What are Shape Memory Alloys? A Guide to Their Properties and Applications

SMA stands for shape memory alloys, which are smart materials that can remember their shape and change according to heat. This article by the authors would inform readers of what SMAs are, how they work, some common materials like Nitinol, applications in aerospace, robotics, medicine, and so on, working with them, challenges, and the ongoing research directions.

The Magic of Shape Memory Alloys: Materials That Remember Their Shape

SMAs, or shape memory alloys, are special metals that tend to change their shape in response to a change in temperature. The atoms in SMAs tend to line up in two different crystal structures.

Atomic Structure

SMAs are made up of really tiny atoms that fit together in patterns called crystal structures. At one temperature, the atoms prefer to fit close together in a squeezed structure called martensite. At a higher temperature, they spread out into an open structure called austenite. Being able to switch between these atomic arrangements is what gives SMAs their special shape memory alloys.

Changing Shape with Heat

If an SMA is bent out of shape while in its martensite structure, it will remember that new shape. But when heated above a certain point, it transforms to its austenite structure and springs back to its original, unbent shape. This is called the shape memory effect. For example, an SMA wire could be bent but then go straight again when heated with hot water.

Getting Stuck in a Bent Shape

Some SMAs exhibit another cool effect called superelasticity or pseudoelasticity. If they are bent past a point while cold, their atoms abruptly shift to the martensite structure to accommodate the new shape without breaking. But when unbent, they smoothly transform back to austenite without needing heat. It’s like the metal gets stuck in the bent shape memory alloys until unbent.

How Nitinol Helps the Body

Nitinol is such a unique 3D print metal that can have memories of their shape. A doctor uses nitinol in the human body for its type of memory known as the shape memory alloys of the metal. It helps fix problems and give medicine.

Open the stenotic part of the artery

Sometimes fatty deposits build up inside arteries and block the flow of bloodThis is called a blockage. Doctors insert tiny mesh tubes called stents to push open clogged arteries. Stents are made from nitinol. They are squeezed small and put into arteries using a thin flexible tube called a catheter. Once in place, the stent gets warmed by body heat and pops open to its unblocked shape, keeping arteries wide open for blood to flow.

Joint Replacements Relieve Pain

Another application is using nitinol to replace parts of a joint, which has become worn out- such as the knee or hip.Joint replacements allow smooth movement. Nitinol implants are bendy so they move like real bones. They are shaped to fit the body during surgery and then remember that shape. This helps implants last a long time without wearing down.

Medicine on Time

Doctors are making tiny containers from nitinol to carry medicine inside the body. Medicine is stored inside until it is time to release a dose. The containers are designed to open at a certain temperature found in one part of the body. This helps give just the right amount of medicine at the right time without surgery to replace a container. Nitinol’s shape memory alloys is precisely timed to help heal illness.

Nitinol Wings that Change Shape

Engineers are using nitinol in airplane wings and parts because it can change shape by itself. Nitinol ‘remembers’ two shapes – what it looks like straight and what it looks like bent. This helps airplanes fly better in different weather.

Wings that Adapt to Wind

Aerospace engineers are making special airplane wings out of nitinol. These wings can adjust their shape memory alloys during flight thanks to tiny nitinol wires inside. When winds get rough, the wires heat up from friction with air. Then the nitinol ‘knows’ to bend the wing a bit. This keeps wings shaped just right so air flows smoothly over them. It lets planes fly strong without shaking even in storms.

Wing Flaps that Move on Their Own

Wing flaps on regular planes are moved by people or motors. But nitinol flaps can change angle by themselves! Engineers make the flaps with nitinol. During takeoff and landing, flaps need to tip downwards to help planes slow down or speed up. Nitinol wires in the flaps sense temperature changes from flying fast or slow. They automatically pull the flaps down into position without any motors. This saves fuel and makes flying more efficient.

From Nozzles to Brackets – Shape-Changing Parts

Many small actuated parts on planes and rockets use nitinol. Examples include rocket engine nozzles that change their exit area for different speeds. Nitinol hinges allow moveable brackets to lock in place without screws. All through its shape memory alloys, nitinol helps aerospace components change shape right when needed for safer, smoother flights.

Robots that Move with Shape Memory

Engineers use shape memory alloys or SMAs to make robots move on their own without batteries or wiring. SMAs “remember” two shapes and can switch between them thanks to temperature changes. This special property lets them act like muscles for robotic systems.

SMA Actuators Power Robotic Motion

Many robots use SMA wires or strips as self-contained actuators. When voltage or hot water is applied, the SMA “remembers” to bend or stretch. This motion allows robot joints to swing, grippers to open and close, and more. For example, one robot hand has SMA actuators in each finger that curl it into a grasping position. Remote-controlled robots underwater or in space can use SMA actuators without electronics too.

Designing More Life-Like “Soft” Robots

With SMAs, robots can have more flexible, lightweight bodies similar to animals or humans. Engineers make “soft” robots with silicone-coated SMA wires embedded in flexible tubes or sheets. When heated, the SMAs contract locally to initiate complex motions like writhing, bending, or grasping without rigid components. These soft robots may someday help with rescue missions or medical procedures by navigating unusual environments.

The shape memory alloys effect of SMAs lets them power lifelike robotic motion. This opens up new possibilities for automation on land, sea, air, space, and even inside the human body.

Using Shape Memory for Building Safety

Civil engineers employ shape memory alloys in structures to help withstand earthquakes, monitor infrastructure wear, and more. SMAs “remember” their shape and change with heat, which benefits construction.

Dampers that Cushion Quakes

SMAs help earthquake-proof buildings. Engineers install SMA-powered dampers in buildings between floors and foundations. During temblors, floors and foundations vibrate at different frequencies. Normally this causes damage over time. But SMA dampers detect vibrations. They contract to absorb and dissipate energy from shaking motions. This cushions the structure from quake strains without extra power.

Checking for Metal Fatigue

SMA wire coils can evaluate fatigue in bridges, tunnels and other infrastructure. Coils placed in critical metal locations slowly contract over many load cycles. Engineers check the coils regularly. If a coil has contracted more than expected, it signals nearby metal sheet fabrication may be weakening from repetitive loading stresses sooner than planned. This prompts closer inspection before failure occurs.

Sensing Deformations after Disasters

After earthquakes or floods, SMA-wrapped sensors placed in roads and building cracks let authorities remotely track any widening. If cracks grow larger over time, it shows more instability exists requiring expensive repairs. Small SMA sensors help prioritize the most urgent reconstruction needs without tedious on-site measurements.

Challenges of Working with Shape Memory Alloys

While SMAs showcase incredible adaptive properties, realizing their full potential requires addressing challenges surrounding durability, thermodynamics, and production.

SMAs undergo stress each time they transition between atomic structures. This fatigue can cause weakening or degradation over many continuous cycles. Improving lifespan remains important.

The shape memory alloys effect also relies precisely on heating and cooling mechanisms. Careful thermal management ensures SMAs reach necessary temperatures uniformly for consistent performance. Real-world conditions can impact thermal control.

Producing SMAs with exact, complex geometric forms for different mechanical roles demands meticulous, often costly manufacturing methods. Engineers research new techniques for cost-effectively configuring SMA properties.

In conclusion, shape memory alloys exhibit remarkable adaptive nature enabling diverse applications. Continued research seeks addressing challenges and advancing possibilities of these “smart” materials. Future applications remain shaped by how well SMAs serve evolving needs through their intrinsic memory of form.

Conclusion

In conclusion, shape memory alloys demonstrate truly unique properties that have sparked immense innovative applications across varied fields. Their ability to automatically “remember” and change shape in response to temperature offers utility unlike conventional materials. SMAs have empowered improved medical devices, more reliable robotics and automation technology, enhanced aerospace and civil engineering structural solutions, and more.

However, fully capitalizing on their adaptive capabilities requires ongoing work to solve challenges. Researchers worldwide continue striving to better understand shape memory alloys material behaviors, refine manufacturing processes, enhance mechanical performance and durability, and advance methods for precise thermal control and energy transfer. Addressing such areas will help push the boundaries of what’s possible with “smart” SMA technology. The future remains brightly shaped by expanding knowledge of these remarkable alloy materials and their potential to empower even more impactful and life-changing applications.

FAQs

Q: How do SMAs change shape?

The most widely used SMA is known as Nitinol and comes under the category of nickel-titanium. Other SMA materials include copper-zinc-aluminum and iron-manganese-silicon alloys.

Q: What are the applications of SMAs?

We use SMAs in applications such as biomedical stents, orthodontic aligners, vascular implants, aircraft actuators, heat-activated fasteners, self-repairing materials, and robotic joints and grippers.

Q: What are the challenges working with SMAs?

Some challenges include fatigue from repeated loading cycles, controlling heating/cooling rates precisely, complex manufacturing techniques and limited force/torque outputs