What are the physical properties and uses of Nitinol superelastic memory alloys?
1. Shape memory (shape memory) Shape memory is when the parent phase of a certain shape is cooled from above the Af temperature to below the Mf temperature to form martensite, the martensite is deformed at a temperature below Mf, and heated to below the Af temperature, With the reverse phase transition, the material automatically returns to its shape in the parent phase. In fact, the shape memory effect is a thermally induced phase transformation process in Nitinol.
2. Superelasticity The so-called superelasticity refers to the phenomenon that the sample produces a strain far greater 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 action of external stress, the stress-induced martensitic transformation occurs, so that the alloy exhibits mechanical behavior different from ordinary materials, its elastic limit is much larger than that of ordinary materials, and it no longer obeys Hu. Gram's Law.
In contrast to shape memory properties, superelasticity has no thermal involvement. All in all, hyperelasticity means that the stress does not increase with the increase of strain within a certain deformation range, and 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 transformation and its inverse transformation during loading and unloading in a certain temperature range above Af, so nonlinear hyperelasticity is also called transformation pseudoelasticity.
The phase transformation pseudoelasticity of NiTi alloy can reach about 8%. The superelasticity of Nitinol can be changed with the change of heat treatment conditions, and when the arch wire is heated above 400oC, the superelasticity begins to decrease.
3. Sensitivity to intraoral temperature change: The orthodontic force of stainless steel wire and CoCr alloy orthodontic wire is basically not affected by the intraoral temperature. The orthodontic force of superelastic nickel-titanium alloy orthodontic wire varies with oral temperature.
When the amount of deformation is constant. As the temperature increases, the healing power 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 blood flow caused by the capillary stagnation caused by the orthodontic device, so that the repair cells are fully nourished during the movement of the teeth, maintain its vitality and normal function. Orthodontists, on the other hand, cannot precisely control or measure orthodontic 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 and titanium, containing about 50% nickel, and nickel is known to have carcinogenic and cancer-promoting effects. In general, the surface layer titanium oxide acts as a kind of barrier, making Ni-Ti alloys have good biocompatibility.
The TiXOy and TixNiOy in the surface layer can inhibit the release of Ni.
6. Soft orthodontic force: Currently commercially used orthodontic metal 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 for these orthodontic wires under tensile test and three-point bending test conditions.
The unloading curve platform of nickel-titanium alloy is the lowest and the flattest, indicating that it can provide the most durable and gentle orthodontic force.
7. Good shock absorption characteristics: The greater the vibration of the arch wire caused by chewing and grinding, the greater the damage to the root and periodontal tissue. 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 NiTi wire, and the initial vibration amplitude of the superelastic NiTi arch wire is only half of that of the stainless steel wire. Tooth health is very important, and traditional arch wires, such as stainless steel wires, tend to aggravate root resorption.