1. Shape memory is that when the parent phase of a certain shape cools from above Af temperature to below MF temperature to form martensite, the martensite will be deformed below MF temperature, and then heated to below Af temperature, accompanied by reverse transformation, the material will automatically restore its shape in the parent phase. In fact, shape memory effect is a heat induced phase transformation process of NiTi alloy.
2. Super elastic refers to the phenomenon that the strain produced by the specimen under the action of external force is far greater than the amount of elastic limit strain, and the strain can be recovered automatically when unloading. That is to say, in the parent phase state, due to the effect of external stress, stress-induced martensitic transformation occurs, so the alloy shows different mechanical behavior from ordinary materials, its elastic limit is far greater than that of ordinary materials, and it no longer complies with Hooke's law. Compared with shape memory, hyperelasticity has no thermal effect. In a word, hyperelasticity means that the internal stress does not increase with the increase of strain in a certain deformation range, but the orthodontic force produced by the arch wire in the deformation process remains constant, and it is no longer lost with the movement of teeth towards the orthodontic direction According to the characteristics of the stress-strain curve corresponding to hyperelasticity, hyperelasticity can be divided into linear hyperelasticity and nonlinear hyperelasticity. In the stress-strain curve of the former, the relationship between stress and strain is nearly linear. Nonlinear hyperelasticity refers to the results of stress-induced martensitic transformation and its inverse transformation in the process of loading and unloading in a certain temperature range above AF. therefore, nonlinear hyperelasticity is also called transformation pseudoelasticity The pseudo elasticity of Ni Ti alloy is about 8% The hyperelasticity of NiTi alloy can be changed with the change of heat treatment conditions. When the bow wire is heated to more than 400oC, the hyperelasticity begins to decrease. When the heat treatment temperature is more than 600oC, the hyperelastic basic hours. According to this feature, in clinic, the non correction area of the arch wire can be heat treated to make it lose hyperelasticity, so as to avoid the influence of the correction process on the teeth in the non correction area, while the arch wire in the correction area still has good elasticity.
3、口腔内温度变化敏感性： 不锈钢丝和 CoCr 合金牙齿矫形丝的矫治力基本不受口腔内温度的影响。超弹性镍钛合金牙齿矫形丝的矫治力随口腔温度的变化而变化。当变形量一定时。温度升高，矫治力增加。一方面，它可以加速牙齿的运动，这是因为口腔内的温度变化会刺激由于矫治器件造成造成毛细滞息的血流停滞部位的血液流动， 从而使得在牙齿移动过程中修复细胞得到充分营养，维持其生机和正常功能。另一方面，正畸医生无法精确控制或测量口腔环境下的矫治力。
3. Sensitivity of intraoral temperature change: the orthodontic force of stainless steel wire and CoCr alloy orthodontic wire is not affected by intraoral temperature. The orthodontic force of superelastic nitinol alloy orthodontic wire changes with the change of oral temperature. When the deformation is fixed. With the increase of temperature, the correction power increased. On the one hand, it can accelerate the movement of teeth, because the temperature change in the mouth will stimulate the blood flow in the stagnant part of blood flow caused by the appliance, so that the repair cells can be fully nourished and maintain their vitality and normal function in the process of tooth movement. On the other hand, orthodontists can not accurately control or measure the orthodontic force in the oral environment.
4. Corrosion resistance: some researches show that the corrosion resistance of NiTi wire is similar to that of stainless steel wire
5、抗毒性： 镍钛形状记忆合金特殊的化学组成 ,即这是一种镍钛等原子合金 ,含约 50% 的镍, 而已知镍有致癌和促癌作用。 一般情况情况下， 表面层钛氧化充当了一种屏障， 使 Ni-Ti合金具有良好的生物相容性。表面层的 TiXOy 和 TixNiOy 能抑制 Ni 的释放。
5. Anti toxicity: the special chemical composition of nitinol shape memory alloy 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 acts as a barrier, which makes Ni Ti alloy have good biocompatibility. TixOy and tixnioy of the surface layer can inhibit the release of Ni.
6、柔和的矫治力： 目前商业上应用的牙齿矫形金属丝包括奥氏体不锈钢丝、钴 -铬-镍合金丝、镍铬合金丝、澳大利亚合金丝、金合金丝和镍钛合金丝。关于这些正畸矫正金属丝在拉伸试验和三点弯曲试验条件的载荷 -位移曲线。镍钛合金的卸载曲线平台最低也最平，说明它最能提供持久柔和的矫治力。
6. Soft orthodontic force: at present, the commercial orthodontic wires include austenitic stainless steel wire, cobalt chromium nickel alloy wire, nickel chromium alloy wire, Australian alloy wire, gold alloy wire and nickel titanium alloy wire. The load displacement curves of these orthodontic wires under tensile test and three-point bending test conditions. The unloading curve platform of nitinol alloy is the lowest and the flattest, which shows that it can provide the most lasting and soft corrective force.
7、良好的减震特性： 由于咀嚼及夜磨牙对于弓丝造成的震动越大，对牙根及牙周组织的损害越大。通过不同弓丝衰减实验的结果研究发现，不锈钢丝震动的振幅比超弹性镍钛丝大，超弹性镍钛弓丝初始震动振幅仅为不锈钢丝的一半 , 弓丝良好的震动和减震特性对于牙齿的健康很重要，而传统弓丝如不锈钢丝，有加重牙根吸收的倾向。
7. Good shock absorption characteristics: the greater the vibration caused by chewing and night molars to the arch wire, the greater the damage to the root and periodontal tissue. The results of different arch wire attenuation experiments show that the vibration amplitude of stainless steel wire is larger than that of super elastic NiTi wire, and the initial vibration amplitude of super elastic NiTi wire is only half of that of stainless steel wire. The good vibration and shock absorption characteristics of arch wire are very important for the health of teeth, while the traditional arch wire, such as stainless steel wire, tends to increase the root absorption.