Shenzhen Wuge Industrial Co., Ltd

Shape memory alloy (SMA) refers to a new type of metallic functional material that has a certain initial shape and can recover its initial shape under thermal, optical, and electrical stimulation after low-temperature shaping. Nickel titanium shape memory alloy (Ni Ti SMA) is a rising star in the field of new functional materials, characterized by its unique shape memory effect. It also possesses superelasticity, good corrosion resistance, and biocompatibility.

As the name suggests, nickel titanium alloy is a binary alloy composed of nickel and titanium, with approximately equal numbers of nickel and titanium atoms. By using temperature difference to change the shape of nickel titanium memory alloy, it can generate inexhaustible power by promoting its external work. The new type of thermistor for various regulators in cars relies on springs made of shape memory alloy to achieve the opening and closing of the thermostat; Stress induced hyperelasticity enables it to be used in the production of self resetting devices, constant elasticity structures, and even load connections in buildings; The corrosion resistance and biocompatibility of nickel titanium memory alloy make it the preferred choice for medical biomaterials. Commonly used for making various functional scaffolds, replicating human tissues and bones, etc.

Why does nickel titanium alloy have shape memory effect?

This starts with the crystal structure. The generation of shape memory effect is due to the existence of two different crystal structure phases. Most alloys with shape memory effect undergo thermoelastic martensitic transformation. After martensitic transformation, the alloy leaves a large plastic deformation space. When the alloy is heated above its final temperature, low-temperature martensite transforms into high-temperature austenite, automatically returning to its initial state. This is actually a phase transition process induced by heat. It should be noted that the austenite state is the state when the load is removed, presenting a cubic structure; And martensite is the state during loading, with a hexagonal structure. Martensitic transformation is a diffusion-free eutectic transformation, in which atoms do not diffuse during the process of transforming from the parent phase to martensite. Therefore, only the crystal structure changes without any compositional changes.

The mutual transformation between austenite (a) and martensite (b)

Application of Nickel Titanium Shape Memory Alloy

(1) Application of Memory Effect in Nickel Titanium Alloy

In order to fully utilize the work done by nickel titanium alloy due to deformation, people invented engines made of nickel titanium alloy. Unlike typical engines, engines made of nickel titanium alloy break free from the constraints of burning oil, gas, or electricity as energy sources. They rely solely on the expansion and contraction of nickel titanium alloy U-shaped belts in hot and cold water to drive the wheels to rotate, achieving the conversion of thermal energy into mechanical energy. We know that scientists have been searching for clean new energy sources for the survival and development of humanity. The emergence of nickel titanium alloys has pointed out a new direction for us to seek new energy, realizing people's pursuit of output energy being greater than input energy. In other words, nickel titanium alloy contains not only a shape memory effect, but also a solid-state energy conversion system. A nickel titanium alloy rod with a cross-sectional area of 1cm2 can generate a force of 8.5t due to temperature changes.

In addition to working, the research team at Saarland University in Germany has also developed an artificial muscle driver using nickel titanium alloy, which is another application of the shape memory effect of nickel titanium alloy. The diameter of this driver is 300-400 microns, which is ultra light and energy-saving. When current passes through a wire made of nickel titanium alloy, the material heats up, the crystal structure undergoes a phase transition, and the wire becomes shorter; After the current is cut off, the circuit cools down and returns to its original length. The research team created these fine 'muscle fibers', which are composed of bundles of ultrafine nickel titanium alloy wires, just like muscle fibers are divided into fiber bundles. Using multiple strands of alloy wire to connect finger joints, simulating muscle fibers to create the anterior flexor and posterior extensor muscles of the fingers, which can quickly contract and relax. They can tighten, bend, and perform more precise movements.

(2) Application of Nickel Titanium Alloy Superelasticity

Superelasticity refers to the ability of a test sample to generate a strain far greater than the elastic limit of conventional materials under external loading, and the deformation of the sample can automatically recover after unloading the external load. The elastic limit of nickel titanium alloy is much greater than that of ordinary materials, and it no longer follows Hooke's law. Within a certain deformation range, the stress does not increase with the increase of strain, but exhibits a nonlinear relationship. Nickel titanium alloys can achieve superelasticity through smelting, hot processing, cold processing, and appropriate aging heat treatment. By utilizing superelasticity, small and exquisite components with high automation and reliable performance can be produced. Such as the widely used hyperelastic self expanding stent and orthodontic archwire in medicine. This type of archwire is clinically manifested as the orthodontic force generated during the deformation process remaining constant and not gradually losing as the teeth move towards the orthodontic direction.

(3) Application of Nickel Titanium Alloy Corrosion Resistance

Corrosion resistance is also a strong point of nickel titanium memory alloys. After oxidation, a layer of TiO2 oxide film is formed on the surface of nickel titanium memory alloy, which can increase the stability of the alloy surface layer and act as a physical and chemical barrier. Due to the chemical inertness of nickel and titanium, nickel titanium alloys are prone to forming a dense oxide layer with a thickness of 2-20 nm. During implantation, this oxide layer grows and absorbs minerals (such as calcium phosphate) and other components of biological fluids, resulting in surface remodeling and good corrosion resistance.

By utilizing these properties, minimally invasive medical devices such as esophagus, trachea, occluder, and vena cava filter made of nickel titanium alloy will not cause rejection reactions when implanted in the human body. The new nickel titanium memory alloy temperature controlled ureteral stent has been produced and used in medical surgery. It is a spiral bracket that expands when heated and softens when cooled. After the stent placement surgery, the nickel titanium memory alloy stent successfully expanded in hot water at 65 ℃, anchored to the narrowed area, and restored the patency of the ureter, bringing good news to the patient. Due to the stiffness and hardness of nickel titanium shape memory alloy being very close to human bone tissue, it is considered the most ideal material for biological internal fixation implantation. It plays an important role in the treatment of rib fractures, joint surgery, and hand surgery diseases, commonly used to make pins and plates for fixing broken bones, and has good biocompatibility.

Nickel titanium shape memory alloy bracket

Nickel titanium alloy, as a shape memory and superelastic material with good comprehensive performance, has long been a top priority in the research of shape memory alloys. From traditional silk and sheet materials to novel thin-walled tubes and films, from traditional microcrystals 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 structurally novel nickel titanium alloys have emerged, such as nickel titanium alloy thin film strips, nickel titanium alloy capillaries, bulk nanocrystalline nickel titanium alloys, etc.