Introduction
Shape Memory Alloys (SMAs) are unique materials that are able to remember their initialization shape after experiencing significant deformation. Unlike traditional metallic alloys, SMAs have the ability to return to their initial state when shifted to a higher temperature without any external force. Thanks to its properties, SMAs may potentially be a substitute for conventional materials in various applications such as medical, mechanical and aerospace engineering.
Definition of Shape Memory Alloys
Shape Memory Alloys are metallic alloys that display the shape-memory effect, which is a thermally induced structural change in materials that cause them to return to their original shape. This effect is observed when a material is deformed by subjecting it to cold temperatures. The alloy may be stretched, compressed, twisted or bent, but will restore to its original form once heated to its transition temperature. Many alloys have been devised to have shape-memory properties including titanium-nickel alloys, copper-based alloys, and iron-based alloys. Other materials will begin to change shape when cooled below a certain temperature but remain that shape upon reheating, a process that Tech Briefs calls pseudoelasticity.
Structure
Shape Memory Alloys come in two different phases: a martensite phase and an austenite phase. The atomic rearrangement between the two phases is an example of a martensitic transformation, where the material is stored as an austenite phase at a high temperature and is then shifted to a martensite phase at a lower temperature. This transformation occurs because of the change in thermal energy which causes the atoms within the structures of the alloy to reposition within the lattice structure, exploring the range of potential energetically favourable configurations. As the arrangement at the lower temperature is no longer thermodynamically stable, reheating the material shifts it back to its original austenite configuration.
Set and Reset
The heat required for a shape memory alloy to remember its original configuration is called the set or reset temperature, which is typically between 100-360°C. This transformation from martensitic to austenitic form requires an amount of energy known as the latent heat of transformation. The reset temperature is typically 48-100°C and is associated with the reverse process, of change from austenite to martensite.
Properties
Shape memory alloys have a high elastic limit compared to other materials, making them highly suitable for applications where the material is subjected to heavy load or strain. They show good fatigue strength, electrical and thermal properties, and can tolerate a wide range of temperatures and stress levels. In addition, SMAs are low density and have a low modulus of elasticity (which determines how much a material deforms under a given load). This means they can be quickly deformed without fracturing, making them a potential candidate for use in aerospace and wearable technologies. SMAs also have good corrosion resistance, making them an ideal choice for marine and medical applications.
Applications
Shape Memory Alloys have many potential uses and applications in a variety of fields including medicine, engineering, and aerospace. In medicine, their unique shape memory properties make them ideal for implantable implants, which can be introduced into the body in one shape and then move back to their original shape upon exposure to higher temperatures. In mechanical engineering, SMAs are used for tasks such as fastening screwed connections with self-locking nuts or for joining pipes and hydraulic components. In aerospace, they can be used to reduce vibration and sound levels in aircraft, and are also being used to create a sort of smart fuselage.
Conclusion
Shape Memory Alloys are a unique material that can remember its original shape after being deformed when exposed to cold temperatures. They possess many attractive qualities that make them advantageous for different applications in various industries, such as their favorable electrical and mechanical properties, their fatigue strength, and the ability to retain their shape when exposed to a higher temperature. Thanks to these properties, shape memory alloys are becoming a popular choice for many medical and engineering applications.
