# Application of Virtual Reality for Learning the Material Properties of Shape Memory Alloys
## 2 SMA Properties and Applications  
Pure metals contain the same kind of atoms which are closely stacked, while alloys are composed of different metal atoms and they may be metal elements in a solid solution with a single phase or a mixture of two or more metallic solutions with multiple phases. Most shape memory alloys have two distinct phases, i.e., martensite and austenite. The former exists at lower temperatures, and the latter exists at higher temperatures [28]. When pure metals are deformed exceeding the elastic range by an external force, they cannot recover the original shapes by heating. In contrast, when the SMA is in the martensite phase and plastically deformed within the limited extent, it can be transformed into the austenite phase by heating to the transition temperature, where the SMA will restore its original shape (Figure 3). If the temperature goes below the normal temperature again, the SMA will still remain the same shape. The shape memory property can be applied to develop a variety of intelligent products. Therefore, SMAs belong to the area of intelligent materials. Ni-Ti alloy is the most important and widely used SMA because of its excellent performance in shape recovery, durability, and corrosion resistance. Other attractive properties include high mechanical strength, super elasticity, biological compatibility, and stable transition temperature, providing it with a high potential for engineering and medical applications. As a result, the Ni-Ti alloy is suitable for such industries as aviation, robotics and medical treatment. However, the Ni element, a toxic metal, may affect human health and increase the risk of cancer, so it is only used in temporary appliances, such as dental braces and vascular stents. The Ni-Ti alloy is composed of about 50% Ni and 50% Ti, and the proportion can be adjusted according to the requirements of different applications, such as the range of transition temperatures and mechanical properties. In general, the transition temperature is lower if the content of Ni is higher [29]. For example, the transition temperature of the alloy with 50% Ni and 50% Ti is 40 \({}^{\circ}\)C, and the alloy with 55% Ni and 45% Ti has shape memory capability at room temperature. If the content of Ni is lower than Ti, the alloy's transition temperature can reach 120 \({}^{\circ}\)C. Basically, the SMA shape memory effects can be divided into the following three types [30] (Figure 4):  
* One-way shape memory effect  
When the SMA is in the martensite phase, it can be deformed and will remain this shape until its temperature is raised above the transition temperature. Upon heating, the SMA transforms into the austenite phase and restores its original shape. The SMA will remain in this shape after cooling until deformed again. Figure 3: Transformation between the martensite and the austenite phases. * Two-way shape memory effect  
When the SMA is deformed with/without an external force and heated past the transition temperature, the SMA will enter the austenite phase and return to the original shape. When the temperature goes below the transition temperature, the SMA will automatically return to the memorized shape in the martensite phase. This is called the intrinsic two-way effect. * All-round shape memory effect  
When there is more Ni in the Ni-Ti alloy, the SMA will have an abnormal memory effect similar to the two-way shape memory effect, but its deformation is more obvious and intense. Therefore, the SMA will automatically shrink when its temperature goes below the transition temperature. When the temperature is lower, it will change to the reverse direction of the deformation. The SMA will restore the memorized shape if the temperature goes above the transition temperature of the austenite phase again. In the automotive industry, SMAs can be applied in actuators, such as pneumatic, hydraulic, and motor-based systems, because of its solid-state and lightweight properties [31].