Nitinol alloy is a shape memory alloy,and the shape memory alloy is a special alloy that can automatically restore its plastic deformation to its original shape at a certain temperature.Its expansion rate is more than 20%, fatigue life is up to the 7th power of 1×10, damping characteristics are 10 times higher than ordinary springs, and its corrosion resistance is better than the current best medical stainless steel, so it can meet the application needs of all kinds of engineering and medicine, and is a very excellent functional material.
In addition to its unique shape memory function, the memory alloy also has excellent characteristics such as wear resistance, corrosion resistance, high damping and super elasticity. Now, let’s summarize the 7 special properties of nitinol alloy：
Shape memory is when the mother phase of a certain shape is cooled from above the Af temperature to below the Mf temperature to form martensite,the martensite is deformed at the temperature below the Mf, and heated to below the Af temperature. With the reverse phase change, the material will automatically restore its shape at the time of the mother phase. In fact, the shape memory effect is a heat-induced phase transition process of nitinol alloy.
The so-called superelasticity refers to the phenomenon that the sample produces a strain much larger than the elastic limit strain under the action of external forces, and the strain can be automatically restored when unloaded.That is, in the parent phase state, due to the action of the applied stress, the stress-induced martensite phase transition occurs, so that the alloy exhibits mechanical behavior different from ordinary materials, its elastic limit is much larger than ordinary materials, and no longer complies with Hook’s law.
Studies have shown that the corrosion resistance of nitinol wire is similar to that of stainless steel wire.
The orthodontic power of stainless steel wire and CoCr alloy dental orthopedic wire is basically not affected by the temperature in the oral cavity.The orthodontic power of super elastic nitinol alloy tooth orthopedic wire varies with the change of oral temperature.When the amount of deformation is certain.The temperature increases, and the correction force increases.On the one hand,it can accelerate the movement of the teeth, because the temperature changes in the mouth will stimulate the blood flow in the stagnant parts of the blood flow due to capillary stagnation caused by the orthodontic device, so that the repair cells are fully nourished during the movement of the teeth, maintaining their vitality and normal function.On the other hand, orthodontists can’t accurately control or measure the orthodontic power in the oral environment.
Nnder normal circumstances, the surface layer of titanium oxidation acts as a barrier, so that Ni-Ti alloy has good biocompatibility.TiXOy and TixNiOy of the surface layer can inhibit the release of Ni.
The current commercial application of dental orthopedic wire includes austenitic stainless steel wire,cobalt-chromium-nickel alloy wire,nickel-chromium alloy wire,Australian alloy wire and titanium alloy wire.About the load-displacement curve of these orthodontic wire in the tensile test and three-point bending test conditions.The unloading curve platform of nitinol alloy is the lowest and flatest, indicating that it can best provide long-lasting and soft correction.
Due to chewing and night molars, the greater the vibration caused by the arch wire, the greater the damage to the root and periodontal tissue.Through the results of different bow wire attenuation experiments,it was found that the amplitude of vibration of stainless steel wire is larger than that of superelastic nitinol wire, and the initial vibration amplitude of superelastic nitinol wire is only half that of stainless steel wire. The good vibration and shock absorption characteristics of bow wire are very important for the health of teeth, while traditional bow wire such as stainless steel wire has a tendency to increase root absorption.