smart memory alloys nitinol wire

Nitinol performance and characteristics

(1) Phase transformation and properties of nickel-titanium alloy

As the name implies, nickel-titanium alloy is a binary alloy composed of nickel and titanium. There are two types of alloys due to changes in temperature and mechanical pressure.

The same crystal structure phase, namely austenite phase and martensite phase. The phase transformation sequence of nickel-titanium alloy during cooling is the mother phase (austenite phase)

-R phase-Martensite phase. When the R phase is rhombohedral and the austenite has a higher temperature (greater than the same: the temperature at which austenite starts)

Wait, or go to the state of the load (external force removal Deactivation), cube, hard. The shape is relatively stable. And the horse

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),

Hexagon is malleable, repetitive, not very stable, and easy to deform.

(2) Special properties of nickel-titanium alloy

1. Shape memory shape memory is when the mother phase of a certain shape is cooled from above Af temperature to Mf temperature

After the martensite is formed below the degree, the martensite is deformed at a temperature below Mf, and heated to below the Af temperature, accompanied by reverse phase transformation, the material

It 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 means that the sample under the action of external force produces much greater than the elastic limit strain

Strain, the phenomenon that strain can recover automatically when unloading. That is, in the state of the parent phase, due to the effect of external stress, it induces stress

Martensitic transformation occurs, so that the alloy exhibits mechanical behaviors different from ordinary materials, and its elastic limit is much greater than ordinary materials.

And no longer obey Hook’s law. Compared with shape memory properties, superelasticity does not involve heat. All in all, super elasticity means

In the range of fixed deformation, the stress does not increase with the increase of strain. Superelasticity can be divided into linear superelasticity and nonlinear superelasticity. before

In the stress-strain curve, the relationship between stress and strain is close to linear. Non-linear superelasticity refers to adding within a certain temperature range above Af

The stress-induced martensitic transformation and its inverse transformation results respectively during loading and unloading, so the nonlinear superelasticity is also called phase transformation pseudo

elasticity. The pseudo-elasticity of the nickel-titanium alloy can reach about 8%. The superelasticity of Nitinol can be changed with the change of heat treatment conditions

When the archwire is heated above 400oC, the superelasticity begins to decrease.

3. Sensitivity of temperature changes in the oral cavity: the curative power of stainless steel wire and CoCr alloy orthodontic wire is basically not affected by the temperature in the oral cavity

ring. The orthodontic power of superelastic nickel-titanium alloy orthodontic wire changes with the change of oral temperature. When the amount of deformation is constant. The temperature rises,

Corrective power increased. On the one hand, it can accelerate the movement of teeth, because the temperature changes in the oral cavity will stimulate due to the correction device

Causes the blood flow in the stagnation of blood flow that causes capillary stagnation, so that the repair of cells during the movement of the teeth is fully

Maintain and 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, that is, this is a nickel-titanium and other atomic alloy, containing about 50% nickel

Know that nickel has carcinogenic and cancer-promoting effects. In general, the titanium oxide on the surface layer acts as a barrier, making the Ni-Ti alloy good

Biocompatibility. TiXOy and TixNiOy of the surface layer can suppress the release of Ni.

6. Soft orthodontic power: currently commercially available orthodontic wires include austenitic stainless steel wire, cobalt-chromium-nickel alloy wire,

Nickel chromium alloy wire, Australian alloy wire, gold alloy wire and ß titanium alloy wire. About these orthodontic correction wires in tensile tests and

Load-displacement curve for three-point bending test conditions. The nickel-titanium alloy’s unloading curve platform is the lowest and flattest, indicating that it can provide the most support

Long and gentle curative power.

7. Good shock absorption characteristics: the greater the vibration caused by the chewing and nocturnal teeth to the arch wire, the more damage to the root and periodontal tissue

Big. 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 nickel-titanium wire, and superelastic nickel

The initial vibration amplitude of the titanium arch wire is only half of that of the stainless steel wire. The good vibration and shock absorption characteristics of the arch wire are very important for the health of the tooth.

Traditional arch wire, such as stainless steel wire, tends to increase the absorption of tooth roots.