Properties of Nitinol alloy
Nickel-titanium alloy is a shape memory alloy, a special alloy that can automatically restore its own plastic deformation to its original shape at a certain temperature, and has good plasticity.
Its elongation rate is above 20%, fatigue life reaches 1*10 to the 7th power, damping characteristics are 10 times higher than ordinary springs, and its corrosion resistance is better than the best medical stainless steel at present, so it can meet various engineering requirements. It is an excellent functional material that is very suitable for the requirements of medical applications.
In addition to its unique shape memory function, memory alloys also have excellent properties such as wear resistance, corrosion resistance, high damping, and superelasticity.
Special properties of nickel-titanium alloy
1. Shape memory characteristics (shape memory) Shape memory is that the parent phase of a certain shape is cooled from above the Af temperature to below the Mf temperature to form martensite, and the martensite is deformed at a temperature below Mf and heated to a temperature below Af. Through the reverse phase transition, the material automatically recovers the shape of its parent phase. In fact, the shape memory effect is a thermally induced phase transition process of Nitinol.
2. Superelasticity The so-called superelasticity refers to the phenomenon that the sample produces a strain far greater than its elastic limit strain under the action of an external force, and the strain energy automatically recovers when unloading. That is, in the state of the parent phase, due to the action of external stress, stress-induced martensitic transformation occurs, so that the alloy exhibits different mechanical behaviors from ordinary materials, and its elastic limit is far greater than that of ordinary materials. Hoogram's Law. In contrast to shape memory properties, superelasticity does not involve heat. All in all, hyperelasticity means that the stress does not increase with the increase of strain within a certain deformation range. Hyperelasticity can be divided into linear hyperelasticity and nonlinear hyperelasticity. In the former stress-strain curve, the relationship between stress and strain is close to linear. Nonlinear superelasticity refers to the result of stress-induced martensitic phase transformation and its inverse transformation in the process of loading and unloading in a certain temperature range above Af, so nonlinear superelasticity is also called phase transformation pseudoelasticity. The phase transition pseudoelasticity of nickel-titanium alloy can reach about 8%. The superelasticity of nickel-titanium alloy can change with the change of heat treatment conditions. When the archwire is heated above 400°C, the superelasticity begins to decrease.
3. Sensitivity to oral temperature changes: The orthodontic force of stainless steel wire and CoCr alloy orthodontic wire is basically not affected by oral temperature. Orthodontic force of superelastic Nitinol orthodontic wire varies with 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 the teeth, because the temperature change in the oral cavity will stimulate the blood flow in the stagnant part of the blood flow caused by the capillary stagnation caused by the orthodontic device, so that the cells of the restored teeth can be fully nourished during the movement and maintain their Vitality and normal function. On the other hand, orthodontists cannot precisely control or measure the corrective forces in the oral environment.
4. Corrosion resistance: Studies have shown that the corrosion resistance of nickel-titanium wire is similar to that of stainless steel wire
5. Anti-toxicity: The special chemical composition of nickel-titanium shape memory alloy, that is, it is an atomic alloy such as nickel-titanium, containing about 50% nickel, and nickel is known to have carcinogenic and cancer-promoting effects. In general, surface titanium oxidation acts as a barrier, making nickel-titanium alloys have good biocompatibility. TiXOy and TixNiOy on the surface can inhibit the release of Ni.
6. Soft orthodontic force: At present, the orthodontic wires used commercially include austenitic stainless steel wire, cobalt-chromium-nickel alloy wire, nickel-chromium alloy wire, Australian alloy wire, gold alloy wire and β-titanium alloy wire. Load-displacement curves of these orthodontic wires under tensile test and three-point bending test conditions. Nitinol had the lowest and flattest unloading curve plateau, indicating that it provided the longest-lasting, softest corrective force.
7. Good shock-absorbing properties: the greater the vibration generated by chewing and molars on the arch wire, the greater the damage to the root and periodontal tissue. According to the results of different archwire attenuation experiments, it is found that the vibration amplitude of the stainless steel wire is larger than that of the superelastic nickel-titanium wire, and the initial vibration amplitude of the superelastic nickel-titanium wire archwire is only half of it. Good vibration and shock-absorbing properties of the arch wire are very important for the health of the teeth, while traditional arch wires such as stainless steel wire tend to aggravate root resorption.
