Shape memory is that when the initial phase of a certain shape is cooled from a temperature above the Af temperature to a temperature below the MF temperature to form martensite, the martensite is deformed below the MF temperature and heated to a temperature below the Af temperature. With the reverse phase transformation, the material will automatically regain its shape in the original phase. In fact, the shape memory effect is the process of phase transformation of the Nitinol alloy caused by heating.
The so-called superelasticity refers to the phenomenon when the deformation of a sample under the action of an external force far exceeds the limit elastic deformation, and the deformation can be automatically restored during unloading. That is, in the initial phase state, due to the influence of external stress, a martensitic transformation occurs, caused by stress, so the alloy shows a different mechanical behavior than ordinary materials, its elastic limit is much greater than that of ordinary materials, and it no longer obeys Hooke’s law.
Corrosion resistance: Studies have shown that the corrosion resistance of Nitinol wire is similar to that of stainless steel wire.
The orthodontic strength of the stainless steel wire and the CoCr alloy orthodontic wire was independent of the oral temperature. The orthodontic strength of the superelastic orthodontic wire made of Ni Ti alloy changed with the change in the temperature of the oral cavity. When the deformation is constant. As the temperature increased, the orthodontic strength increased. On the one hand, it can speed up the movement of the teeth, because the change in temperature in the mouth will stimulate the blood flow of the stagnant part of the blood flow caused by the orthodontic device, so that the repair cells can receive sufficient nutrition and maintain their vitality and normal function during the movement of the teeth. On the other hand, orthodontists cannot accurately monitor or measure the orthodontic strength in the oral cavity.
Nitinol shape memory alloy has a special chemical composition, that is, it is a kind of Nitinol and other atomic alloy containing about 50% nickel, which is known to have carcinogenic and carcinogenic effects. In general, the oxidation of titanium on the surface layer acts as a barrier, which makes the NiTi alloy have good biocompatibility. The surface layer of TixOy and tixnioy can inhibit the release of Ni.
Currently commercially used orthodontic wires include austenitic stainless steel wire, cobalt chromium nickel alloy wire, nickel alloy wire, Australian alloy wire, gold alloy wire, and nickel titanium alloy wire. The load displacement curves of these orthodontic wires were obtained under the conditions of the tensile test and the three-point bending test. The Nitinol alloy discharge curve platform is the lowest and flattest, indicating that it can provide the most lasting and gentle orthodontic force.
The greater the vibration of the arc wire caused by chewing and molars, the greater the damage to the root and periodontal tissues. The results of various experiments on the weakening of the arch wire show that the vibration amplitude of the stainless steel wire is greater than that of the superelastic nitinol wire, and the initial vibration amplitude of the superelastic nitinol wire is only half of the vibration amplitude of the stainless steel wire. The good vibration and shock absorption characteristics of arch wire are very important for dental health, while traditional arch wire, such as stainless steel wire, tends to impair root absorption.