Nickel-titanium alloy (Nitinol) is a special metal alloy composed of two elements, nickel (Ni) and titanium (Ti). Its name "Nitinol" is a combination of its composition and the abbreviation of the Naval Ordnance Laboratory in Maryland, USA (Ni-Ti-NOL) where it was discovered. Nickel-titanium alloy uses temperature difference to change the shape of nickel-titanium memory alloy, prompting it to do work externally, and can generate inexhaustible power. The new thermal sensitive elements of various regulators on cars rely on springs made of shape memory alloy to realize the opening and closing of the thermostat; stress-induced superelasticity makes it used to make self-resetting devices, constant elastic structures and load-balancing connections in buildings; the corrosion resistance and biocompatibility of nickel-titanium memory alloy make it the first choice for medical biomaterials. It is often used to make various functional stents, replicate human tissues and bones, etc.

▶Nickel-titanium alloy memory effect

Why does the alloy have a shape memory effect? ​​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 plastic deformation space. 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, and it has a cubic structure; while the martensite state is the state when loaded, and it has a hexagonal structure. The martensitic phase transformation is a diffusionless coherent shear phase transformation. In the process of transforming from the parent phase to martensite, the atoms do not diffuse, so only the crystal structure changes, and there is no change in composition.

 

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

(Picture from China Science and Technology Museum)

▶Typical alloy grades of nickel-titanium alloys:

▶Characteristics of nickel-titanium alloys:

Shape memory effect: This is the core characteristic of nickel-titanium alloys. Shape memory is when a certain shape of the parent phase is cooled from above Af temperature (phase transition temperature of nickel-titanium alloy) to below Mf temperature (martensite transformation temperature) to form martensite, and then the martensite is deformed at a temperature below Mf, and heated to below Af temperature, accompanied by reverse phase transformation, the material will automatically restore its shape in the parent phase. In fact, the shape memory effect is a heat-induced phase transition process of nickel-titanium alloy.

Superelasticity: Nickel-titanium alloy has extremely strong superelasticity, and its elasticity can reach 8%, which is much higher than that of ordinary metal materials. This superelasticity enables nickel-titanium alloy to undergo martensitic transformation when subjected to large stress, from martensite to austenite, thereby absorbing a large amount of energy and deforming, and when the stress is removed, it can return to its original shape. The superelasticity exhibited by nickel-titanium alloy makes it widely used in the medical field.

Wear and corrosion resistance: A dense titanium dioxide (TiO2) oxide film will form on the surface of nickel-titanium alloy, which acts as a physical and chemical barrier, making nickel-titanium alloy have excellent corrosion resistance. Studies have shown that the corrosion resistance of nickel-titanium wire is similar to that of stainless steel wire. Due to the superelasticity and martensitic adaptive behavior of nickel-titanium alloy, good strain hardening ability and fatigue resistance, its wear resistance is higher than that of general materials.

Biocompatibility: In terms of biocompatibility, compared with the clinical reference control material AISI 316L stainless steel, nickel-titanium has no cytotoxicity, neurotoxicity, genotoxicity or allergy, and the stress-strain behavior of nickel-titanium is very similar to that of bones and tendons. Therefore, the use of nickel-titanium can shorten the healing time, reduce trauma to surrounding tissues, and is compatible with human tissues without causing obvious rejection or allergic reactions. This makes it safe for the manufacture of various implants and medical devices, such as implants, stents, catheters, etc.

Anti-toxicity: Nickel-titanium shape memory alloy is an alloy of nickel and titanium atoms, containing about 50% nickel, and nickel is known to have carcinogenic and cancer-promoting effects. In general, the surface titanium oxide layer acts as a barrier, making Ni-Ti alloys have good biocompatibility, and the TixOy and TixNiOy of the surface layer can inhibit the release of Ni.

High strength and low density: Ni-Ti alloys have the characteristics of high strength and light weight, and can provide sufficient strength while keeping the instruments and implants lightweight. This makes it very useful in manufacturing medical devices and implants that require strength and light weight.

▶ Application areas of Ni-Ti alloys:

01 Biomedical industry Another important feature of Ni-Ti is that its elastic modulus is as low as that of natural bone materials, and it has good biocompatibility and unique stress/strain response, which makes its compressive strength higher than that of natural bone materials. Therefore, it is widely used in various biomedical applications, such as vascular stents and heart valves in medical devices, Ni-Ti braces in dental corrections, and catheters in vascular interventional surgery.

02 Aerospace industry Ni-Ti alloys have been expanded in the aerospace and defense fields due to their excellent mechanical properties and lightweight characteristics, such as key components such as wing regulators, fuel pipes and sensors. Nickel-titanium alloys are also used as smart materials in the aerospace field for adaptive structures, shock absorption systems, and shape-changing components.

03 Smart Materials and Devices Industry

Nickel-titanium alloys are used to manufacture flexible electronic devices, using the flexibility and durability of nickel-titanium alloy materials to make bendable displays and sensors.