The boundary between shape memory and superelasticity: Where does the alloy's "memory" come from?

What kind of sparks will be created when intangible cultural heritage meets new technology? Recently, the TV program "China in Intangible Cultural Heritage" jointly launched by China Central Radio and Television and the Ministry of Culture and Tourism showed the first dynamic interactive Suzhou embroidery work of "intangible cultural heritage + technology" in China, which is refreshing. With the support of shape memory materials and temperature sensing interaction technologies, a light touch of the fingertips on the embroidery can trigger the butterfly to flutter its wings and the petals to stretch. Shape memory materials add a lot of charm to exquisite embroidery. So why do shape memory materials produce such a magical phenomenon?

Shape memory alloy refers to a new type of metal functional material that has a certain initial shape and can restore its original shape under the stimulation of heat, light and electricity after low-temperature shaping. Nickel-titanium shape memory alloy is a "rising star" of new functional materials. It is a binary alloy composed of nickel and titanium. The shape memory effect is its unique characteristic. It also has superelasticity, good corrosion resistance and biocompatibility.

Why do alloys have shape memory effect? ​​This has to start with the crystal structure. The shape memory effect is caused by the existence of two different crystal structure phases. Most alloys with shape memory effect undergo thermoelastic martensitic phase transformation. After the martensitic phase transformation, the alloy leaves a large space for plastic deformation. When the alloy is heated above the final temperature, the low-temperature martensite transforms into the high-temperature austenite and automatically returns to the initial state. This is actually a phase transformation process induced by heat. It should be noted that the austenite state is the state when the load is removed, with a cubic structure, while the martensite state is the state when the load is loaded, with a hexagonal structure. During the deformation process, the atoms do not diffuse, only the crystal structure changes.

In order to make full use of the deformation of nickel-titanium alloy to do external work, people invented engines made of nickel-titanium alloy. Unlike ordinary engines, engines made of nickel-titanium alloy are free from the constraints of traditional energy. They only rely on the expansion and contraction of the nickel-titanium alloy U-shaped belt in hot and cold water to drive the wheels to rotate, realizing the conversion of thermal energy into mechanical energy. The emergence of nickel-titanium alloy has pointed out a new direction for us to seek new energy.

In addition to doing work, the research team at Saarland University in Germany also used nickel-titanium alloy to make artificial muscle actuators. This is another level of application of the shape memory effect of nickel-titanium alloy. The fine "muscle fibers" created by the research team are composed of bundles of ultra-fine nickel-titanium alloy wires. When current passes through wires made of nickel-titanium alloy, the material heats up and the crystal structure undergoes a phase change, which shortens the wire; after the current is cut off, the circuit cools and returns to its original length. Multiple strands of alloy wire are used to connect the mechanical finger joints to simulate muscle fibers, forming muscle groups similar to the front flexors and back extensors of the fingers, so as to perform more precise movements.

Superelasticity means that the test sample can produce a strain far greater than the elastic limit of conventional materials under the action of an external load. After the external load is unloaded, the deformation of the sample can automatically recover. The elastic limit of nickel-titanium alloy is far greater than that of ordinary materials, and it no longer obeys Hooke's law. The stress shows a nonlinear relationship within a certain deformation range. Superelasticity can be used to produce small, exquisite, highly automated, and reliable components, such as superelastic self-expanding stents that are widely used in medicine, and corrective archwires that will not gradually lose strength as they move in the direction of correction.

Corrosion resistance is also a strong point of nickel-titanium memory alloy. After oxidation, a titanium dioxide oxide film is formed on the surface of nickel-titanium memory alloy, which can increase the stability of the alloy surface layer. Due to the chemical inertness of nickel and titanium, nickel-titanium alloy is easy to form a dense oxide layer with a thickness of 2-20 nanometers. During the implantation process, the oxide layer grows and absorbs minerals and other components of biological fluids, resulting in surface remodeling, which once again strengthens its corrosion resistance.

Taking advantage of these properties, minimally invasive medical devices such as esophagus, occluder, vena cava filter, etc. made of nickel-titanium alloy are widely used in people's lives. The new nickel-titanium memory alloy temperature-controlled ureteral stent has been produced and used in medical surgery. This is a spiral stent that expands when heated and softens when cooled. After the stent implantation surgery, the nickel-titanium memory alloy stent successfully expands in 65°C hot water and anchors in the narrow area, and the ureter is restored to patency, bringing good news to the patient.

As a shape memory material with good comprehensive performance, nickel-titanium alloy has long been the top priority in shape memory alloy research. From traditional micron-crystals to nanocrystals and amorphous states, nickel-titanium shape memory alloys keep pace with the times and are at the forefront of development. At the same time, a large number of nickel-titanium alloys with novel structures, such as nickel-titanium alloy capillaries and bulk nanocrystalline nickel-titanium alloys, have emerged. In the future, the use of digital three-dimensional reconstruction technology or other new technical means to perform delicate and exquisite processing on nickel-titanium alloys is the direction of its development.

(The first author is a professor and doctoral supervisor at Northwest Normal University, and the second author is a graduate student at Northwest Normal University)