What Are The Physical Properties And Uses Of Nickel-titanium Superelastic Memory Alloys?

1, shape memory characteristics (shape memory) shape memory is when a certain shape of the parent phase by Af temperature above the cooling to the Mf temperature below the formation of martensite, the martensite will be below the Mf temperature deformation, by heating to the Af temperature below, accompanied by the inverse phase transition, the material will automatically restore its shape in the parent phase when the shape. In fact, the shape memory effect is a heat-induced phase transition process in Ni-Ti alloys.

2,Superelastic The so-called superelasticity refers to the phenomenon that the strain produced by the sample under the action of external force is much larger than the strain of the elastic limit, and the strain can be recovered automatically when unloading. That is, in the parent phase state, due to the role of the applied stress, resulting in stress-induced martensitic phase transition occurs, so that the alloy shows different mechanical behavior from ordinary materials, its elastic limit is much larger than that of ordinary materials, and no longer comply with Hooke's law.

In contrast to shape memory properties, hyperelasticity has no thermal involvement. To summarize, hyperelasticity refers to the fact that stress does not increase with strain within a certain range of deformation, and can be classified into two categories: linear hyperelasticity and nonlinear hyperelasticity. In the former, the stress and strain in the stress-strain curve are close to a linear relationship. Nonlinear hyperelasticity refers to the result of stress-induced martensitic phase transformation and its inverse phase transformation during loading and unloading in a certain temperature interval above Af, so nonlinear hyperelasticity is also known as phase transformation pseudoelasticity.

The phase transition pseudoelasticity of Ni-Ti alloys can reach about 8%. The superelasticity of Ni-Ti alloy can be changed with the change of the conditions of heat treatment, and the superelasticity starts to decrease when the archwire is heated to more than 400oC.

3, intraoral temperature change sensitivity: the orthodontic force of stainless steel wire and CoCr alloy dental orthopedic wire is basically unaffected by intraoral temperature. The orthodontic force of superelastic Ni-Ti alloy dental orthopedic wire changes with the change of oral temperature.
When the amount of deformation is certain. When the temperature increases, the orthodontic force increases. On the one hand, it accelerates the movement of the teeth, which is because the temperature change in the oral cavity stimulates the blood flow in the stagnant area of blood flow caused by capillary stagnation due to the orthodontic appliance pieces, which makes the restorative cells receive sufficient nutrition to maintain their vitality and normal function in the process of tooth movement. On the other hand, orthodontist can not accurately control or measure the orthodontic force in the oral environment.

4, corrosion resistance: some studies have shown that the corrosion resistance of nickel titanium wire is similar to stainless steel wire

5, anti-toxicity: nickel-titanium shape memory alloy special chemical composition, that is, this is a nickel-titanium isoatomic alloy, containing about 50% of nickel, and nickel is known to have carcinogenic and cancer-promoting effects. In general, the surface layer of titanium oxide acts as a barrier to make the Ni-Ti alloy biocompatible.

The surface layers TiXOy and TixNiOy inhibit the release of Ni.

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

Nickel-titanium alloy has the lowest and flattest unloading curve plateau, indicating that it is the most capable of providing long-lasting and gentle orthodontic force.

7, good vibration damping characteristics: due to chewing and grinding teeth for the archwire caused by the greater the vibration, the greater the damage to the roots and periodontal tissues. Through the results of different archwire attenuation experiments, it is found that the vibration amplitude of stainless steel wire is larger than that of super-elastic nickel titanium wire, and the initial vibration amplitude of super-elastic nickel titanium archwire is only half of that of stainless steel wire. Good vibration and shock absorption characteristics of archwire are very important for the health of the teeth, whereas traditional archwires such as stainless steel wire have a tendency to exacerbate the resorption of the root of the teeth.