Nitinol memory features principle
The unique properties of Nitinol alloy are due to a reversible solid phase transition called a martensitic phase transition, which can produce mechanical stresses of 10,000 to 20,000 psi between two different phase transition crystals. At higher temperatures, memory alloys form a parent phase called austenite. At lower temperatures, the memory alloy spontaneously switches to the more complex martensitic structure (Daughter phase).
The temperature at which austenite and martensite convert to each other is usually called transformation temperature. More specifically, there are four transition temperatures. The transition temperature at which martensite begins to change into austenite is called Ms. At a constant temperature below Ms, a small part of austenite structure changes rapidly, but does not continue. Only when the temperature drops further do more of the austenite change to martensite. Finally, the temperature reaches the end of martensitic transition temperature Mf, the end of the martensitic transition. On the contrary, austenite is formed when the alloy is heated after martensite is completely formed. The initial and final temperatures of this transformation are called As and Af respectively.
Shape Memory: Nitinol
have the feature of shape memory. Shape memory is that when the parent phase of a certain shape is cooled from above Af temperature to below Mf temperature to form martensite, the martensite is deformed at a temperature below Mf and heated to below Af temperature. With the reverse phase transformation, the material will automatically restore its shape at the parent phase. Shape memory effect is a thermally induced phase transition in Nitinol.
: The so-called superelasticity refers to the phenomenon that the strain of the specimen under the action of external force is far greater than the elastic limit strain variable, and the strain can be automatically recovered during unloading. That is, in the parent state, due to the action of applied stress, the stress-induced martensitic transformation occurs, and the alloy exhibits mechanical behavior different from that of ordinary materials. Its elastic limit is far greater than that of ordinary materials, and it no longer obeyes Hooke's law.
Oral temperature sensitivity
: Stainless steel wire and CoCr alloy tooth orthotic wire orthotic force are not affected by oral temperature. The orthodontic force of superelastic Nitinol orthodontic wire varies with the change of oral temperature when the amount of deformation is constant. As the temperature rises, the orthodontic force increases.
: Research shows that the corrosion resistance of nickel-titanium wire and stainless steel wire.
: The special chemical composition of nickel-titanium shape memory alloy, that is, it is a nickel-titanium and other atomic alloys, containing about 50% nickel, and nickel is known to have carcinogenic and cancer-promoting effects. In general, the surface layer of titanium oxidation acts as a barrier, making the Ni-Ti alloy have good biocompatibility.
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