1. Shape memory The shape memory is that when a certain shape of the mother phase is cooled from the Af temperature to below the Mf temperature to form martensite, the martensite is deformed at a temperature lower than Mf and heated to below the Af temperature. With the reverse phase change, the material automatically recovers its shape at the parent phase. In fact, the shape memory effect is a thermally induced phase transition process of Nitinol.
2 superelastic (superelastic) The so-called superelasticity refers to the phenomenon that the sample produces a strain far greater than the elastic limit strain under external force, and the strain can be automatically recovered during unloading. That is, in the mother phase state, the stress-induced martensitic transformation occurs due to the applied stress, so that the alloy exhibits a mechanical behavior different from that of ordinary materials, and its elastic limit is far greater than that of ordinary materials, and the tiger is no longer observed. Gram law. Compared to shape memory characteristics, superelasticity does not involve heat. In summary, superelasticity means that the stress does not increase with the increase of strain within a certain deformation range, and the superelasticity can be divided into two types: linear superelasticity and nonlinear superelasticity. The stress-strain curve in the former has a linear relationship between stress and strain. Nonlinear superelasticity is the result of stress-induced martensitic transformation and reverse phase transformation during loading and unloading in a certain temperature range above Af. Therefore, nonlinear superelasticity is also called phase transformation pseudoelasticity. The phase-change pseudo-elasticity of Ni-Ti alloy can reach about 8%. The superelasticity of Nitinol can be changed as the conditions of the heat treatment change. When the archwire is heated above 400oC, the superelasticity begins to decrease.
1. Corrosion resistance: Some studies have shown that the corrosion resistance of nickel-titanium wire is similar to that of stainless steel wire.
2. 4. Anti-toxicity: The special chemical composition of nickel-titanium shape memory alloy, that is, an atomic alloy such as nickel-titanium, containing about 50% Nickel, which is known to have carcinogenic and cancer-promoting effects. Under normal circumstances, the surface layer of titanium oxide acts as a barrier to make the Ni-Ti alloy have good biocompatibility. The surface layer of TiXOy and TixNiOy can suppress the release of Ni.
5. Sensitivity of temperature change in the oral cavity: The corrective force of the stainless steel wire and CoCr alloy orthodontic wire is basically not affected by the temperature inside the mouth. The orthodontic force of a superelastic nickel-titanium tooth orthodontic wire varies with the temperature of the oral cavity. When the amount of deformation is constant. As the temperature increases, the corrective force increases. On the one hand, it can accelerate the movement of the teeth, because the temperature changes in the mouth stimulate the blood flow in the stagnant part of the blood flow caused by the stagnation of the device, so that the repaired cells are fully nourished during the movement of the teeth. Maintain its vitality and normal function. On the other hand, orthodontists cannot precisely control or measure the corrective force in the oral environment.
6. Gentle orthodontic force: Currently used orthodontic wires for commercial use include austenitic stainless steel wire, cobalt-chromium-nickel alloy wire, nickel-chromium alloy wire, Australian alloy wire, gold alloy wire and ß titanium alloy wire. The load-displacement curves for these orthodontic correction wires under tensile test and three-point bending test conditions. The unloading curve platform of Nitinol is the lowest and flattest, indicating that it provides the most durable and gentle correction.
