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Property and characteristics of Nititanium alloy

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[Abstract]:
Introduction to nickel-titanium alloy

(1) Phase change and properties of nickel-titanium alloy

As the name suggests, nickel-titanium alloy is a binary alloy composed of nickel and titanium, and two different crystal structure phases exist due to the changing temperature and mechanical pressure, which are the austensite and martensite phases. The phase transition order of nickel-titanium alloy during cooling is the parent phase (austensite phase) -R phase-martensite phase. The R phase is a diamond square, the austenite is the state of the higher temperature (greater than the same: the starting temperature of the austenite), or the local load (the external force removes the Deactivation), the cube is hard. The shape is relatively stable. The martensite phase is a state of relatively low temperature (less than Mf: the temperature at the end of martensite) or loading (activated by external forces), hexagonal, extended, repetitive, less stable, and easy to deform.

(2) Special properties of nickel-titanium alloy

1. Shape memory characteristics (shape memory) Shape memory is when a certain shape of the parent phase is cooled from Af temperature above to below Mf temperature formation, martensite temperature below Mf deformation, after heating to Af temperature below, with the reverse phase change, the material will automatically restore its shape in the parent phase. Actually shape memory effect is a heat-induced phase transition process of nittitanium alloy.

2. Hyperelasticity (superelastic) The so-called superelasticity refers to the phenomenon that the sample strain produces much greater than the elastic limit variable under the action of external force, and the strain can be automatically recovered when unloading. That is, in the parent phase state, due to the action of external stress, the stress-induced martensite phase transition occurs, so that the alloy shows the mechanical behavior that is different from ordinary materials, its elasticity limit is far greater than that of ordinary materials, and no longer abide by Huke's law. In contrast to the shape memory properties, hyperelasticity has no thermal involvement. In a word, hyperelasticity refers to that the stress does not increase with the increase of strain within a certain range of deformation, and it can be divided into linear superelasticity and nonlinear superelasticity. The stress and strain are nearly linear in the former stress-strain curve. Nonlinear hyperelasticity refers to the result of the stress-induced Martensite phase transition and its reverse phase transition respectively during loading and unloading in a certain temperature interval above Af. Therefore, the nonlinear hyperelasticity is also called the phase transition pseudoelasticity. The phase transition elasticity of nittitanium alloy can reach about 8%. The super elasticity of nittitanium alloy can change with the conditions of heat treatment and begins to decline when the bow wire is heated above 400oC.

3. Sensitivity of intraoral temperature change: the orthodontic force of stainless steel wire and CoCr alloy tooth orthopedic wire is basically not affected by the temperature in the oral cavity. The orthodontic force of superelastic nickel-titanium alloy teeth varies with the change of oral temperature. When the deformation amount 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 change in the mouth can stimulate the blood flow in the stagnant blood flow caused by the appliance, thus making the repair cells fully nourished during the tooth movement to maintain their vitality and normal function. On the other hand, the orthodontists are unable to accurately control or measure the orthodontic forces in the oral environment.

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

5. Anti-toxicity: The special chemical composition of nickel titanium shape memory alloy, that is, this is a atomic alloy of nickel titanium and other atomic alloy, containing about 50% nickel, and nickel is known to have carcinogenic and cancer-promoting effects. In general, the surface-layer titanium oxidation serves as a barrier for the good biocompatibility of Ni-Ti alloys. The TiXOy and TixNiOy of the surface layer can inhibit the Ni release.