What are the physical properties and uses of nickel titanium alloy super elastic memory alloy?

1. Shape memory refers to the deformation of a martensite formed by cooling a certain shape of the parent phase from above Af temperature to below Mf temperature. The martensite is then heated to below Af temperature, accompanied by a reverse phase transformation, and the material automatically restores its shape in the parent phase. In fact, the shape memory effect is a thermally induced phase transition process in nickel titanium alloys.

2. Superelasticity refers to the phenomenon in which a specimen produces a strain far greater than the elastic limit strain under external forces, and the strain can automatically recover during unloading. In the mother phase state, due to the effect of external stress, stress induced martensitic transformation occurs, resulting in the alloy exhibiting mechanical behavior different from ordinary materials. Its elastic limit is much greater than that of ordinary materials, and it no longer follows Hooke's law.

Compared to shape memory properties, superelasticity has no thermal involvement. In summary, hyperelasticity refers to the situation where stress does not increase with strain within a certain deformation range, and can be divided into two types: linear hyperelasticity and nonlinear hyperelasticity. The stress-strain curve of the former shows a close linear relationship between stress and strain. Nonlinear hyperelasticity refers to the results of stress induced martensitic transformation and its reverse transformation during loading and unloading processes within a certain temperature range above Af, and is therefore also known as phase transition pseudoelasticity.

The phase transformation pseudo elasticity of nickel titanium alloy can reach about 8%. The superelasticity of nickel titanium alloy can change with changes in heat treatment conditions. When the wire is heated above 400oC, the superelasticity begins to decrease.

3. Sensitivity to temperature changes in the oral cavity: The corrective force of stainless steel wire and CoCr alloy orthodontic wire is basically not affected by the temperature in the oral cavity. The orthodontic force of super elastic nickel titanium alloy dental orthodontic wire varies with changes in oral temperature.

When the deformation is constant. As the temperature rises, the corrective force increases. On the one hand, it can accelerate the movement of teeth, because the temperature changes in the oral cavity can stimulate the blood flow in the stagnant areas caused by capillary stagnation caused by orthodontic devices, thereby providing sufficient nutrition for repairing cells during tooth movement, maintaining their vitality and normal function. On the other hand, orthodontists are unable to accurately control or measure the orthodontic force 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. Antitoxicity: Nickel titanium shape memory alloy has a special chemical composition, which is a type of atomic alloy such as nickel titanium, containing about 50% nickel. Nickel is known to have carcinogenic and pro cancer effects. In general, the surface layer of titanium oxide acts as a barrier, making Ni Ti alloys have good biocompatibility.

The surface layer of TiXOy and TixNiOy can inhibit the release of Ni.

6. Soft orthodontic force: Currently, commercially used dental orthodontic metal wires include austenitic stainless steel wire, cobalt chromium nickel alloy wire, nickel chromium alloy wire, Australian alloy wire, and titanium alloy wire. The load displacement curves of these orthodontic metal wires under tensile and three-point bending test conditions.

The unloading curve platform of nickel titanium alloy is the lowest and smoothest, indicating that it can provide the most long-lasting and gentle corrective force.

7. Good shock absorption characteristics: The greater the vibration caused by chewing and grinding on the arch wire, the greater the damage to the root and periodontal tissue. Through the results of different arch wire attenuation experiments, it was found that the vibration amplitude of stainless steel wire is larger than that of super elastic nickel titanium wire. The initial vibration amplitude of super elastic nickel titanium wire is only half of that of stainless steel wire. The good vibration and shock absorption characteristics of arch wire are important for tooth health, while traditional arch wires such as stainless steel wire tend to aggravate root resorption.