Nitinol is a kind of Nickel-Titanium alloy demonstrates both shape memory functionality and superelasticity due to it being able to undergo phase changes in the solid state. It was first developed by William Beuhler from the US Naval lab, with the composition of 55% nickel (Ni) and 45% titanium (Ti). The name of Nitinol represents its elemental components and origin place. Obviously, the ‘Ni’ and ‘Ti’ are atomic symbols. The ‘nol’ stands for the Naval Ordinance Laboratory where it was discovered. [1] Today, the proportions of nickel and titanium in various Nitinol alloys vary slightly. These differences lead to different phase transition temperatures, some slight differences in properties and wide applications. The principle of the two typical characteristics, manufacturing method, processing technology and applications of Nitinol will be described below.

Shape Memory Effect (SME)

Shape memory effect (SME) is an ability to “remember” the parent shape after being plastically deformed. Alloys with this ability are called shape memory alloys (SMA), and Nitinol is a representative example. Nitinol exhibits SME by reversible phase transformation between austenite and martensitic. At higher temperatures above the transition temperature, nitinol exists in austenitic phase, whereas below this temperature it exists in the martensite phase. The transition temperature various depending on the elements composition, and can be from -50℃ to 150℃. [3] The original shape, also known as parent shape of Nitinol, is defined in the high temperature austenitic phase and remembered by the material. After being plastically deformed in lower temperature, Nitinol can restore to its parent shape by heating or other external stimulus as long as the deformation is within the recoverable range. [3, 4] The recoverable range means strain limit, for Nitinol it is about 8.5%. Figure 1 shows the process of deformation and recovery. Shape Memory NiTi alloys exploit the ability of the materials to recover a trained shape upon heating above their transformation temperatures. Therefore, the most critical property to specify is the transformation temperature. The Active Af represents the finish of the transformation from martensite to austenite upon heating and, therefore, the temperature at which the shape recovery is complete.

Figure1. The sketch of deformation and recovery process [5]

Superelasticity

Superelasticity is a special characteristic of SMA. If SMA deforms at a temperature slightly above its transition temperature, it will immediately return to shape. This elasticity is called superelasticity and is used in many commercial applications. The principle of superelasticity is as follows: above the phase transition temperature, the material is in high temperature or austenite phase. When stress is applied, deformation transforms austenite into martensite. When the applied stress is relieved, the material immediately springs back and the crystalline form returns to the austenite phase.  According to this principle, superelastic nitinol uses stress to induce martensitic transformation to achieve an incredible amount of flexibility and kink resistance.  Nitinol has superelasticity at room temperature, and at the same time, this material also shows good superelasticity at human body temperature (37 degrees Celsius). [6]

Application

In addition, Nitinol has a unique position in biomedicine due to its excellent biocompatibility and corrosion resistance. [7] Compared with stainless steel, another common biomaterial, Nitinol has superelastic properties more similar to bones, so it has made great progress in orthopedic applications. With high damping effect, Nitinol has become a new implant material, including cardiac stents and plastic surgery.  Metal implants containing biocompatible metals or used in combination with other biomaterials are generally considered as the standard for many implant types. In general, Nitinol's biomedical applications include stents, heart valve tools, bone anchors, bone nails, nasal septum defect devices, and implants. However, because nickel element may dissolve out under physiological conditions, it has potential toxicity to cells, tissues and organs, and may induce inflammatory reactions; its clinical application is still greatly limited. Based on this, it is particularly important to improve the corrosion resistance of Nitinol and avoid the precipitation of toxic particles. At present, there are many feasible surface modification methods that can effectively inhibit the dissolution of nickel ions, such as surface inert coating, surface oxidation, surface activation and surface grafting of macromolecules. [8] This can improve the corrosion resistance of Nitinol and is of great significance to improve its biocompatibility.

In addition to various applications in the medical field, Nitinol is also widely used in many other fields. It generally takes Nitinol wire, Nitinol stent and Nitinol basket as three basic forms, and is widely applied from household appliances to automobiles, aerospace and industry. Nitinol provides designers with incredible flexibility because it can replace traditional materials and has better performance, thus gaining a strong foothold. In the industrial field, Nitinol is used to control systems and valves, especially temperature control, due to its shape memory characteristics and superelasticity. [9] In the field of aeronautics and astronautics, the compressed Nitinol antenna is carried into space. After it is heated to the phase change temperature by solar radiation, the compressed antenna is restored to its original state (see Figure 2). [10] In addition, it can also be used in the temperature control system of spacecraft to keep the temperature constant. In daily life, glasses frames, spoons and golf made of Nitinol materials are very common. In a word, Nitinol's effective shape memory capability provides extensive and far-reaching applications in aerospace and industrial fields, and it is a very useful and excellent material.

Figure 2. Sketch of Nitinol antenna.

Processing Route

Due to the high reactivity of titanium, the effective use of vacuum method or inert gas is very important in the preparation of Nitinol. The recipe for vacuum preparation is here, and the two primary melting methods in this section will be briefly introduced.  Vacuum induction melting (VIM) uses alternating magnetic field to heat the raw materials in crucible in vacuum. Vacuum arc remelting (VAR) method is to melt the raw materials in a water-cooled copper mold under high vacuum condition to form an arc between the raw materials and the water-cooled copper punching plate. [11] In general, VIM is the first choice because the materials prepared by it have better performance.  These will be described in detail in the production route section.

Manufacture

For manufacturing, the cold working of Nitinol is difficult, and hotworking is appropriate. [12] The processing and molding methods of the three basic raw material forms mentioned above will be explained. Various forming methods of Nitinol are briefly introduced. For the first Nitinol wire, Nitinol raw materials are first melted, forged and rolled into bars at high temperature. This step belongs to hotwork. Then through a series of steps such as cold drawing and annealing, the wire with the required diameter is produced. Nitinol tube is another net material. The center of the rod is drilled through a loophole to create the original hollow tube form. The last is Nitinol flake. Nitinol ingot is forged into slab by hotworking technology. More detailed information will be displayed in specific sections.

Safety Information

Nickel-titanium alloy is safe to use. Although nickel is at risk of dissolution when used as mentioned above, the current technology has solved this problem by passivation and other methods. According to Nitinol's safety data sheet (SDS), if properly stored, it can maintain good stability without special protection for contacts. [13] At present, Nitinol has been modified as a research hotspot in the related research field of memory alloys to be endowed more excellent properties. This magical nickel metal will have a great impact on our daily life, medical treatment, entertainment and the development of the space industry.

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[2] Yamauchi, K., Ohkata, I., Tsuchiya, K., & Miyazaki, S. Shape memory and superelastic alloys.

[3] B. O’Brien, F.M. Weafer, M.S. Bruzzi. (2017). 1.3 Shape Memory Alloys for Use in Medicine. Comprehensive Biomaterials II, 1: 50-78

[4] CES edupack, Granta design, 2019, Cambridge, UK

[5] Otsuka, K.; Ren, X. (2005). "Physical Metallurgy of Ti-Ni-based Shape Memory Alloys". Progress in Materials Science. 50 (5): 511–678.

[6] 2. Group T. An Overview of Nitinol: Superelastic and Shape Memory [Internet]. Medicaldesignbriefs.com.  [cited 14 June 2020]. Accessed from: https://www.medicaldesignbriefs.com/component/content/article/mdb/features/articles/23077

[7] Kapoor, D. (2017). Nitinol for Medical Applications: A Brief Introduction to the Properties and Processing of Nickel Titanium Shape Memory Alloys and their Use in Stents. Johnson Matthey Technology Review, 61(1), 66-76.

[8] Wadood, A. (2016). Brief Overview on Nitinol as Biomaterial. Advances in Materials Science and Engineering, 1-9.

[9] Duerig, T., Pelton, A., & Stöckel, D. (1999). An overview of nitinol medical applications. Materials Science and Engineering: A, 273-275, 149-160 [cited 13 June 2020]. Accessed from: https://doi.org/10.1016/s0921-5093(99)00294-4

[10] Costanza, G., & Tata, M. (2020). Shape Memory Alloys for Aerospace, Recent Developments, and New Applications: A Short Review. Materials, 13(8), 1856.

[11] Villa E. (2015). Manufacturing of Shape Memory Alloys. Shape Memory Alloy Engineering. pp. 79-96.

[12] Bethel, Menlo Park, New Hartford (2017), Introduction to Nitinol [cited 10 June 2020]. Accessed from: https://www.memry.com/.

[13] Safety Data Sheet acc. to OSHA HCS, Retrieved from https://chemicalsafety.com/sds-search/. Accessed on 6 June, 2020