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Basic knowledge of nickel-titanium memory alloy springs

Author: Time:2019-07-23Pageviews:82

Basic knowledge of nickel-titanium memory alloy springs


       In recent years, the field of application of shape memory alloys has continued to expand. For example, hydraulic system ducts that have been made into jet fighters; solid-state engines that use low-energy energy; collapsible aerospace lines on aerospace engineering; dental orthodontic archwires for medical use; Hastelloy bars for correcting vertebrae; Automatic contacts, security devices; thermal sensors on control, temperature switches; up to toys and household items.


Basic knowledge of nickel-titanium memory alloy springs


      In recent years, the field of application of shape memory alloys has continued to expand. For example, hydraulic system ducts that have been made into jet fighters; solid-state engines that use low-energy energy; collapsible aerospace lines on aerospace engineering; dental orthodontic archwires for medical use; Hastelloy bars for correcting vertebrae; Automatic contacts, security devices; thermal sensors on control, temperature switches; up to toys and household items.




     The heat treatment of the shape memory alloy is mainly carried out around its thermoelastic martensitic transformation. The meaning of shape memory effect is that some materials with thermoelastic martensitic phase change alloys, in the martensite state, undergo a certain degree of deformation or deformation induced martensite, then in the subsequent heating process, when the temperature exceeds When the martensite phase disappears, the material can fully recover to the shape and volume before deformation.




      Martensite transformation is initially found in steel and is studied as a basis for heat treatment of steel. The memory effect of shape memory alloy is produced by the thermoelastic martensitic transformation in the material. It has become a research topic in the field of martensitic transformation and has opened up a new field of application research for martensite. At present, there are more than ten series of Ti-Ni, Au-Cd, Cu-Zn, Ag-Cd, Ni-Al, Co-Ni, Fe-Ni. Martensitic transformation is a solid phase transition, which is a phase change caused by pseudo shear to cause short-range diffusion of atoms. Through the study of shape memory alloys, it is considered that only the martensitic transformation is thermoelastic and martensite is a lattice structure with low symmetry, while the parent phase crystal is a cubic lattice structure with high symmetry, and There are memory effects when the conditions are mostly ordered.




Memory spring classification and application


Shape memory alloys can be divided into three types:


(1) One-way memory effect. The shape memory alloy deforms at a lower temperature and can restore the shape before deformation after heating. This shape memory phenomenon existing only during heating is called a one-way memory effect.


(2) Two-way memory effect. Some alloys recover the shape of the high temperature phase when heated, and can recover the shape of the low temperature phase when cooled, which is called the two-way memory effect.


(3) Full-memory effect. The shape of the high-temperature phase is restored when heated, and the shape of the low-temperature phase having the same shape and opposite orientation when cooled is called the full-time memory effect.




Specific applications of shape memory alloys:


      So far, more than 20 alloys with shape memory effects have been found, but four hundred Ni-Ti and Cu-Zn-Al alloys have been put into practical use. The former is good in corrosion resistance. High fatigue life, suitable for human implant, biological, aerospace and atomic engineering. The latter is inexpensive (only 1/10 of the former), has good processing performance and can be widely used in various fields.




(1) One-way shape recovery using one-way shape memory effect. Such as pipe joints, antennas, collars, etc.


(2) Exogenous two-way memory recovery. That is, the single-pass shape memory effect is used and repeated actions are performed with the external force with the temperature rise and fall, such as a heat sensitive element, a robot, a binding post, and the like.


(3) Intrinsic two-way memory recovery. That is to use the two-way memory effect to repeat the action with the temperature rise and fall, such as heat engine, thermal element and so on. However, such applications have fast memory decay and poor reliability, which are not commonly used.


(4) Super elastic application. Such as springs, binding posts, frames, etc.


(5) Medical applications. The biocompatibility of the TiNi alloy is very good, and there are quite a few medical examples using its shape memory effect and superelasticity. Such as thrombus filter, spinal orthopedic rod, orthodontic wire, cerebral aneurysm clip, bone plate, intramedullary nail, artificial joint, contraceptive, heart repair component, micro pump for artificial kidney.




Memory spring heat treatment process


      Alloys with shape memory effects are called memory alloys, and the main cause of shape memory effects is phase transition. The phase transition of most shape memory alloys is a reversible thermoelastic martensitic transformation, and temperature and stress are two independent variables of thermoelastic martensitic transformation. Therefore, the heat treatment of shape memory alloys affects its shape. One of the key factors of memory effect. The heat treatment process mainly has the following aspects.




Quenching heat treatment




      The rapid quenching of the parent phase (austenite) at high temperatures is enhanced by the interaction of quenching vacancies and dislocations. The higher the temperature, the more obvious the strengthening, and the increase of the quenching cooling rate will also strengthen the mother phase, but the excessive strengthening will affect the martensite transformation, thus affecting the memory recovery transition. Generally, different quenching media should be selected according to different materials. .




2. Thermal pre-deformation


      In order to strengthen the resistance of the mother phase (austenite) to improve the sliding deformation, but at the same time it can not make the martensite transformation difficult, in addition to the role of alloying elements, thermal pre-deformation is also an effective method, that is, After obtaining the austenite phase at high temperature and then performing thermal pre-deformation at a temperature higher than the Ms point, the parent phase austenite can be strengthened without martensite, so that the memory effect of the alloy is significantly improved. . However, if the hot pre-deformation temperature is too high, the opposite effect will occur, and the strength of the parent phase will decrease. Slip occurs during the strain process, thereby reducing the memory effect. Similarly, when the thermal pre-deformation is too large, the number of defects in the parent phase increases and the memory effect is reduced.




3. Cyclic heat treatment


      Shape memory alloys are subjected to multiple cycles of heat treatment in a certain temperature range and then deformed at room temperature to have different degrees of bidirectional memory effects at the recovery temperature. However, aging and constraining aging refers to the application of certain aging to the alloy, and is also a good method to induce and improve the two-way shape memory effect.




E.g:




1. Heat treatment of hydrogen storage alloy


      As the best secondary energy in the future, hydrogen has been receiving more and more attention. Even in a contemporary energy-sufficient environment, the use of hydrogen energy is conducive to the environmental protection of the planet and reduces the threat of the greenhouse effect. A series of theoretical and technical problems, such as oxygen development, transportation, and energy conversion, need to be solved. Hydrogen storage alloys are produced under such circumstances.




       Metal hydrides can be classified into three types according to their hydrogen bond properties: covalent bonds, ionic bonds, and metal bonds. The microstructure and mechanical properties (hardness) of hydrogen storage alloys affect their hydrogen storage properties to varying degrees. Therefore, the purpose of heat treatment of the hydrogen storage alloy is to improve its hydrogen storage property by improving its structure, mainly in the following categories.




l>Quick quenching heat treatment during solidification


      Rapid cooling during solidification (30m/s copper wheel or water-cooled copper mold) can obtain a fine columnar crystal structure, which makes the hydrogen pressure platform of the hydrogen storage alloy PCT curve tilt less, and the cycle life and waterization speed are also greatly improve. This is because a large number of grain boundaries can release lattice stress, alleviate the volume change of hydrogen absorption, and can serve as a diffusion channel for hydrogen absorption and desorption, thereby increasing the activation speed. At the same time, rapid cooling also inhibits chemical composition non-uniformity and improves atomic order.




2> Low temperature stress relief heat treatment


      The rapid cooling of hydrogen storage alloy during solidification will lead to the formation of a large number of crystal defects and hardness in the structure. The low temperature treatment can eliminate the defects of the quenching matrix, reduce the hardness of the alloy, improve its toughness, and inhibit the powdering and Cracking, thereby increasing alloy and cycle life.




3> high temperature diffusion treatment


      The structure of the hydrogen storage alloy in the as-cast state is not uniform, and there is a component segregation zone. The high-temperature diffusion treatment facilitates the homogenization of the composition of the matrix phase, thereby slowing down the attenuation of the cycle capacity and increasing the cycle life.


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