nitinol metal properties nitinol wire
Main uses: fishing line,super elastic memory alloy mobile phone antennas,optical glasses frames, Bluetooth headsets, medical.
Titanium nickel alloy wire concept
Nitinol is a shape memory alloy. Shape memory alloy is a special alloy that can automatically restore its plastic deformation to its original shape at a certain temperature. Its expansion rate is more than 20%, fatigue life is 107 times, damping characteristics are 10 times higher than ordinary springs, its corrosion resistance is better than the best medical stainless steel at present, so it can meet the needs of various engineering and medical applications. It is a very good functional material.
In addition to its unique shape memory function, memory alloys also have excellent characteristics such as wear resistance, corrosion resistance, high damping and super elasticity.
The characteristics of nickel-titanium alloy wire and its clinical application in the field of orthodontics. Nickel-titanium alloy is widely used for its superior superelasticity, shape memory function, corrosion resistance, good biocompatibility and shock absorption characteristics. Orthodontics field.
(1) Phase transformation and properties of Nitinol
As the name suggests, a nickel-titanium alloy is a binary alloy composed of nickel ions and titanium ions. Due to changes in temperature and mechanical pressure, there are two different crystal structure phases, namely, austenite phase and martensite phase (Martensite phase). ). The phase transformation sequence of nickel-titanium alloy during cooling is the mother phase (austenite phase) -R phase-martensite phase. The R phase is rhombohedral, and austenite is a state when the temperature is higher (greater than the same: the temperature at which austenite starts), or when the load is removed (external force is removed from Deactivation). The shape is relatively stable. The martensite phase is the state when the temperature is relatively low (less than Mf: the temperature at which the martensite ends) or loaded (activated by external force), hexagonal, malleable, repeatable, less stable, and easier Deformed. Therefore, determining the phase transition temperature of Nitinol arch wire clinically has positive guiding significance, so that clinicians can better use the performance of Nitinol alloy for clinical orthodontic treatment.
Special properties of Nitinol
1. Shape memory (shape memory) shape memory is when a certain shape of the parent phase is cooled from above Af temperature to below Mf temperature to form martensite, then the martensite is deformed at a 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 heat-induced phase transformation process of nickel-titanium alloy.
2. Superelastic (superelastic) The so-called superelastic refers to the phenomenon that the sample produces a strain that is much larger than the elastic limit strain under the action of external force, and the strain can automatically recover when unloaded. That is, in the state of the parent phase, due to the effect of applied stress, the stress-induced martensite transformation occurs, so that the alloy exhibits a mechanical behavior different from that of ordinary materials, and its elastic limit is much greater than that of ordinary materials, and no longer observes the tiger. Law. Compared with shape memory properties, superelasticity does not involve heat. In short, superelasticity means that the stress does not increase with the increase of strain within a certain deformation range. Clinically, it shows that the correction force generated by the arch wire during the deformation process remains constant and no longer gradually with the movement of the tooth in the direction of correction Lost. According to the characteristics of the stress-strain curve corresponding to superelasticity, superelasticity can be divided into linear superelasticity and nonlinear superelasticity. The stress-strain curve in the former is close to a linear relationship between stress and strain. Non-linear superelasticity refers to the results of stress-induced martensitic transformation and its inverse phase transformation during loading and unloading in a certain temperature range above Af. Therefore, nonlinear hyperelasticity is also called phase transition pseudoelasticity. The pseudo-elasticity of the nickel-titanium alloy can reach about 8%. The superelasticity of nickel-titanium alloy can be changed with the change of heat treatment conditions. When the archwire is heated above 400oC, the superelasticity begins to decrease. When the heat treatment temperature exceeds 600oC, the superelasticity is basically small. According to this feature, the non-corrected area of the arch wire can be heat-treated clinically to make it lose its superelasticity, which can avoid the influence of the correction process on the teeth in the non-corrected area, and the arch wire in the corrected area still has good elasticity.
3. Sensitivity of temperature change in oral cavity: The curative power of stainless steel wire and CoCr alloy orthodontic wire is basically not affected by the temperature in oral cavity. The orthodontic power of superelastic nickel-titanium alloy orthodontic wire changes with the change of oral temperature. When the amount of deformation is constant. As the temperature rises, the correction power increases. On the one hand, it can accelerate the movement of teeth, because the temperature changes in the oral cavity will stimulate the blood flow in the stagnation of blood flow caused by the correction device, so that the repair cells are fully nourished during the movement of the teeth. Maintain its vitality and normal function. On the other hand, orthodontists cannot accurately control or measure the correction power in the oral environment.
4. Corrosion resistance: Some 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, which is a kind of nickel-titanium and other atomic alloy, contains about 50% nickel, and nickel is known to have carcinogenic and cancer-promoting effects. In general, the titanium oxide on the surface layer acts as a barrier to make Ni-Ti alloys have good biocompatibility. TiXOy and TixNiOy of the surface layer can suppress the release of Ni.
6. Soft orthodontic power: The currently used orthodontic metal wires include austenitic stainless steel wire, cobalt-chromium-nickel alloy wire, nickel-chromium alloy wire, Australian alloy wire, gold alloy wire and titanium alloy wire. The load-displacement curve of these orthodontic correction wires under tensile test and three-point bending test conditions. Nitinol’s unloading curve platform is the lowest and flattest, indicating that it can provide long-lasting and gentle healing power.
7. Good shock absorption characteristics: The greater the shock to the arch wire caused by chewing and nocturnal teeth, the greater the damage to the roots and periodontal tissues. Through the results of different arch wire 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 arch wire is only half of that of the stainless steel wire. The good vibration and shock absorption characteristics of the arch wire The health of teeth is very important, and traditional arch wire, such as stainless steel wire, has a tendency to increase the absorption of tooth roots.