SMA (shape memory alloys) materials are a kind of smart material that undergoes shape changes with respect to temperature variations. In this chapter, the authors focus on the microstructure, classifications, fabrication, and future applications of these materials in brief. They discuss the four main types of SMA processing techniques, which include mechanical, thermomechanical, powder metallurgy, and melting route. Finally, its applications in various fields like transportation, civil structures, robotics, aerospace, automation, and medical fields will be briefly discussed.
Top1. Introduction
Shape Memory Alloys (SMAs) are a special material that can regain shape when the temperature rises (Hu et al., 2005). SMAs are also widely referred to as memory alloy, memory metal, muscle wire, and smart metal. These one-of-a-kind materials continued to enhance the achievement in accordance with the demand for the engineering sector in general; form Because of qualities such as increased strength, higher strain recovery, lightweight, high stability, and so on, memory alloys are used in heavy engineering (Zhao et al., 2019). Ni-Ti is the most often used SMAs (Liu et al., 2014). Ag-Cd, In-Th, and Cu-based alloys are some further examples. The sophisticated material called form memory alloy exhibits a shape-memory effect and exceptional elasticity. These qualities distinguish them from other materials (Li et al., 2016). The shape memory effect is concerned with recovering a distorted material to its previous shape during heating; in other words, the material's capacity to restore its shape under thermal load. This feature allows them to restore their original shape after being distorted by heating to a crucial temperature (Velvaluri et al., 2021). During a mechanical loading and unloading cycle, super elasticity deals with non-linear recoverable stresses (Kim et al., 2021). Figure 1 shows the shape memory effect's microscopic phenomenology, and Figure 2 shows the pseudo elastic effect connected with microscopic phenomena.
Shape memory effect-wise, SMAs are classified as one-way and two-way shape memory alloys. Previous demonstrations show that the metal may be bent while unconscious. This will keep the form until the heating level exceeds what is known as the changeover temperature. The materials in the latter exhibit one of their qualities while cold and another when heated (Li et al., 2016) (Kim et al., 2021). Due to the SMAs materials availability having improved characteristics and low manufacturing cost, the use has grown in recent years. Therefore, it has recently prompted an expansion in many technical domains, new economic prospects establishment and research disciplines (Zhang et al., 2017). Among the various uses are civil engineering, medical devices, mechanical engineering, aerospace, and military.
Figure 1. Phenomenology of the microscopic shape memory effect
Difference between SMAs and conventional alloys
The martensite to austenite transition is solely affected by temperature and stress (Shashanka et al., 2014). It is not affected by time since diffusion is not present. Steel lacks shape memory because martensite may be produced by the fast-cooling carbon steel from austenite, an irreversible process. In addition, ordinary steel has a higher yield strength than SMAs; however, some are superior to aluminium or plastic. The amount of recoverable plastic strain that can be created via SMAs makes them stand out (Mohd Jani et al., 2014). Some SMAs can bear the highest recoverable strain of 8% without permanent deformation, which is much greater than the maximum strain in typical steels. Stainless steel has a greater density, thermal conductivity, heat capacity, melting temperature, and vaporisation temperature than Nitinol. Both materials have the same latent heat of melting and evaporation.
Top2. Properties Of Sma
As previously stated, Shape Memory Alloys have distinct characteristics prior to the Martensitic and Austenitic transition. Due to this change, the SMAs material’s mechanical macroscopic behaviour may be divided into two groups, as shown below.