Nitinol alloy is a special alloy that can automatically restore its plastic deformation to its original shape at a specific temperature. Its expansion rate is more than 20%, its fatigue life reaches 1*10 to the 7th power, its 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 the requirements of various engineering and It is a very excellent functional material for medical application needs.
In addition to its unique shape memory function, memory alloys also have excellent features such as wear resistance, corrosion resistance, high damping and superelasticity.
1. Performance and features:
(1) Phase transformation and properties of nitinol alloy:
As the name suggests, nitinol alloy is a binary alloy composed of nickel and titanium. Due to changes in temperature and mechanical pressure, there are two different crystal structure phases, namely the austenite phase and the martensite phase. The phase transformation sequence of nickel-titanium alloy when cooled is parent phase (austenite phase) - R phase - martensite phase. The R phase is rhombus-shaped, and austenite is in a state when the temperature is higher (greater than the temperature at which austenite begins) or when the load is removed (external force removes Deactivation). It is cubic and hard. The shape is relatively stable. The martensite phase is a state when the temperature is relatively low (less than Mf: the temperature at which martensite ends) or when it is loaded (activated by external force). It is hexagonal, ductile, repeatable, less stable, and easier to Deformation.
2. Special properties of nitinol alloy:
1). Shape memory characteristics (shapememory) 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, accompanied by In reverse phase change, the material automatically returns to its shape in the parent phase. In fact, the shape memory effect is a thermally induced phase change process of Nitinol Alloy.
2). Superelasticity: 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 automatically recover when unloaded. That is, in the parent phase state, due to the action of external stress, stress-induced martensitic transformation occurs, so the alloy exhibits mechanical behavior different from ordinary materials. Its elastic limit is much larger than ordinary materials, and it no longer adheres to the tiger Gram's law. In contrast to shape memory properties, superelasticity has no thermal involvement. All in all, superelasticity means that the stress does not increase with the increase of strain within a certain deformation range. Superelasticity can be divided into two categories: linear superelasticity and nonlinear superelasticity. In the stress-strain curve of the former, the relationship between stress and strain is close to linear. Nonlinear superelasticity refers to the result of stress-induced martensitic phase transformation and its reverse phase transformation respectively during loading and unloading in a certain temperature range above Af. Therefore, nonlinear superelasticity is also called phase transformation pseudoelasticity. The phase transition pseudoelasticity of nickel-titanium alloy can reach about 8%. The superelasticity of Nitinol Alloy can change with changes in heat treatment conditions. When the archwire is heated above 400ºC, the superelasticity begins to decrease.
3). Sensitivity to temperature changes in the oral cavity: The correction power of stainless steel wires and CoCr alloy dental orthodontic wires is basically not affected by the temperature in the oral cavity. The orthodontic force of superelastic nickel-titanium alloy dental orthodontic wire changes with changes in oral temperature. When the deformation amount is constant. As the temperature rises, the correction force increases. On the one hand, it can accelerate the movement of teeth, because the temperature change in the oral cavity will stimulate the blood flow in the areas where the blood flow is stagnant due to 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 functions. Orthodontists, on the other hand, cannot precisely control or measure orthodontic forces in the oral environment.
4). Anti-corrosion performance: Studies have shown that the anti-corrosion performance of nickel-titanium wire is similar to that of stainless steel wire.
5). Anti-toxicity: Nitinol Alloy has a special chemical composition, that is, it is a nickel-titanium and other atomic alloy, containing about 50% nickel, and nickel is known to be carcinogenic and cancer-promoting. Under normal circumstances, the oxidation of titanium on the surface layer acts as a barrier, making Ni-Ti alloys have good biocompatibility. The TiXOy and TixNiOy in the surface layer can inhibit the release of Ni.
6). Gentle orthodontic force: Currently commercially used dental orthodontic wires include austenitic stainless steel wire, cobalt-chromium-nickel alloy wire, nickel-chromium alloy wire, Australian alloy wire, gold alloy wire and beta titanium alloy wire. Load-displacement curves of these orthodontic wires under tensile test and three-point bending test conditions. Nitinol Alloy has the lowest and flattest unloading curve platform, indicating that it can best provide long-lasting and gentle correction force.
7). Good shock-absorbing properties: The greater the vibration caused by chewing and night grinding to the archwire, the greater the damage to tooth roots and periodontal tissues. Through the results of different archwire attenuation experiments, it was found that the vibration amplitude of stainless steel wire is larger than that of superelastic nitinol wire. The initial vibration amplitude of superelastic nitinol wire is only half of that of stainless steel wire. The good vibration and shock-absorbing characteristics of archwire are good for teeth. Your health is very important, and traditional archwires such as stainless steel wires tend to aggravate tooth root resorption.
Therefore, Nitinol Alloy can meet the needs of various engineering and medical applications and is a very excellent functional material.
