(a) nickel titanium alloy phase change and performance, as the name implies, nickel titanium alloy by binary alloy of nickel and titanium, due to the change of temperature and mechanical stress and there are two different crystal structure, namely austenitic and martensitic phase. The phase transition sequence of nickel titanium alloy cooling is the parent phase (austenite phase) -R phase - martensite phase. R phase is ling square, austenite is higher temperature (greater than in the same way: namely austenitic temperature), or a place to load (force to remove the shelf, cube, hard. The shape is more stable. And martensite phase is relatively low temperature (less than Mf: at the end of the martensite temperature) or load (by external activation) state, hexagon, is malleable, repeatability, unstable, easily deformation.
(b) the special properties of the nickel titanium alloy 1, shape memory characteristics (shape memory) shape memory is when the mother of a certain shape by Af temperature cooled to Mf temperature above the following after the formation of martensite, the martensite under Mf deformation temperature, the heating below Af temperature, along with the inverse transformation, material will automatically restore its shape in the parent phase. In fact, shape memory effect is a heat-induced phase transition process of nickel-titanium alloy.
(c) the classification of the nickel titanium alloy wire EvansandDurning classification 1) in 1940, the gold silk bow, cobalt chromium alloy wire and stainless steel round wire (2) in 1960, martensite stabilization alloy: many for nickel titanium alloy after deformation martensite state. This kind of bow wire has low rigidity, which can produce light correction force. There is no martensitic phase transition caused by stress or temperature, so there is no memory effect and superelasticity. 3) in 1980, the Chinese nickel titanium alloy and the nickel titanium arch wire, alloy for austenitic activation: the present state of austenite in any condition, placed inside the mouth and outlets are not caused by the temperature of martensite state, martensite state can only be caused by stress, with super elasticity, but I don't have shape memory function. This kind of arch wire has excellent resilience and low stiffness, which can produce weak correction force.
