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Metal surprisingly has memory: nickel titanium alloy

Nickel titanium alloy is a type of shape memory alloy, which is a special alloy that can automatically restore its plastic deformation to its original shape at a specific temperature and has good plasticity.

Memory alloys not only have unique shape memory functions, but also have excellent characteristics such as wear resistance, corrosion resistance, high damping, and super elasticity.

Performance and characteristics

（1） Phase Transformation and Properties of Nickel Titanium Alloy

As the name suggests, nickel titanium 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 austenite phase and martensite phase. The phase transformation sequence of nickel titanium alloy during cooling is the parent phase - R phase - martensitic phase. The R-phase is rhombic, while the austenite is in the state at high temperatures or when the load is removed, cubic, and hard. The shape is relatively stable. The martensitic phase is in a relatively low temperature or loading state, hexagonal, with ductility, repeatability, instability, and easy deformation.

（2） Special properties of nickel titanium alloy

Shape memory characteristics: Shape memory refers to the process in which a certain shape of the parent phase is cooled from above Af temperature to below Mf temperature to form martensite. The martensite is then deformed below Mf temperature and heated to below Af temperature, accompanied by a reverse phase transformation. The material will automatically recover its shape in the parent phase.

Superelasticity: The so-called superelasticity refers to the phenomenon where a specimen produces a strain far greater than its elastic limit strain under external forces, and the strain can automatically recover during unloading.

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.

Corrosion resistance: Studies have shown that the corrosion resistance of nickel titanium wire is similar to that of stainless steel wire

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.

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, alloy wire, and? Titanium alloy wire.

Good shock absorption characteristics: The greater the vibration caused by chewing and night grinding on the arch wire, the greater the damage to the root and periodontal tissue.

（3） Evans and Durning classification method for nickel titanium alloy wire

1) In 1940, gold bow wire, cobalt chromium alloy wire, and stainless steel round wire

2) In 1960, martensitic stabilized alloys were mostly made of nickel titanium alloys after deformation in a martensitic state. This type of wire has low stiffness and can generate relatively light corrective force.

3) In 1980, Chinese nickel titanium alloy and Japanese nickel titanium alloy wire were made into austenitic activated alloys: they exhibited an austenitic state in any state and did not exhibit a martensitic state caused by temperature when placed inside or outside the mouth.

4) In 1990, martensite activated nickel titanium alloy: that is, TTR is lower than or very close to the oral temperature, exists in a multicomponent state at room temperature, and is prone to deformation.

5) Graded thermodynamic: Added thermodynamic nickel titanium alloy: The TTR temperature is raised above the oral temperature, about 40 ℃, so that when the nickel titanium arch wire is placed in the oral cavity, it remains in a multi-component state and the arch wire is relatively soft.

（4） Clinical application of nickel titanium alloy wire:

1. For early alignment and leveling of patient dentition, due to the super elasticity and shape memory performance of nickel titanium alloy arch wire, as well as its lower stress-strain curve, nickel titanium alloy arch wire is currently commonly used as the initial arch wire included in the orthodontic system in clinical practice.

2. Nickel titanium spring: Nickel titanium push and pull springs are a type of spring used for orthodontic treatment, with the special properties of nickel titanium superelasticity, suitable for orthodontic treatment to open up gaps between teeth and pull teeth in different directions.

3. L-H bow wire is a Japanese doctor "LH", developed by Soma et al., is named after "LowHysteresis", which means that the difference between the stress generated when the wire is ligated onto the bracket, that is, when the wire is activated, and the stress generated when the wire slowly returns to its original state when moving teeth, is very small.

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