1. Shape memory characteristic shape memory is that 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 at the temperature below MF and heated to below Af temperature. With reverse phase transformation, the material will automatically recover its shape in the parent phase. In fact, shape memory effect is a thermally induced phase transformation process of NiTi alloy.
2. Superelasticity refers to the phenomenon that the strain produced by the specimen under the action of external force is much larger than the elastic limit strain, and the strain can recover automatically when unloading. That is, in the parent phase state, due to the effect of external stress, the stress-induced martensitic transformation occurs, so the alloy exhibits a mechanical behavior different from that of ordinary materials. Its elastic limit is far greater than that of ordinary materials, and it no longer complies with Hooke's law.
Compared with shape memory properties, superelasticity has no thermal involvement. In a word, hyperelasticity refers to that the internal stress does not increase with the increase of strain in a certain deformation range. Hyperelasticity can be divided into linear hyperelasticity and nonlinear hyperelasticity. 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 martensitic transformation and its inverse transformation during loading and unloading in a certain temperature range above AF. therefore, nonlinear hyperelasticity is also called phase transformation pseudo elasticity.
The pseudo elasticity of Ni Ti alloy can reach about 8%. The superelasticity of nitinol alloy can be changed with the change of heat treatment conditions. When the arch wire is heated above 400oC, the superelasticity begins to decrease.
3. Sensitivity to temperature change in the oral cavity: the orthodontic force of stainless steel wire and CoCr alloy orthodontic wire is basically not affected by the temperature in the oral cavity. The orthodontic force of superelastic nitinol orthodontic wire changes with the change of oral temperature.
When the amount of deformation is constant. The temperature increased and the corrective force increased. On the one hand, it can accelerate the movement of the teeth, because the temperature change in the oral cavity will stimulate the blood flow at the blood flow stagnation site caused by the corrective devices, so that the repair cells can be fully nourished during the tooth movement and maintain their vitality and normal functions. On the other hand, orthodontists cannot accurately control or measure the orthodontic force in the oral environment.
4. Corrosion resistance: some studies show that the corrosion resistance of nickel titanium wire is similar to that of stainless steel wire
5. Anti toxicity: nitinol shape memory alloy has a special chemical composition, that is, it is an atomic alloy such as nitinol, containing about 50% nickel, and nickel is known to have carcinogenic and cancer promoting effects. In general, the oxidation of titanium on the surface layer acts as a barrier, which makes Ni Ti alloys have good biocompatibility. TixOy and tixnioy in the surface layer can inhibit the release of Ni.
6. Soft orthodontic force: Currently, the commercially used dental orthodontic wires include austenitic stainless steel wire, cobalt chromium nickel alloy wire, nickel chromium alloy wire, Australian alloy wire and titanium alloy wire. Load displacement curves of these orthodontic wires under tensile test and three-point bending test conditions. The unloading curve platform of nitinol alloy is the lowest and the most flat, indicating that it can provide the most durable and soft corrective force.
7. Good shock absorption characteristics: the greater the vibration caused by chewing and grinding on the arch wire, the greater the damage to the root and periodontal tissue. According to the results of different arch wire attenuation experiments, it is found that the vibration amplitude of stainless steel wire is larger than that of superelastic nickel titanium wire. The initial vibration amplitude of superelastic nickel titanium arch wire is only half of that of stainless steel wire. The good vibration and shock absorption characteristics of arch wire are very important for the health of teeth, while the traditional arch wire, such as stainless steel wire, tends to increase the root absorption.
