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1. Shape memory refers to that after the parent phase of a certain shape is cooled from above Af temperature to below Mf temperature to form martensite, the martensite will be deformed at the temperature below Mf and heated to below Af temperature. With the reverse phase change, the material will automatically restore its shape in the parent phase. In fact, the shape memory effect is a thermally induced phase transition in Nitinol alloys.
2. Super elastic the so-called super elastic refers to the phenomenon that the specimen generates a strain far greater than the elastic limit strain under the action of external forces, and the strain can be recovered automatically during unloading. In other words, under the parent state, the stress induced martensite phase transition occurs due to the action of external stress, so that the mechanical behavior of the alloy is different from that of ordinary materials. Its elastic limit is far greater than that of ordinary materials, and it no longer obeys hooke's law. Compared with shape memory, hyper elasticity has no thermal involvement. In a word, hyper elasticity refers to that the stress does not increase with the increase of strain within a certain deformation range. It can be divided into linear hyper elasticity and nonlinear hyper elasticity. The stress - strain curve of the former is nearly linear. Nonlinear hyper elasticity refers to the results of stress-induced martensite phase transition and its inverse phase transition respectively in the loading and unloading processes within a certain temperature range above Af. Therefore, nonlinear hyper elasticity is also known as phase transition pseudo-elasticity. The phase-change pseudo elasticity of nickel-titanium alloy can reach about 8%.The hyper elasticity of nickel-titanium alloy can be changed with the change of heat treatment conditions. When the bow wire is heated to over 400oC, the hyper elasticity begins to decline.
3. Sensitivity to temperature changes in the oral cavity: the correction force of stainless steel wire and CoCr alloy dental orthodontic wire is basically not affected by the temperature in the oral cavity. The orthodontic force of the super elastic Nitinol dental 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 can accelerate the movement of teeth, because the temperature change in the mouth will stimulate the blood flow in the stagnant part of the blood flow caused by capillary stagnation caused by the orthodontic device, so that the repair cells can be fully nourished during the tooth movement to maintain their vitality and normal function. On the other hand, orthodontists cannot precisely control or measure the orthodontic force in the oral environment.
4. Corrosion resistance: studies have shown that the corrosion resistance of niti 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 nickel-titanium and other atomic alloy, containing about 50% nickel, and nickel is known to have carcinogenic and carcinogenic effect. In general, titanium oxidation on the surface layer ACTS as a barrier, making the ni-ti alloy have good biocompatibility. TiXOy and TixNiOy in the surface layer can inhibit the release of Ni.
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