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

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.

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The effects of nitrogen and oxygen absorption on lattice expansion of Ti-6Al-4V have been investigated by high-temperature X-ray diffractometry. Experiments have been performed on stress-free powder for its high surface-volume ratio and to avoid possible effects on diffraction patterns due to texture change and recovery of defective structures induced by the measurement cycle at temperature. Cell parameters a and c, measured at increasing temperatures up to 600°C, show linear trends with different slopes. As temperature increases, the cell volume expands and the c/a ratio changes. This is due to both lattice thermal expansion and absorption of oxygen and nitrogen. Part of the gas remains entrapped in the metal after cooling to room temperature causing a residual lattice distortion. Results have been compared with literature data obtained in analogous tests performed on bulk Ti-6Al-4V alloy both in vacuum and in inert gas atmosphere.

Effects on metal targets after an explosion include the following: fracture, plastic deformation, surface modifications, and microstructural crystallographic alterations with ensuing mechanical properties changes. In the case of small charge explosions, macroscopic effects are restricted to small charge-to-target distances, whereas crystal alterations can still be observed at moderate distances. Microstructural variations, induced on gold-alloy disk samples, as compared to previous results on AISI 304Cu steel samples, are illustrated. The samples were subjected to blast-wave overpressures in the range of 0.5 to 195 MPa. Minimum distances and peak pressures, which could still yield observable alterations, were especially investigated. Blast-related microstructural features were observed on the explosion-exposed surface and on perpendicular cross sections. Analyses using X-ray diffraction (XRD) were performed to identify modifications of phase, texture, dislocation density, and frequency of mechanical twins, before and after the explosions. Optical metallography (OM) and scanning electron microscopy (SEM) observations evidenced partial surface melting, zones with recrystallization phenomena, and crystal plastic deformation marks. The latter marks are attributed to mechanical twinning in the stainless steel and to cross-slip (prevalent) and mechanical twinning (possibly) in the gold alloy.

The shape recovery efficiency of Ni-Ti shape memory springs has been investigated upon the application up to 6 X 105 thermo-activation cycles. The hysteretic behaviour of the Martensitic-Austenitic phase transition has been characterized by resistivity measurements and infrared thermography. A loss in the recovery efficiency of the original shape has been observed and has been ascribed to functional fatigue leading to the formation of the R phase upon sample heating. Nevertheless, one way shape memory effect was found to exhibit an asymptotic stable behaviour which makes possible the realization of Ni-Ti actuators able to operate for a relative large number of activation cycles.

Solar sails are propellantless systems where the propulsive force is given by the momentum exchange of reflecting photons. Thanks to the use of shape memory alloys for the self-actuation of the system, complexity of the structure itself has decreased and so has the weight of the whole structure. Four self-deploying systems based on the NiTi shape memory wires have been designed and manufactured in different configurations (wires disposal and folding number). The deployed solar sails surfaces have been acquired by a Nextengine 3D Laser Scanner based on the Multistripe Triangulation. 3D maps have been pre-processed through Geomagic Studio and then elaborated in the Wolfram Mathematica environment. The planarity degree has been evaluated as level curves from the regression plane highlighting marked differences between the four configurations and locating the vertices as the most critical zones. These results are useful in the optimization of the best folding solution both in the weight/surface reduction and in the planarity degree of the solar sail.

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