The mission of shape memory alloys

Shape memory alloys (SMA for short) are alloys with "memory": they can return to their original shape after being deformed by pressure or temperature changes. SMA has a wide range of applications, including metallurgy, manufacturing, biomedicine and children's handicrafts, and its application range continues to expand to various fields.

What is a shape memory alloy?

Shape memory alloy is a material that undergoes phase change when subjected to mechanical stress or temperature changes. SMA will "remember" its original shape and return to its original shape when conditions return to normal.

The two crystal structures of SMA materials are called austenite and martensite. The former is the structure of SMA at a higher temperature, and the latter is the structure at a lower temperature. The transformation of austenite to martensite is the reason for this "memory" characteristic (and vice versa).

It's All Coincidence: The Discovery of Shape Memory Alloys

There is another story about the discovery of the properties of SMA materials. Early research on this material began in the 1930s, when scientists studied some unexpected properties exhibited by different metals. Swedish chemist Arne Ölander discovered a pseudo-elasticity phenomenon while observing gold-cadmium alloys and described it. However, it was not until a laboratory accident about 30 years later that people really started to use the term "shape memory alloy"...

By the late 1950s and early 1960s, the US Naval Weapons Laboratory conducted metallurgical research, as described in Reference 1. One day, a scientist named William J. Buehler was smelting and casting nickel-titanium rods. While waiting for the rod to cool, he dropped a cooling rod on the concrete floor and heard a dull crash. He thought it was weird, and then threw a hot nickel-titanium rod on the floor. At this time, he heard a ringing sound. Buehler was worried about a problem with the casting process, so he ran to the water dispenser and put the hot nickel-titanium rod in cold water to cool it down. Then he threw the freshly cooled nickel-titanium rod to the ground, when he heard a dull crash again.

Surprise: I dropped a nickel-titanium rod on the floor and discovered its unique memory characteristics.

This effect was later confirmed at a meeting of the Naval Weapons Laboratory. Buehler's assistant distributed thin nickel-titanium alloy strips to the crowd. The thin strips have been stretched, bent and folded, just like an accordion. When Dr. David S. Muzzey got the alloy bar, he took out a lighter to heat it, and the alloy bar quickly unfolded and returned to the original thin bar shape. After recognizing the characteristics and characteristics of Nitinol under different temperature conditions, people call this material Nitinol, which is a kind of SMA.

Judging from recent and future development trends, shape memory materials are no longer limited to alloys. People have developed shape memory polymers and various other forms of shape memory materials and used them for different commercial purposes.

SMA manufacturing realizes cross-industry applications

The unique properties of SMA make it an attractive material of choice for manufacturing products and components in various industries (Ref. 2).

Aerospace

In the aerospace industry, SMA is used to develop lightweight, quiet, and efficient designs: these three factors have always been the focus of aircraft design. The variable section fan nozzle, shock absorber and actuator are all made of SMA material. These devices are austenite at normal temperature and transform into martensite (and the desired shape) when cooled due to temperature changes caused by airflow around the aircraft or changes in ambient temperature during normal flight.

The temperature change that causes the phase change can be produced in different ways, either by heating the SMA device through an electronic component, or by excess air from other parts of the aircraft.

Samples of shape memory materials used in aircraft development.

The latest technological innovation of shape memory materials is wing deformation technology. SMA is used to develop an adaptive wing that can bend and change shape during flight.

car

Motor vehicles on the ground also benefit from SMA, but its use is more in terms of comfort and ease of use than operation. For example, some cars have SMA valves for the airbags in the seats. Under certain pressure, the lumbar support on the seat can perfectly fit the waist of the driver or passenger.

In addition, SMA is also used to make actuators to make the car trunk easier to close, and to make valves to control noise and vibration in the engine (this is an important performance indicator in the automotive industry), and improve noise, vibration and sound vibration Roughness (noise, vibration, and harshness, referred to as NVH).

building

Another application area of SMA is architectural design. For example, SMA rods in concrete beams help to prestress bridges or buildings. On a smaller scale, shape memory materials can be used to enhance the reliability of pipe fittings in piping systems.

medicine

The application of SMA to biomedicine can reduce the patient's need for medical intervention. For example, medical stents can be implanted in arteries to improve blood flow in heart patients in a minimally invasive manner. Micro-actuators and artificial muscles rely on SMA to make mechanical prostheses, helping amputees to move more freely.

Heart stent is a minimally invasive heart treatment method, usually made of SMA material. (Note: The picture is taken from a teaching case of plastic deformation during the expansion of a biomedical stent, which does not include SMA, which is purely to illustrate the application discussed above).

On a smaller scale, SMA is also used for orthodontics, such as braces; it is also used for optometry, such as glasses. If the spectacle frame is made of shape memory material, it does not need to be replaced even if it is bent and deformed. You can heat the spectacle frame to restore it to its original shape.

Other uses of SMA

Recently, shape memory materials have been applied to consumer electronic products. For example, the autofocus components of smartphone cameras and some mobile antennas are made of SMA.

Some crafts and toys also use SMA. "Flexible bracelets" are an example. This kind of bracelets are made of shape memory materials that can be bent and twisted, and then they can easily restore their original shape. (Unfortunately, Slinky® toys are made of a kind of steel. After being entangled, they cannot return to tightly wound coils. This is a common problem faced by children everywhere.)

Disadvantages and design considerations

When developing a design or component using SMA as a raw material, people need to consider some factors and risks. The main disadvantage of SMA is the risk of fatigue failure. The number of times a certain SMA material can roughly return to its original shape after bending and deformation is limited (too many times, it may break).

Another disadvantage is that for some SMAs, the lag time of the phase change is quite long. If you search for "shape memory alloy" videos on the Internet, you will find that the speed of the material returning to its original shape may be slow and unpredictable.

Defects such as lag time and fatigue can cause problems in the SMA phase transition cycle.

From a manufacturing point of view, the production cost of SMA can be very high, which limits the use of SMA by manufacturers and consumers. Not only that, because most of these materials depend on temperature to produce deformation, there are certain risks in using SMA in equipment operating under uncontrollable or unstable temperature conditions. Take the SMA in automotive applications as an example. It must be able to work normally under all temperature conditions that the car may endure.

Modeling shape memory alloys in COMSOL Multiphysics®

Because the phase transition of SMA is very complicated, its structure is difficult to describe. This complexity makes modeling very difficult.

The COMSOL® software "Nonlinear Structural Material Module" contains the two most common SMA material models: Lagoudas and Souza-Auricchio. Using these material models in the simulation, you can define the austenite and martensite properties and phase transformation properties of SMA. With the built-in coupling between the Heat Transfer in Solids and Solid Mechanics interfaces, you can also easily analyze the heat transfer in SMA.

The uniaxial load teaching example of shape memory alloy demonstrates how to use the SMA material model in the COMSOL Multiphysics® software.

In this teaching case, the Nitinol cylinder is subjected to axial tension, and three independent studies have been performed:

Parametric scanning, showing pseudo-elastic effects at different constant temperatures

The specified displacement scan shows that the pseudo-elastic effect is a partial unloading-partial loading cycle

Shape memory effect, manifested after temperature rise

This model shows that SMA has a temperature-dependent stress limit. When the axial tension reaches the stress limit, the material structure transforms from austenite to martensite, that is, a "positive" transformation (deformation) occurs.

The stress and strain of SMA at different temperatures.

In the process of axial stress unloading, the material will undergo reverse transformation. This "reverse" transformation occurs at a stress level lower than the stress limit of the forward transformation, and the result is that the material returns to its original shape.

The stress and strain curves illustrate the shape memory effect in the alloy.