ABSTRACT
Ni-Ti archwires are shape memory alloys, and that is the main reason they are used in orthodontics. The property of superelasticity allows applying low constant forces to the dentition over a long period of activation and the final result is tooth movement to desired position by combining rotation and translatation. Superelasticity results from a stress-induced phase transformation. The formation of martensitic phase in the alloy is initiated by cooling material below Ms which can be defined as the temperature at which the martensitic transformation begins. Mf is temperature at which the martensitic transformation ends. This transformation is reversible, being the As the temperature at which the reverse austenitic transformation starts upon heating, and Af the temperature at the end of the reverse austenitic transformation. When a stress is applied to the parent phase material above its Af temperature, a mechanically elastic martensite is stress-induced in alloy. That means that the deformed material reverts to its original shape when the stress is released. This effect is called pseudoelasticity or superelasticity. The critical stress increases with increasing temperature while the yield
stress of the parent phase decreases with increasing temperature. Due to their superelastic properties Ni-Ti archwires are able to produce teeth movement with greater efficiency and in a shorter time in comparison to other orthodontic alloys.They are especially indicated in situations that require large deflections of an orthodontic archwire such as the leveling and alignment stage of orthodontic therapy (Gil et al. 2012).
