1. Shape memory: Shape memory is when the parent phase of a certain shape is cooled from above Af temperature to below Mf temperature to form martensite, the martensite is deformed below MF temperature, heated below Af temperature, accompanied by inverse phase transformation, the material will automatically restore its shape at the parent phase. In fact, shape memory effect is a heat-induced phase transition process in Ni-Ti alloy.
2. Super elastic: Super elastic refers to the phenomenon that the strain produced by the specimen under external force is much larger than the initial elastic limit strain, and the strain can automatically recover when unloading. That is to say, in the parent phase state, stress-induced martensitic transformation occurs due to the effect of external stress, which results in the mechanical behavior of the alloy different from that of ordinary materials. Its elastic limit is far greater than that of ordinary materials, and it no longer obeys Hooke's law. Compared with shape memory characteristics, hyperelasticity has no thermal involvement. In a word, superelasticity means that the stress does not increase with the increase of strain in a certain range of deformation. It can be divided into linear hyperelasticity and non-linear hyperelasticity. In the stress-strain curve of the former, the relationship between stress and strain is close to linear. Nonlinear superelasticity is the result of stress-induced martensitic transformation and its inverse transformation during loading and unloading at a certain temperature range above Af, so it is also called phase transformation pseudoelasticity. The pseudoelasticity of phase transformation of Ni-Ti alloy can reach about 8%. The superelasticity of Ni-Ti alloy can be changed with the change of heat treatment conditions. When the arch wire is heated to 400-ordm and above C, the superelasticity begins to decrease.
3. Sensitivity of temperature change in oral cavity: The orthodontic force of stainless steel wire and CoCr alloy orthodontic wire was not affected by temperature in oral cavity. The orthodontic force of hyperelastic nickel-titanium alloy orthodontic wire varies with the change of oral temperature. When the deformation is constant. The orthodontic force increased with the increase of temperature. On the one hand, it can accelerate the movement of teeth, because the temperature changes in the oral cavity can stimulate the blood flow in the blood stagnation area caused by the capillary arrest caused by the orthodontic device, so that the repair cells can be fully nourished and maintain their vitality and normal functions during the tooth movement. On the other hand, orthodontists can not accurately control or measure the orthodontic force in the oral environment.
4. Corrosion resistance: Some studies have shown that the corrosion resistance of nickel-titanium wire is similar to that of stainless steel wire.
5. Anti-toxicity: Nickel-titanium shape memory alloy is a special chemical composition, that is, it is a nickel-titanium isoatomic alloy, containing about 50% of nickel, and nickel is known to have carcinogenic and carcinogenic effects. In general, the oxidation of titanium on the surface layer acts as a barrier, which makes the Ni-Ti alloy have good biocompatibility. TiXOy and TixNiOy in the surface layer can inhibit the release of Ni.
6. Soft orthodontic force: At present, commercial orthodontic wires include austenitic stainless steel wire, cobalt-chromium-nickel alloy wire, nickel-chromium alloy wire, Australian alloy wire, gold alloy wire and titanium alloy wire. The load-displacement curves of these orthodontic wires under tension test and three-point bending test conditions are discussed. The unloading curve platform of Ni-Ti alloy is the lowest and the flattest, which indicates that it can provide the most durable and soft orthodontic force.
7. Good shock absorption characteristics: The greater the vibration caused by chewing and night molars to the archwire, the greater the damage to the root and periodontal tissue. The results of different archwire attenuation experiments show that the vibration amplitude of stainless steel wire is larger than that of superelastic nickel-titanium wire, and the initial vibration amplitude of superelastic nickel-titanium archwire is only half of that of stainless steel wire. Good vibration and vibration reduction characteristics of archwire are very important for the health of teeth, while traditional archwire such as stainless steel wire tends to increase root absorption.
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