1. Shape memory refers to that when the parent phase of a certain shape is cooled from Af temperature to Mf temperature to form martensite, the martensite is deformed below Mf temperature, and heated to below Af temperature. With reverse phase transformation, the material will automatically recover its shape in the parent phase. In fact, the shape memory effect is a heat induced phase transformation process of Ni Ti alloy.

2. The so-called superelasticity refers to the phenomenon that the specimen produces a strain far greater than the elastic limit strain under the action of external force, and the strain can recover automatically when unloading. That is, in the parent phase state, due to the effect of external stress, stress induced martensite transformation occurs, so the alloy shows a mechanical behavior 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 properties, superelasticity has no thermal involvement. In a word, hyperelasticity refers to that the stress does not increase with the increase of strain in a certain deformation range. Superelasticity can be divided into linear hyperelasticity and nonlinear hyperelasticity. The relationship between stress and strain in the stress-strain curve of the former is close to linear. Nonlinear hyperelasticity refers to the result of stress induced martensitic transformation and its reverse transformation during loading and unloading in a certain temperature range above Af. Therefore, nonlinear hyperelasticity is also called transformation pseudoelasticity. The pseudo elasticity of Ni Ti alloy is about 8%. The hyperelasticity of NiTi alloy can change with the change of heat treatment conditions. When the arch wire is heated to more than 400 º C, the hyperelasticity starts to decrease.

3. Sensitivity to temperature changes in the oral cavity: The correction force of stainless steel wire and CoCr alloy dental orthopedic wire is basically not affected by the temperature in the oral cavity. The orthodontic force of super elastic Ni Ti alloy orthodontic wire varies with the change of oral temperature. When the deformation is fixed. With the increase of temperature, the corrective force increases. On the one hand, it can accelerate the movement of the teeth, because the temperature change in the mouth will stimulate the blood flow in the blood flow stagnation part caused by the appliance, so that the repair cells can be fully nourished during the tooth movement, and maintain their vitality and normal function. On the other hand, orthodontists cannot accurately control or measure the orthodontic force in the oral environment.

4. Corrosion resistance: research shows that the corrosion resistance of nickel titanium wire is similar to that of stainless steel wire

5. Toxicity resistance: NiTi shape memory alloy has a special chemical composition, that is, it is an atomic alloy such as NiTi, containing about 50% nickel. Nickel is known to have carcinogenic and cancer promoting effects. In general, the surface layer of titanium oxide acts as a barrier, making Ni Ti alloys have good biocompatibility. TiXOy and TixNiOy in the surface layer can inhibit the release of Ni.

6. Soft orthodontic force: currently commercially used dental orthopedic wires include austenitic stainless steel wire, cobalt chromium nickel alloy wire, nickel chromium alloy wire, Australian alloy wire, alloy wire and titanium alloy wire. The load displacement curves of these orthodontic wires under tensile test and three-point bending test conditions. The unloading curve platform of NiTi alloy is the lowest and the flattest, which indicates that it can provide the most lasting and gentle corrective force.

7. Good shock absorption characteristics: the greater the vibration caused by chewing and night grinding to the arch wire, the greater the damage to the root and periodontal tissue. Through the research on the results of different arch wire attenuation experiments, it is found that the vibration amplitude of stainless steel wire is larger than that of hyperelastic nickel titanium wire, and the initial vibration amplitude of hyperelastic nickel titanium arch wire is only half of that of stainless steel wire. The good vibration and shock absorption characteristics of arch wire are important for the health of teeth, while traditional arch wire, such as stainless steel wire, tends to increase the root absorption.