Shape memory characteristics
When a certain shape of the parent phase is cooled from above Af temperature to below Mf temperature to form martensite, the martensite is deformed at a temperature below Mf, and after heating to below Af temperature, the material will automatically return to its parent shape of phase. In fact, the shape memory effect is a thermally induced phase transformation process of Nitinol.
The so-called superelasticity refers to the phenomenon that the specimen produces a strain much larger than the elastic limit strain under the action of external force, and the strain can automatically recover when unloaded. The superelasticity of nickel-titanium alloy can change with the change of heat treatment conditions. When the archwire is heated above 400°C, the superelasticity begins to decrease.
Sensitivity to temperature changes in the mouth
The corrective force of stainless steel wire and CoCr alloy orthopedic wire is basically not affected by the temperature in the oral cavity. The corrective force of the super-elastic nickel-titanium alloy orthopedic wire changes with the change of the oral cavity temperature. 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. On the other hand, orthodontists cannot precisely control or measure the orthodontic force in the oral environment.
Studies have shown that the corrosion resistance of nickel-titanium wire is similar to that of stainless steel wire.
Nitinol shape memory alloy has a special chemical composition, that is, it is an atomic alloy such as nickel-titanium, containing about 50% nickel, and nickel is known to have carcinogenic and cancer-promoting effects. Under normal circumstances, the surface layer of titanium oxide acts as a barrier, so that Ni-Ti alloy has good biocompatibility. TiXOy and TixNiOy in the surface layer can inhibit the release of Ni.
Among the currently commercially used orthopedic wires, the unloading curve platform of Nitinol is the lowest and the flattest, indicating that it can provide the most durable and soft orthodontic power.
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