What Is Nickel Titanium?

Nitinol is a shape memory alloy. Shape memory alloy is a special alloy that can automatically restore its plastic deformation to its original shape at a certain temperature, and has good plasticity.

Nitinol

Nitinol is a shape memory alloy. Shape memory alloy is a special alloy that can automatically restore its plastic deformation to its original shape at a certain temperature. Its expansion and contraction rate is more than 20%, the fatigue life is 7 × to the power of 1 * 10, the damping characteristics are 10 times higher than ordinary springs, and its corrosion resistance is better than the best medical stainless steel at present, so it can meet various engineering and Medical application requirements are a very good functional material.

In addition to the unique shape memory function, the memory alloy also has excellent characteristics such as wear resistance, corrosion resistance, high damping and super elasticity.

(I) Phase transition and properties of nickel-titanium alloy

As its name implies, nickel-titanium alloy is a binary alloy composed of nickel and titanium. Due to changes in temperature and mechanical pressure, there are two different crystal structure phases, namely an austenite phase and a martensite phase. The transformation order of the nickel-titanium alloy during cooling is the parent phase (austenite phase) -R phase-martensite phase. The R phase is rhombohedral, and the austenite is harder when the temperature is higher (greater than: the temperature at which the austenite starts) or when the load is removed (deactivation by external force). The shape is relatively stable. The martensite phase is a relatively low temperature (less than Mf: the temperature at which martensite ends) or a state under load (activated by external force). The hexagon is ductile, repeatable, less stable, and easier. Deformation.

(II) Special properties of nickel-titanium alloy

1. Shape memory (shape memory) Shape memory is when a certain shape of the mother phase is cooled from above Af temperature to below Mf temperature to form martensite, martensite is deformed below Mf temperature, and heated to below Af temperature With the inverse phase change, the material will automatically recover its shape in the parent phase. In fact, the shape memory effect is a thermally induced phase transition process of nickel-titanium alloys.

2. Superelasticity The so-called superelasticity refers to the phenomenon that the specimen under the external force generates a strain that is much larger than its elastic limit strain, and the strain can automatically recover when unloaded. That is, in the mother phase state, due to the effect of applied stress, stress-induced martensite transformation occurs, so that the alloy exhibits different mechanical behavior than ordinary materials, its elastic limit is much larger than ordinary materials, and it no longer adheres to Hu Law of grams. Compared to shape memory properties, superelasticity has no thermal involvement. In a word, superelasticity means that the stress does not increase with the increase of strain in a certain deformation range. Superelasticity can be divided into two types: linear hyperelasticity and nonlinear hyperelasticity. The former stress-strain curve has a nearly linear relationship with stress. Non-linear superelasticity refers to the results of stress-induced martensitic transformation and its inverse phase transformation during loading and unloading in a certain temperature range above Af. Therefore, non-linear superelasticity is also called phase transition pseudoelasticity. The phase transition pseudo-elasticity of nickel-titanium alloy can reach about 8%. The superelasticity of the nickel-titanium alloy can be changed with the change of the heat treatment conditions. When the arch wire is heated above 400 & ordm; C, the superelasticity begins to decrease.

3. Sensitivity to temperature changes in the oral cavity: The correction force of stainless steel wire and CoCr alloy tooth orthopedic wire is basically not affected by the temperature in the oral cavity. The correction force of superelastic nickel-titanium alloy orthodontic wire changes with the change of oral temperature. When the amount of deformation is constant. As the temperature increases, the healing power increases. On the one hand, it can accelerate the movement of teeth. This is because temperature changes in the oral cavity can stimulate blood flow in the stagnant blood flow stasis caused by the correction device, so that the repair cells are fully nourished during the tooth movement. Maintain its vitality and normal function. On the other hand, orthodontists cannot accurately control or measure the corrective force in the oral environment.

4. Corrosion resistance: Studies have shown that the corrosion resistance of nickel-titanium wires is similar to that of stainless steel wires.

5. Anti-toxicity: The special chemical composition of nickel-titanium shape memory alloy, which is an atomic alloy such as nickel-titanium, contains about 50% of nickel, and nickel is known to have carcinogenic and cancer-promoting effects. In general, the surface layer of titanium oxide acts as a barrier, making Ni-Ti alloys have good biocompatibility. TiXOy and TixNiOy on the surface layer can inhibit the release of Ni.

6. Gentle orthodontic force: The currently used dental orthopedic wire includes austenitic stainless steel wire, cobalt-chromium-nickel alloy wire, nickel-chromium alloy wire, Australian alloy wire, gold alloy wire and ß titanium alloy wire. About the load-displacement curves of these orthodontic correction wires under tensile test and three-point bending test conditions. The unloading curve platform of the nickel-titanium alloy is the lowest and the flattest, which indicates that it can provide long-lasting and soft healing power.

7. Good shock absorption characteristics: The greater the vibration of the archwire caused by chewing and night molars, the greater the damage to the root and periodontal tissue. According to the results of different bow wire attenuation experiments, it is found that the vibration amplitude of stainless steel wire is larger than that of super elastic nickel-titanium wire. The initial vibration amplitude of super elastic nickel-titanium bow wire is only half of that of stainless steel wire. Dental health is important, and traditional archwires, such as stainless steel wires, have a tendency to aggravate root root absorption.

(Three) classification of nickel-titanium alloy wire EvansandDurning classification

1) In 1940, gold bow wire, cobalt chromium wire and stainless steel round wire

2) In 1960, martensite-stabilized alloys: Most were made of nickel-titanium alloys after being deformed in the martensitic state. This kind of arch wire has low rigidity and can produce light correction power. There is no martensitic transformation due to stress or temperature, so it does not exhibit memory effects and superelasticity.

3) In 1980, China's nickel-titanium alloy and Japan's nickel-titanium alloy arch wire were austenite-activated alloys: that is, in any state, they showed austenite state, and there was no martensite caused by temperature inside and outside the mouth Body state, martensite state can only be caused by stress, it is super elastic, but it does not have shape memory function. This kind of arch wire has excellent resilience and low stiffness, and can produce weaker correction force. The main feature of the masterpiece is that from the initial start to the final stage, the force produced by it is constantly constant, and the effect is better when the teeth are not uniform at the early stage of treatment. The disadvantage is that it cannot be bent at normal temperature and it is not easy to weld. If the company is used as the main arch wire, it can often cause undesired expansion or contraction, and it is difficult to establish a good premolar and molar arrangement.

4) In 1990, martensite-activated nickel-titanium alloy: that is, TTR is lower than or close to the oral temperature, and exists in a multi-state at room temperature, which is easy to deform. The transformation of martensite to austenite caused by room temperature has shape memory function and super elasticity. It is easy to deform at normal temperature (about 25 ° C) and below. When it reaches a certain temperature (about 32 ° C), it will return to its original pre-formed shape, showing shape memory plus super elasticity. Beijing Shengmart Technology Co., Ltd.'s Smart brand and 3M's NitinolHA brand are typical representative products. Because of this characteristic, heat-activated nickel-titanium arch wire can be easily manipulated and maintained at normal temperature and below. It can be placed in the bracket, and can be produced when it is activated by the temperature and heat in the mouth. Shape restoring force, in turn, provides the required strength for orthopedics. Because of the characteristics of "heat-activated nickel-titanium orthopedic filaments that become softer when cold and become more elastic when activated by heat", patients can change the correction force by using cold and hot water in the mouth under the guidance of a doctor. Facilitate the correction of the corrector and reduce the discomfort of the initial correction.

5) Gradedthermodynamic: increased thermodynamic nickel-titanium alloy: the TTR temperature is higher than the oral temperature, about 40 ° C. In this way, when the nickel-titanium archwire is placed in the oral cavity, it is still in a multiple state, and the archwire is softer, and in the mouth The austenite phase transition occurs only with hot water. Therefore, the correction force is 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 Japanese low-latency LH nickel-titanium arch wire have such properties.

(IV) Clinical application of nickel-titanium alloy wire:

1. For early alignment and flattening of dentition in patients. Because of the superelasticity and shape memory properties of nickel-titanium alloy archwires and the low stress-strain curve, nickel-titanium alloy archwires are routinely incorporated into the correction system as the initial stage in clinical practice. In this way, the patient's discomfort will be greatly reduced. Due to the existence of several straight wire arch correction techniques, MBT technology recommends using 0.016-inch heat-activated nickel-titanium arch wire (HANT wire), and DEMON self-locking bracket technology recommends using copper-containing heat-activated nickel-titanium produced by Omcro Corporation Alloy arch wire (phase transition temperature is about 40 degrees), O-PAK correction technology recommends using 0.016-inch super-elastic nickel-titanium alloy arch wire for early alignment and flattening.

2. Nitinol springs: Nitinol springs and tension springs are springs used for orthodontics. They have special properties of nickel-titanium superelasticity. They are suitable for orthodontic treatment to open the gap between teeth and pull teeth in different directions. A nickel-titanium coil spring with an extension of 1 mm produces a force of approximately 50 g. Nickel-titanium coil springs have very high elastic properties and can produce a softer and stable continuous force under tension. The attenuation of the force is small, and it can produce the ideal orthodontic force required for clinically moving teeth. Meet physiological requirements. The high elasticity and permanent deformation rate of the nickel-titanium wire tension spring are extremely low. Compared with the stainless steel wire of the same diameter, the released correction force is 3.5-4 times. Therefore, in the application of orthodontic treatment, patients not only have light pain, soft and lasting feeling power, but also reduce the time for follow-up visits, shorten the treatment period, and improve the curative effect.

3. LH arch wire is researched and developed by Dr.Soma and others in Japan and produced by Tomy. "LH" is named after "LowHysteresis", that is, when the archwire is ligated to the bracket, that is, the stress generated when the archwire is activated and the stress generated when the tooth is moved when the archwire is slowly restored The gap is small. That is, the lag is small. SOMA et al compared the stress-strain curves of LH archwires and other nickel-titanium alloy wires. The lag range of LH archwires is the smallest. This characteristic makes the archwires have the advantages of low load and sustained light force. This shows that the arch wire has low rigidity, and the hysteresis curves of the other types of nickel-titanium alloy arch wires indicate that it is relatively rigid. Obviously, LH arch wires have obvious mechanical advantages. Because the proportion of titanium in the nickel-titanium component of the LH wire is higher than that of ordinary nickel-titanium arch wires, it is called a titanium-nickel wire, and experiments have proved that its shock absorption effect is strong. Another feature of LH nickel-titanium wire is that it can be bent and heat-set with heat treatment equipment. Therefore, the LH nickel-titanium wire can also be leveled, aligned, opened to close, and closed at the final stage. That is to say, the treatment can be completed, as long as the arch wire is taken out and bent to the required shape at each stage, and then it is shaped with a heat treatment instrument to strengthen the hardness. At present, LH archwires are used in the clinic to expand the arch, and the treatment of open jaw, partial jaw, and reverse jaw is effective because of its continuous stability and softness. At the same time, J hooks are often used in combination to improve the weakness of softer archwires. Although MEAW technology also has an ideal effect on the correction of the above misaligned jaws, the complicated arch wire bending often prohibits many doctors. Therefore, some doctors use a rocking chair type nickel-titanium arch wire similar to the mechanical system and vertical traction of the front teeth. Although this has similar effects, they always feel that when compared with MEAW, the individual teeth are not as good as the MEAW technology. The rocking chair type nickel-titanium wire is a continuous arch wire, and there is no way to bend it. Therefore, the angle of bracket bonding and the curvature of the rocking chair of the archwire determine the angle of each tooth. Unlike the MEAW technology, the angle of each tooth has more room for individual adjustment. The use of LH nickel-titanium to bend the rocking chair, and then use the arch wire shaper to bend in the mouth and then lean back or lean forward, the effect is quite satisfactory.