Ni-Ti alloy is a shape memory alloy, shape memory alloy is able to deformation of their own plastic at a particular temperature automatically restored to the original shape of the special alloy.
Performance and characteristics
(一) Phase transition and properties of nickel-titanium alloy
As the name suggests, nickel-titanium alloy is composed of nickel and titanium binary alloy, due to the temperature and mechanical pressure changes exist in two different crystal structure phase, namely the austenite phase and martensite phase. The phase transition order of the nickel-titanium alloy is the mother phase (austenite phase) -R phase-martensite phase. R phase is a rhombus, the austenite is a higher temperature (greater than the same: that is, the temperature of the austenite start), or when the load (external removal Deactivation) state, cube, hard. Shape more stable. And the martensite phase is relatively low temperature (less than Mf: that is, the end of martensite temperature) or loading (by external force activation) state, hexagonal, with ductility, repeatability, less stable, easier Deformation.
(二) the special properties of nickel-titanium alloy
1, shape memory characteristics (shape memory) shape memory is a certain shape of the mother phase from Af temperature above the cooling to Mf temperature below the formation of martensite, the martensite Mf below the temperature deformation, heated to Af below the temperature, With the inverse phase transformation, the material will automatically restore its shape in the parent phase. In fact, the shape memory effect is a thermally induced phase transition process of the nickel-titanium alloy.
2, superelastic (superelastic) The so-called superelasticity refers to the specimen under the action of external force is far greater than the elastic limit strain of the strain, the load can be automatically restored when the phenomenon of recovery. That is, in the matrix state, due to the role of external stress, resulting in stress induced martensitic transformation occurs, so the alloy exhibits a different mechanical properties than ordinary materials, its elastic limit is much larger than ordinary materials, and no longer follow the tiger Gram law. Compared with the shape memory properties, superelasticity does not involve heat. In general, superelasticity refers to the increase of stress in a certain deformation range without increasing the strain. The superelasticity can be divided into two types: linear superelasticity and nonlinear superelasticity. The linear relationship between the stress and strain in the stress - strain curve of the former. The nonlinear superelasticity is the result of stress induced martensitic transformation and its inverse transformation in the process of loading and unloading in a certain temperature range above Af. Therefore, the nonlinear superelasticity is also called phase change pseudo-elasticity. The phase change of pseudo - elasticity of nickel - titanium alloy is about 8%. The superelasticity of the nickel-titanium alloy may vary with the conditions of the heat treatment, and the superelasticity begins to decrease when the arch wire is heated to above 400ºC.