1.Shape memory property(shape memory)Shape memory refers to the phenomenon where,after the parent phase of a certain shape cools from above the Af temperature to below the Mf temperature,martensite is formed.If the martensite is deformed at a temperature below Mf and then heated to below the Af temperature,accompanied by reverse phase transformation,the material will automatically recover its shape in the parent phase.In essence,the shape memory effect is a thermally induced phase transformation process in nickel-titanium alloys.

2.Superelasticity.The so-called superelasticity refers to the phenomenon where a specimen produces a strain far exceeding its elastic limit strain under external force,and the strain can automatically recover upon unloading.That is,in the parent phase state,due to the effect of external stress,stress-induced martensitic transformation occurs,resulting in the alloy exhibiting mechanical behavior different from that of ordinary materials.Its elastic limit is much higher than that of ordinary materials,and it no longer obeys Hooke's law.

Compared to shape memory properties,superelasticity does not involve thermal participation.In summary,superelasticity refers to the phenomenon where stress does not increase with the increase of strain within a certain range of deformation.Superelasticity can be divided into two categories:linear superelasticity and nonlinear superelasticity.The stress-strain curve of the former exhibits a nearly linear relationship between stress and strain.Nonlinear superelasticity refers to the result of stress-induced martensitic transformation and its reverse transformation during loading and unloading processes within a certain temperature range above Af,hence it is also known as transformation pseudoelasticity.

The pseudoelasticity of nickel-titanium alloy during phase transformation can reach approximately 8%.The superelasticity of nickel-titanium alloy can vary with changes in heat treatment conditions.When the archwire is heated above 400°C,the superelasticity begins to decrease.

3.Sensitivity to Temperature Changes in the Oral Cavity:The orthodontic force of stainless steel wires and CoCr alloy orthodontic wires is essentially unaffected by temperature changes in the oral cavity.However,the orthodontic force of super-elastic nickel-titanium alloy orthodontic wires varies with changes in oral temperature.

When the amount of deformation is constant,an increase in temperature leads to an increase in orthodontic force.On the one hand,it can accelerate the movement of teeth,as temperature changes in the oral cavity stimulate blood flow in areas where orthodontic devices cause capillary stasis,thereby providing sufficient nutrition to repair cells during tooth movement,maintaining their vitality and normal function.On the other hand,orthodontists cannot precisely 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 comparable to that of stainless steel wire

5.Toxicity resistance:The special chemical composition of nickel-titanium shape memory alloy,which is an equiatomic alloy of nickel and titanium containing about 50%nickel,is known to have carcinogenic and tumor-promoting effects.Generally,the titanium oxidation on the surface layer acts as a barrier,making the Ni-Ti alloy have good biocompatibility.

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

6.Gentle orthodontic force:Currently,commercially available orthodontic 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 wires under tensile testing and three-point bending testing conditions.

The unloading curve platform of nickel-titanium alloy is the lowest and flattest,indicating that it can provide the most durable and gentle orthodontic force.

7.Excellent shock absorption characteristics:The greater the vibration caused by chewing and grinding on the archwire,the greater the damage to the tooth root and periodontal tissue.Research on the attenuation experiments of different archwires has found that the amplitude of vibration of stainless steel wire is greater than that of superelastic nickel-titanium wire.The initial vibration amplitude of superelastic nickel-titanium archwire is only half of that of stainless steel wire.The good vibration and shock absorption characteristics of the archwire are important for dental health,while traditional archwires such as stainless steel wire tend to aggravate root resorption.