Nickel-titanium alloy, also known as shape memory alloy, is a special alloy with unique properties. The distinctive feature of this alloy is its ability to automatically return to its original shape after undergoing plastic deformation, provided it is exposed to a specific temperature condition. This phenomenon is known as the shape memory effect, one of the core characteristics of shape memory alloys. Nickel-titanium alloys have a very high elongation rate, which can exceed 20%, meaning they can still return to their original state even after experiencing significant deformation. Additionally, the fatigue life of nickel-titanium alloys is astonishingly high, reaching 1*10^7 cycles, indicating that they can maintain their shape memory function even with repeated use and deformation. The damping characteristics of nickel-titanium alloys are also outstanding, being ten times higher than those of ordinary springs, which makes them promising for applications in vibration control and shock absorption. Even more remarkable is that the corrosion resistance of nickel-titanium alloys is superior to the best medical stainless steel currently available on the market, giving them extensive application potential in the medical field, especially in implant devices and surgical procedures. Due to these unique properties, nickel-titanium alloys have become an excellent functional material, widely used in various engineering and medical fields. Beyond its unique shape memory function, nickel-titanium alloys also possess excellent characteristics such as wear resistance, corrosion resistance, high damping, and superelasticity. These features make nickel-titanium alloys indispensable in high-tech fields such as aerospace, automotive industry, biomedical engineering, and many others.
7. Good vibration damping characteristics: The greater the vibration caused by chewing and night grinding on the archwire, the greater the damage to the tooth root and periodontal tissue. Research findings from different archwire attenuation experiments show that the amplitude of vibration of stainless steel wires is larger than that of superelastic nickel-titanium wires. The initial vibration amplitude of superelastic nickel-titanium archwires is only half that of stainless steel wires. The archwire’s good vibration and damping characteristics are important for the health of teeth, while traditional archwires such as stainless steel wires tend to aggravate root resorption.
(3) Classification of nickel-titanium alloy wires
1) In 1940, gold archwires, cobalt-chromium alloy wires, and stainless steel round wires were used.
2) In 1960, martensite-stabilized alloys: mostly made from nickel-titanium alloys deformed in the martensite state. This type of archwire has low stiffness and can produce lighter corrective forces. There is no stress or temperature-induced martensitic transformation, so it does not exhibit memory effects or superelasticity.
3) In 1980, Chinese nickel-titanium alloy and Japanese nickel-titanium alloy archwires, austenite-activated alloys: that is, they are in the austenite state under any condition, neither in the oral cavity nor outside the oral cavity, and can only be caused by stress, not by temperature, to the martensite state, with superelasticity but without shape memory function. This type of archwire has excellent resilience and lower stiffness, capable of producing weaker corrective forces. Its significant feature is that from the initial start to the final stage, the force it generates remains constant, which is better for treating early misaligned teeth. The disadvantage is that it cannot be bent into shape at room temperature and is not easy to weld. If used as the main archwire, it often causes unwanted expansion or contraction of the arch and makes it difficult to establish a good premolar and molar alignment.
4) In 1990, martensite-activated nickel-titanium alloy: that is, the TTR (Transformation Temperature Range) is lower than oral temperature or very close to it, existing in a multi state at room temperature, easy to deform. When placed in the oral cavity, both the stress-induced and room temperature-induced martensite transform into austenite, that is, there is shape memory function and superelasticity. At room temperature (around 25°C) and below, it is easy to deform, and when it reaches a certain temperature (around 32°C) or above, it will return to its original preformed shape, exhibiting shape memory plus superelastic characteristics. The Smart brand of Beijing Saintmate Technology Co., Ltd. and the NitinolHA brand of 3M Company are typical representative products. Heat-activated nickel-titanium archwires precisely because of this characteristic, maintaining them at room temperature and below can easily be shaped and placed into brackets, and when activated by body heat in the oral cavity, they can produce shape-restoring forces, providing the necessary force for orthodontic correction. Due to the “become softer when cold, and become more elastic when heated” characteristics of heat-activated nickel-titanium orthodontic wires, patients can, under the guidance of a doctor, use the method of holding cold or hot water in their mouths to change the corrective force, making orthodontic correction more convenient and reducing the discomfort of initial correction.
5) Graded thermodynamic: Increased thermodynamic nickel-titanium alloy: the TTR temperature is higher than oral temperature, about 40°C. In this way, when the nickel-titanium archwire is placed in the oral cavity, it remains in a multistate, and the archwire is relatively soft. Only when holding hot water in the mouth does the austenite phase transformation occur. Therefore, the corrective force is even weaker and can be used as the initial archwire for adult patients and patients with periodontal disease. The copper-containing nickel-titanium wire produced by Omcro Company and the low-hysteresis L-H nickel-titanium archwire produced by Japan have this performance.
(4) Clinical application of nickel-titanium alloy wires:
1. Used for the early alignment and leveling of patients’ dentition Due to the superelasticity and shape memory performance of nickel-titanium alloy archwires, as well as their lowerNickel-titanium alloy is a shape memory alloy, which is a special alloy capable of automatically restoring its original shape after plastic deformation at a certain specific temperature. Its elongation rate is over 20%, fatigue life reaches 1*10^7 cycles, damping characteristics are 10 times higher than those of ordinary springs, and its corrosion resistance is superior to the best medical stainless steel currently available. Therefore, it can meet the application requirements of various engineering and medical fields, making it an excellent functional material. In addition to its unique shape memory function, memory alloys also possess excellent characteristics such as wear resistance, corrosion resistance, high damping, and superelasticity.
Performance and Characteristics
(1) Phase transformation and properties of nickel-titanium alloy
As the name suggests, nickel-titanium alloy is a binary alloy composed of nickel and titanium. Due to changes in temperature and mechanical pressure, it exists in two different crystal structures, namely the austenite phase and the martensite phase. The phase transformation sequence of nickel-titanium alloy upon cooling is parent phase (austenite phase) – R phase – martensite phase. The R phase is rhombic, austenite is cubic and stable at higher temperatures (above the starting temperature of austenite), or when the load is removed (the state when external forces are removed), while martensite is hexagonal, ductile, reversible, relatively unstable, and prone to deformation at lower temperatures (below Mf, the ending temperature of martensite) or when loaded (activated by external forces).
(2) Special properties of nickel-titanium alloy
1. Shape memory characteristic (shape memory) – Shape memory occurs when a parent phase cools below Af temperature to form martensite, which is then deformed below Mf temperature. Upon heating to below Af temperature, accompanied by a reverse phase transformation, the material automatically restores its shape from the parent phase. In essence, the shape memory effect is a phase transformation process of nickel-titanium alloy induced by heat.
2. Superelasticity (superelasticity) – Superelasticity refers to the phenomenon where a sample undergoes a strain far greater than its elastic limit under an external force, and the strain can automatically recover upon unloading. That is, in the parent phase state, due to the action of an applied stress, a stress-induced martensitic transformation occurs, resulting in mechanical behavior different from that of ordinary materials. Its elastic limit is much greater than that of ordinary materials, and it no longer obeys Hooke’s law. Compared with shape memory characteristics, superelasticity does not involve heat. In summary, superelasticity refers to a phenomenon where stress does not increase with the increase of strain within a certain range of deformation, which can be divided into linear superelasticity and nonlinear superelasticity. Linear superelasticity refers to a stress-strain curve where stress and strain are approximately linearly related. Nonlinear superelasticity refers to the result of stress-induced martensitic transformation and its reverse transformation during loading and unloading within a certain temperature range above Af, and therefore nonlinear superelasticity is also known as transformation pseudoelasticity. The transformation pseudoelasticity of nickel-titanium alloy can reach about 8%. The superelasticity of nickel-titanium alloy can change with the conditions of heat treatment. When the archwire is heated above 400°C, superelasticity begins to decrease.
3. Sensitivity to temperature changes within the oral cavity: The corrective force of stainless steel wires and CoCr alloy orthodontic wires is basically not affected by the temperature within the oral cavity. The corrective force of superelastic nickel-titanium alloy orthodontic wires changes with the temperature within the oral cavity. When the amount of deformation is constant, as the temperature rises, the corrective force increases. On the one hand, it can accelerate the movement of teeth, as temperature changes within the oral cavity stimulate blood flow in areas of capillary stasis caused by corrective devices, thereby providing sufficient nutrition for repair cells during tooth movement, maintaining their vitality and normal function. On the other hand, orthodontists cannot precisely control or measure the corrective force under oral conditions.
4. Corrosion resistance: Studies have shown that the corrosion resistance of nickel-titanium wires is similar to that of stainless steel wires.
5. Antitoxicity: The special chemical composition of nickel-titanium shape memory alloy, that is, this is a nickel-titanium equiatomic alloy containing about 50% nickel, and it is known that nickel has carcinogenic and cocarcinogenic effects. Under normal circumstances, the surface layer of titanium oxide acts as a barrier, giving Ni-Ti alloy good biocompatibility. The surface layers of TiXOy and TixNiOy can inhibit the release of Ni.
6. Gentle corrective force: Currently, commercially available orthodontic metal wires include austenitic stainless steel wires, cobalt-chromium-nickel alloy wires, nickel-chromium alloy wires, Australian alloy wires, gold alloy wires, and β-titanium alloy wires. Regarding the load-displacement curves of these orthodontic correction metal wires under tensile tests and three-point bending tests, the unloading curve platform of nickel-titanium alloy is the lowest and the flattest, indicating that it can provide the most persistent and gentle corrective force.
7. Good vibration damping characteristics: The greater the vibration caused by chewing and night grinding on the archwire, the greater the damage to the tooth root and periodontal tissue. Research findings from different archwire attenuation experiments show that the amplitude of vibration of stainless steel wires is larger than that of superelastic nickel-titanium wires. The initial vibration amplitude of superelastic nickel-titanium archwires is only half that of stainless steel wires. The good vibration and damping characteristics of archwires are important for the health of teeth, while traditional archwires such as stainless steel wires tend to aggravate root absorption.
(3) Classification of nickel-titanium alloy wires
1) In 1940, gold archwires, cobalt-chromium alloy wires, and stainless steel round wires.
2) In 1960, martensitic stabilized alloys: mostly made from nickel-titanium alloy deformed in the martensitic state. This type of archwire has low stiffness and can produce lighter corrective forces. There is no stress or temperature-induced martensitic transformation, so it does not exhibit memory effects and superelasticity.
3) In 1980, Chinese nickel-titanium alloy and Japanese nickel-titanium alloy archwires, which are austenite-activated alloys: that is, they are in the austenitic state under any condition, whether inside or outside the mouth, and do not exhibit martensitic state caused by temperature, but only by stress, with superelasticity but without shape memory function. This type of archwire has excellent resilience and low stiffness, capable of producing weak corrective forces. Its significant feature is that from the initial start to the final stage, the force it generates remains constant, which is effective in the early treatment of crooked teeth. The disadvantage is that it cannot be bent into shape at room temperature, is not easy to weld, and if used as the main archwire, it often causes unwanted expansion or contraction of the arch, and it is difficult to establish a good premolar and molar alignment.
4) In 1990, martensitic-activated nickel-titanium alloy: that is, the TTR (Transformation Temperature Range) is lower than oral temperature or very close to it, existing in a multi-state at room temperature, easy to deform, and when placed in the oral cavity, both the stress-induced and room temperature-induced martensite transform into austenite, that is, exhibiting shape memory function and superelasticity. At room temperature (around 25°C) and below, it is easy to deform, and when it reaches a certain temperature (around 32°C), it will return to its original preformed shape, exhibiting shape memory plus superelastic characteristics. The Smart brand of Beijing Saintmart Technology Co., Ltd. and the NitinolHA brand of 3M Company are typical representative products. Heat-activated nickel-titanium archwires precisely because of this characteristic, maintaining them at room temperature and below can easily be manipulated into shape and placed into brackets, and when activated by body heat in the mouth, they can produce shape-restoring force, providing the necessary force for orthodontic correction. Due to the “become soft when cold, become elastic when heated” characteristics of heat-activated nickel-titanium orthodontic wires, patients can, under the guidance of a doctor, use the method of holding cold or hot water in their mouths to change the corrective force, making orthodontic correction more convenient and reducing the discomfort of initial orthodontic correction.
5) Graded thermodynamic: Increased thermodynamic nickel-titanium alloy: the TTR temperature is higher than oral temperature, about 40°C. In this way, when the nickel-titanium archwire is placed in the oral cavity, it remains in multi-state, and the archwire is relatively soft. Only when holding hot water in the mouth does the austenitic phase transformation occur. Therefore, the corrective force is even weaker and can be used as the initial archwire for adult patients and patients with periodontal disease. Omcro Company’s copper-containing nickel-titanium wire and Japan’s low hysteresis L-H nickel-titanium archwire have this performance.
(4) Clinical application of nickel-titanium alloy wires:
1. Used for the early alignment and leveling of patients’ dentition Due to the superelasticity and shape memory performance of nickel-titanium alloy archwires, as well as their lower stress-strain curve