Shape memory is that after 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 and heated to below Af temperature. With inverse phase transformation, the material will automatically restore its shape in the parent phase.In fact, shape memory effect is a heat-induced phase change process of Nitinol alloy.
The so-called superelasticity refers to the phenomenon that the strain of the sample under the action of external force is much larger than the strain of elastic limit strain, and the strain can automatically recover when unloading.In other words, under the condition of parent phase, due to the action of external stress, the stress-induced martensitic transformation occurs, and the alloy shows the mechanical behavior different from that of ordinary materials. Its elastic limit is much larger than that of ordinary materials, and it no longer obeys hooke's law.Hyperelasticity has no thermal involvement compared to shape memory properties.In a word, hyperelasticity refers to that the stress does not increase with the increase of strain within a certain range of deformation, which is clinically manifested as the orthodontic force generated by the arch wire during deformation remains constant, and it is no longer gradually lost with the movement of teeth towards the orthodontic direction.According to the characteristics of stress-strain curve corresponding to superelasticity, the superelasticity can be divided into linear and nonlinear superelasticity.In the former stress-strain curve, the relationship between stress and strain is nearly linear.Nonlinear hyperelasticity refers to the results of stress-induced martensitic transformation and its inverse transformation during the loading and unloading of Af above a certain temperature range. Therefore, nonlinear hyperelasticity is also known as phase change pseudo-elasticity.The pseudo-elasticity of Nitinol alloy is up to 8%.The superelasticity of Nitinol alloy can be changed with the change of heat treatment conditions. When the bow wire is heated to above 400oC, the superelasticity starts to decrease.When the heat treatment temperature exceeds 600oC, the super-elastic basic hour.According to this characteristic, the arch wire can be heat-treated in the non-orthodontic area to make it lose its hyperelasticity, so as to avoid the influence of the orthodontic process on the teeth in the non-orthodontic area, and the arch wire in the orthodontic area still has good elastic energy.
Sensitivity of oral temperature change: the orthodontic force of stainless steel wire and CoCr alloy dental orthopedic wire is basically not affected by the oral temperature.The orthodontic force of hyperelastic Nitinol alloy dental wire varies with the change of oral temperature.When the amount of deformation is constant.As the temperature increases, the corrective force increases.On the one hand, it can speed up the movement of the teeth. This is because the temperature change in the oral cavity can stimulate the blood flow in the stagnant blood flow area caused by the orthodontic devices, so that the repair cells can get adequate nutrition during the tooth movement to maintain their vitality and normal function.On the other hand, orthodontists cannot accurately control or measure orthodontic forces in the oral environment.
Corrosion resistance: research shows that the corrosion resistance of niti wire is similar to that of stainless steel wire
anti-toxicity: niti shape memory alloy special chemical composition, that is, it is a niti and other atomic alloy, containing about 50% nickel, and nickel is known to have carcinogenic and carcinogenic effect.In general, the surface layer of titanium oxidation ACTS as a barrier, so that the ni-ti alloy has a good biocompatibility.TiXOy and TixNiOy of the surface layer can inhibit the release of Ni.
Soft orthodontic force: currently commercially available orthodontic wires include austenitic stainless steel wire, cobalt-chrome-nickel wire, nickel-chromium wire, Australian wire, gold wire and nickel-titanium wire.The load-displacement curves of these orthodontic wires under tensile test and three-point bending test conditions were studied.Nickel-titanium alloy has the lowest and smoothest unloading curve, which indicates that it provides the most durable and gentle correction.
Good shock absorption characteristics: the greater the vibration caused by mastication and night molar to the arch wire, 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.