loads. The measurement of this length has been performed at successive thermomechanical complete working cycles (150, 5350, 43,000 and 600,000) under a constant applied load necessary to full recovery in the martensitic phase. It has been found that the higher the thermomechanical cycles the lower the reached maximum length. In particular the length loss is negligible at higher cycles. Starting from these considerations and the knowledge of the mechanical characteristics of the springs, a linear actuator (SMA spring-harmonic steel spring) for high-cycle duty can be designed. The right working conditions have been verified too.