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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, after heating to below Af temperature, accompanied by reverse phase transformation, the material will automatically restore its shape in the parent phase. In fact, shape memory effect is a thermally induced phase transition process of Nitinol.
Superelasticity refers to the phenomenon where a specimen is subjected to an external force with a strain far greater than the elastic limit and the strain automatically recovers upon unloading. That is, in the mother phase state, the stress induced martensitic transformation occurs due to the effect of the applied stress, so that the alloy exhibits mechanical behavior different from that of ordinary materials, and its elastic limit is far greater than that of ordinary materials, and it no longer obeys Hooke's law. Compared with shape memory properties, hyperelasticity has no thermal involvement.In a word, superelasticity means that the stress does not increase with the increase of strain within a certain range of deformation. Superelasticity can be divided into linear superelasticity and nonlinear superelasticity. The relationship between stress and strain in the stress-strain curve of the former is nearly linear. Nonlinear hyperelasticity refers to the results of stress-induced martensite transformation and its inverse transformation in the loading and unloading processes within a certain temperature range above Af. Therefore, nonlinear hyperelasticity is also called phase transition pseudo-elasticity. The pseudo-elasticity of phase transition of Nitinol alloy is about 8%.The superelasticity of Nitinol alloy can be changed with the change of heat treatment conditions. Above C, the hyperelasticity begins to decrease.
1, Sensitivity to intraoral temperature change: The orthodontic force of stainless steel wire and CoCr alloy orthodontic wire is not affected by intraoral temperature. The orthodontic force of hyperelastic Nitinol dental orthodontic wire changes with the change of oral temperature. When the amount of deformation is constant. As the temperature increases, the correction force increases. On the one hand, it speeds up tooth movement because the temperature change in the mouth stimulates blood flow in the area of blood stasis caused by the appliance, which causes capillary stagnation, so that the repair cells can be adequately nourished during tooth movement to maintain their vitality and normal function. On the other hand, orthodontists cannot precisely control or measure orthodontic force in the oral environment.
2, corrosion resistance: studies have shown that the corrosion resistance of nickel titanium wire and stainless steel wire similar
3, anti-toxicity: nickel-titanium shape memory alloy special chemical composition, that is, it is a nickel-titanium atomic alloy, containing about 50% nickel, and nickel is known to have carcinogenic and carcinogenic effect. In general, the surface layer titanium oxidation acts as a barrier to make Ni-Ti alloys have good biocompatibility. The TiXOy and TixNiOy of the surface layer can inhibit the release of Ni.
4. Soft orthodontic force: the current commercial application of dental orthodontic wire includes austenitic stainless steel wire, cobalt-chrome-nickel alloy wire, nickel-chromium alloy wire, Australian alloy wire, gold alloy wire and ß titanium alloy wire. Load - displacement curves of these orthodontic correction wires under tensile test and three point bending test conditions. Nickel-titanium alloy has the lowest and most flat unloading curve platform, indicating that it can provide the most durable soft correction force.
5. Good shock absorption characteristics: The greater the vibration caused by chewing and night molar to the arch cable, the greater the damage to the root and periodontal tissue. Through different attenuation experiment results, the study found the arch wire stainless steel wire vibration amplitude is larger than hyperelastic nickel titanium silk, super elastic nickel-titanium arch wire the initial vibration amplitude is only half of stainless steel wire, good vibration and shock absorption characteristics of the arch wire is important for the health of the teeth, and traditional arch wire such as stainless steel wire, tend to increase the root absorption.
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