[Edit this paragraph] What is a memory alloy?
/kloc-in the 1970s of 0/9, an alloy with "memory" shape function appeared in the world material science. Memory alloy is a special kind of metal belt, which is easy to bend. We put it in a glass jar filled with hot water, and the metal strip rushed forward. Put it in cold water and the metal strip will recover. In a glass jar filled with cold water, stretch a spring, and when the spring is put into hot water, it automatically closes again. In cold water, the spring returns to its original state, but in hot water, it will contract. The spring can stretch and contract infinitely, and then contract and open. These are all made of a kind of intelligent metal with memory. Its microstructure has two relatively stable states. At high temperature, this alloy can be changed into any shape you want. At a lower temperature, the alloy can be stretched, but if it is reheated, it will remember its original shape and change back. This material is called memory metal. It is mainly made of nickel-titanium alloy. For example, a spiral superalloy is in a spiral state after high temperature annealing. At room temperature, it is forcibly straightened with great strength, but as long as it is heated to a certain "abnormal temperature", this alloy seems to think of something and immediately returns to its original spiral shape. What's going on here? Does alloy also have human memory?
That's not true! This only makes use of the law that the crystal structure of some alloys changes with temperature when they are in solid state. For example, the crystal structure of Ni-Ti alloy is different between above 40oC and below 40oC, but when the temperature changes around 40oC, the alloy will shrink or expand, thus changing its morphology. Here 40oC is the "abnormal temperature" of NiTi memory alloy. All kinds of alloys have their own abnormal temperatures. The deformation temperature of the above superalloys is very high. It is made into spiral shape at high temperature and is in a stable state. When forcibly straightened at room temperature, it is in an unstable state. So as long as it is heated to an abnormal temperature, it will immediately return to the original stable spiral shape.
Classification and application
Shape memory alloys can be divided into three types:
(1) One-way memory effect
Shape memory alloy is deformed at low temperature and can recover its shape before deformation after heating. This shape memory phenomenon, which only exists in the heating process, is called one-way memory effect.
(2) bidirectional memory effect
Some alloys recover their high-temperature phase state when heated and their low-temperature phase state when cooled, which is called bidirectional memory effect.
(3) global memory effect
When heating, it restores the shape of high-temperature phase, and when cooling, it becomes the shape of low-temperature phase with the same shape but opposite orientation, which is called the whole memory effect.
The following figure shows three memory effects.
At present, the successfully developed shape memory alloys include TiNi-based shape memory alloys, copper-based shape memory alloys and iron-based shape memory alloys.
The earliest report on shape memory effect was published by Chang in 1952. They observed the reversibility of the phase transition of Au-Cd alloy. Later, the same phenomenon was found in copper-zinc alloy, but it did not attract widespread attention at that time. Until 1962, Buehler and his collaborators observed the memory effect with macroscopic shape change in TiNi alloy with equal atomic ratio, which attracted the attention of materials science and industry. By the early 1970s, the shape memory effect related to martensite transformation was also found in CuZn, CuZnAl, CuAlNi and other alloys. For decades, the study of shape memory alloys has gradually become an important topic in international phase transformation conferences and material conferences, and many symposiums have been held for this purpose, which have continuously enriched and improved the martensitic transformation theory. With the deepening of theoretical research, the application research of shape memory alloy has also made great progress, and its application scope involves many fields such as machinery, electronics, chemical industry, aerospace, energy, medical care and so on.
The specific applications of shape memory alloys are as follows.
Industrial application:
(1) One-way shape restoration using one-way shape memory effect. Such as pipe joints, antennas, collars, etc.
(2) Exogenous bidirectional memory recovery. That is, using one-way shape memory effect, with the help of external force, with the rise and fall of temperature to do repetitive actions, such as thermal sensors, robots, terminals and so on.
(3) Endogenous bidirectional memory recovery. That is, using the bidirectional memory effect to do repetitive actions with the rise and fall of temperature, such as heat engine, heat sensitive element and so on. However, this kind of application has fast memory decay and poor reliability, so it is not commonly used.
(4) Application of superelasticity. Such as springs, terminals, spectacle frames, etc.
Medical application:
TiNi alloy has good biocompatibility, and there are many medical examples using its shape memory effect and superelasticity. Such as thrombus filters, spinal orthopedic rods, dental orthopedic wires, aneurysm clips, bone plates, intramedullary needles, artificial joints, birth control pills, heart repair elements, micropumps for artificial kidneys, and the like.
High-tech application prospect:
The 20th century is an era of mechatronics. Sensor integrated circuit driver is the most typical mechatronics control system, but it is complex and huge. Shape memory materials have dual functions of sensing and driving, and can realize miniaturization and intelligence of control systems, such as holographic robots and millimeter-scale ultra-micro manipulators. 2 1 century will be the era of materials electronics. Shape memory alloy robots are not affected by any environmental conditions except temperature, and are expected to show their talents in high-tech fields such as reactors, accelerators and space laboratories.
When it comes to memory alloys, of course, we should talk about the most interesting alloys-memory alloys. Metals have memories, which is an accidental discovery. In the early 1960s, a research team of the US Navy brought some NiTi alloy wires from the warehouse for experiments. They found that these alloy wires were crooked and inconvenient to use, so they straightened them one by one. During the experiment, a strange phenomenon happened. They found that when the temperature rises to a certain value, these straightened NiTi alloy wires will suddenly return to their original bending state. They are observant people, and repeated experiments have proved that these wires really have "memory".
This discovery of the United States Naval Research Institute has aroused great interest in the scientific community, and a large number of scientists have conducted in-depth research on it. It is found that Cu-Zn alloy, Cu-Al-Ni alloy, Cu-Mo-Ni alloy and Cu-Au-Zn alloy also have this peculiar ability. People can change the shapes of these alloys according to their needs within a certain range. At a certain temperature, they will automatically return to their original shape, and this "change-recovery" can be repeated many times. No matter how they change, they can always remember the shape at that time. At this temperature, they will accurately reproduce the original shape. People call this phenomenon shape memory effect, and the metal with this shape memory effect is called shape memory alloy.
Why do these alloys have this shape memory effect? How do they remember their true colors? It is difficult to explain this memory effect of alloys with general metal bond theory and free electron theory. Memory alloy can return to its original shape at a certain temperature, which provides an excellent example for the movement of electrons outside the nucleus-the movement with temperature change.
It is precisely because the alloy is formed by mutual melting of liquid metals at high temperature, and because the structural elements of liquid metals are excluded, the structural elements of this element are evenly distributed with those of another metal. After solidification, its microstructure is that different structural elements are arranged orderly in proportion, and electromagnetic force is the main cohesion of alloy objects.
The electromagnetic force is formed by the valence state and the movement of electrons, and the movement rate of electrons changes with the temperature condition, so the electromagnetic force (size, direction and action point) in an object also changes with the temperature condition. Therefore, the internal force of metal objects changes with temperature conditions, but these changes are not obvious in a small temperature difference range, and will only be shown when the temperature changes greatly (several hundred degrees Celsius).
General metal can produce plastic deformation after being stressed. For example, when a wire is bent, the electromagnetic force is disturbed by external force at the bending position, which leads to a slight adjustment of the price of electromagnetic force and the running plane of electrons, and a plastic deformation is completed.
Because different kinds of structural elements are evenly mixed with each other, although the size and electromagnetic force of structural elements are different, they accelerate their own prices and operations and are adjacent to each other at a certain temperature. After being subjected to external force, the electromagnetic force is disturbed by external force, and the plane of motion of valence electrons is slightly adjusted, resulting in plastic deformation of the object. In this plastic deformation, the partially adjusted valence electrons do not stretch. When the temperature conditions change, the price and speed of electrons also change. When the temperature returns to the condition of phase-safe stretching (transition temperature), the unstretched price and the movement of electrons immediately return to the speed at that time, and the electromagnetic force changes accordingly, so that the price and movement of adjacent structural elements are also adjusted, and all of them return to the original stretching state, so the whole object also returns to the original state. This is the memory process of memory alloy.
In fact, the memory of metal has long been discovered: bend a straight iron wire at a right angle (90), once it is released, it will recover a little and form an angle greater than 90. To straighten the bent wire, it must be bent above 180 before it is loosened, so that it can just be restored to a straight line. This is overkill in China's idiom. Another alloy with better memory is spring. Here is a steel spring, and steel is an iron-carbon alloy. The spring firmly remembers its shape. When the external force is removed, it immediately returns to its original appearance, but the memory temperature of the spring is very wide, unlike the memory alloy which has a specific transition temperature, so it has some special functions.
Using the deformation function of memory alloy at a specific temperature, various temperature control devices, temperature control circuits, temperature control valves and temperature control pipeline connections can be made. People use memory alloy to make automatic fire hydrants-when the fire temperature rises, the memory alloy deforms to open the valve and spray water to extinguish the fire. Do a good job in connecting the mechanical parts and pipelines. After the interface of aircraft aerial refueling is tied with memory alloy-two engine oils, the temperature is changed by electric heating, so that the memory alloy at the interface is deformed and the interface drops (oil) tightly without leakage. The space station's self-expanding antenna has an area of several hundred square meters-first, make a large-area parabolic or planar antenna on the ground, fold it into a ball, and take it into space by spaceship. When the temperature changes, it will expand into its original large area and shape.
At present, there are dozens of kinds of memory alloys, which are used in aviation, military, industry, agriculture, medical care and other fields, and the development trend is very considerable. It will make great achievements and benefit mankind.
So far, there are more than a dozen memory alloy systems. Include Au-Cd, Ag-Cd, Cu-Zn, Cu-Zn-Al, Cu-Zn-Sn, Cu-Zn-Si, Cu-Zn-Ga, In-Ti, Au-Cu-Zn, NiAl, Fe-Pt, Ti-Ni and ti-pt.
Catalogue of shape memory alloys
History:
Classification:
Memory alloy system discovered so far
Features:
[Edit this paragraph] History:
1932, the Swede Aurand first observed the "memory" effect in the Au-Cd alloy, that is, once the alloy is heated to a certain transition temperature, it can magically change back to its original shape. People call the alloy with this special function shape memory alloy. The development of memory alloy has only been more than 20 years, but it has attracted worldwide attention because of its special functions in various fields, and is known as "magical functional materials".
There are many successful examples of the application of memory alloys in the aerospace field. The huge antenna on the artificial satellite can be made of memory alloy. Before launching the artificial satellite, the parabolic antenna is folded and put into the satellite. After the rocket launch sends the satellite into the predetermined orbit, it only needs to be heated. Because the folded satellite antenna has the function of "memory", it naturally unfolds and restores the parabolic shape.
Memory alloy is widely used in clinical medicine, such as artificial bone, bone fixation pressurizer, orthodontic appliance, various endoluminal stents, embolizers, cardiac prostheses, thrombus filters, interventional guide wires and surgical sutures. Memory alloy plays an irreplaceable role in modern medical care.
Memory alloys are also closely related to our daily life. Take the spring made of memory alloy as an example. If you put this spring in hot water, the length of the spring will be extended immediately, and then put it in cold water, it will be restored immediately. Shape memory alloy spring can be used to control the water temperature of bathroom water pipes. When the temperature of hot water is too high, the water supply pipe can be adjusted or closed by the "memory" function to avoid scalding. It can also be made into fire alarm devices and safety devices for electrical equipment. When a fire breaks out, the spring made of memory alloy is deformed, and the fire alarm device is started to achieve the purpose of alarm. You can also put a spring made of memory alloy in the heating valve to keep the temperature of the greenhouse, and automatically open or close the heating valve when the temperature is too low or too high.
As a new functional material, many new uses of memory alloys are being continuously developed. For example, glasses frames made of memory alloy can be restored to their original state if they are accidentally bent, as long as they are heated in hot water. In the near future, the car shell can also be made of memory alloy. If you accidentally hit it, you can restore it to its original state by heating it with a hair dryer, which saves money and effort and is very convenient.
[Edit this paragraph] Classification:
(1) One-way memory effect:
Shape memory alloy is deformed at low temperature and can recover its shape before deformation after heating. This shape memory phenomenon, which only exists in the heating process, is called one-way memory effect.
(2) Two-way memory effect:
Some alloys recover their high-temperature phase state when heated and their low-temperature phase state when cooled, which is called bidirectional memory effect.
(3) global memory effect
When heating, it restores the shape of high-temperature phase, and when cooling, it becomes the shape of low-temperature phase with the same shape but opposite orientation, which is called the whole memory effect.
The shape memory effect of shape memory alloy comes from thermoelastic martensite transformation. Once this martensite is formed, it will grow with the decrease of temperature, and if the temperature increases, it will decrease and disappear in the opposite process. The difference between the two free energies is the driving force of phase transition. The temperature T0 when the two free energies are equal is called the equilibrium temperature. Martensite transformation will occur only when the temperature is lower than the equilibrium temperature T0, and conversely, reverse transformation will occur only when the temperature is higher than the equilibrium temperature T0.
In SMA, martensitic transformation is not only caused by temperature, but also by stress, which is called stress-induced martensitic transformation, and the transformation temperature is linearly related to stress.
The memory alloy system discovered so far
Gold cadmium, silver cadmium, copper zinc, copper zinc aluminum, copper zinc tin, copper zinc silicon, copper zinc gallium, indium titanium, gold copper zinc, nickel, iron platinum, titanium nickel, titanium nickel.
[Edit this paragraph] function:
1, large bending, high plasticity.
2. Restore the shape before above the memory temperature.
The earliest discovered memory alloy seems to be 50% Ti+50% Ni.
Application:
Shape memory alloys have many excellent properties and are widely used in aerospace, mechatronics, biomedicine, bridge construction, automobile industry and daily life.
Application of (1) in aerospace industry [5]
Shape memory alloys have been used in aerospace equipment. For example, the low-temperature bonding connector used in the hydraulic system of military aircraft, and the shape memory alloy material for the intelligent horizontal rotor of helicopter is being developed in Europe and America. Because the use of helicopter high vibration and noise is limited, the main sources of noise and vibration are blade eddy current interference and slight deviation of blade profile. This requires a device to balance the pitch of the blades so that each blade can rotate accurately in the same plane. At present, a blade trajectory controller has been developed, which uses a small dual-tube shape memory alloy actuator to control the position of winglet on the blade edge trajectory to minimize its vibration [6].
It can also be used to make a moon antenna and explore the mysteries of the universe. People use shape memory alloy to make an antenna at high temperature, and then compress it into a small iron ball at low temperature, so that its volume can be reduced to one thousandth of the original, so that it can be transported to the moon conveniently, and the strong radiation of the sun will restore it to its original state and send back valuable cosmic information to the earth as required.
In addition, a shape memory release device with an openable container is used in satellites to protect sensitive germanium detectors from pollution during assembly and launch.
(2) Application in mechanical and electronic products [7]
1970, the United States made the F- 14 low-temperature butt connector with shape memory alloy and put it into battle, and then millions of connectors were used [5]. Shape memory alloy is used as low temperature joint in aircraft hydraulic system and small petroleum, petrochemical and electrical industrial products. Another shape of the connector is welded net wire, which is used to make the safety joint of the metal wire braided layer of the conductor. The connector has been used in sealing devices, electrical connection devices and electronic engineering machinery devices, and can work reliably at -65~300℃. The open sealing system device can be used to connect electrical components in harsh environment [6].
Shape memory alloy is made into a spring, which can open and close the shutter to protect the fog lamp from flying debris. Used for manufacturing precision instruments or precision lathes, once deformed due to vibration, collision and other reasons, only heating can eliminate the fault. In the process of mechanical manufacturing, various stamping and mechanical operations often need to transfer parts from one machine to another. Now, shape memory alloy is used to develop fixtures instead of manual or hydraulic pressure. This device is called driving cylinder, which has the characteristics of high efficiency, flexibility and large clamping force.
(3) Biomedical applications [8]
Medical TiNi shape memory alloy should not only use its shape memory effect or superelasticity, but also meet the requirements of chemistry and biology, that is, good biocompatibility. TiNi can form a stable passive film with organisms. At present, the main medical applications of TiNi alloy are:
(1) Orthodontic wire Orthodontic wire is made of superelastic TiNi alloy wire and stainless steel wire, among which superelastic TiNi alloy wire is the most suitable. Stainless steel wire CoCr alloy wire is usually used in orthodontics, but these materials have the disadvantages of high elastic modulus and small elastic strain. In order to give proper correction force, it should be processed into an arch before correction, and ligation and fixation need to be skilled. If TiNi alloy is used as dental orthopedic wire, even if the strain is as high as 10%, plastic deformation will not occur. Stress-induced martensite makes the elastic modulus nonlinear, that is, the correction force does not fluctuate greatly when the strain increases. The material is not only simple to operate and has good curative effect, but also can relieve the discomfort of patients.
(2) Scoliosis corrects various scoliosis (congenital, habitual, nervous, rickets, idiopathic, etc.). ) the disease not only seriously damages the body and mind, but also oppresses the internal organs, so surgery is needed to correct it. At present, stainless steel Harun bar is used for orthopedic treatment in this operation. When placing orthopedic rods during operation, it is required that the correction force to the spine after fixation should be kept below 30~40kg. Once the force is too large, the orthopedic rod will be damaged, and as a result, not only the spine will be damaged, but also the nerves will be damaged. At the same time, the corrective force will change with time after the orthopedic rod is placed. When the corrective force drops to the initial 30%, surgery is needed to adjust the corrective force, which will bring great mental and physical pain to patients. Hareton bar made of shape memory alloy can be fixed only by placing orthopedic bar once. If the corrective force of the orthopedic rod changes, heating the shape memory alloy in vitro to raise the temperature to about 5℃ higher than the body temperature can restore enough corrective force.
In addition, TiNi shape memory alloy is used in surgery to manufacture various bone connectors, vascular clamps, blood coagulation filters and vasodilators. At the same time, it is also widely used in stomatology, orthopedics, cardiovascular, thoracic surgery, hepatobiliary, urology, gynecology and so on. With the development of shape memory, medical applications will be more extensive.
(4) Daily life application [5]
(1) Anti-scald valve In family life, the developed shape memory valve can be used to prevent accidental scalding of hot water in sinks, bathtubs and bathrooms; These valves can also be used in hotels and other suitable places. If the water temperature from the faucet reaches a temperature that may burn people (about 48℃), the shape memory alloy drives the valve to close, and the valve will not be reopened until the water temperature drops to a safe temperature.
(b) TiNi alloy is pasted on the nose and ears of the glasses frame, which can make people feel comfortable and wear-resistant. Because of its flexibility, TiNi alloy has been widely used to change the fashion world of glasses. Using superelastic TiNi alloy wire as spectacle frame, the shape memory alloy wire can clamp the lens through superelastic constant force even if the lens is thermally expanded. Glasses frames made of these superelastic alloys have great deformability, while ordinary glasses frames cannot.
(c) Using superelastic TiNi wire as mobile phone antenna for mobile phone antenna and fire inspection valve is another application of shape memory alloy. In the past, stainless steel antennas were often damaged by bending. Mobile phone antenna using TiNi shape memory alloy wire is popular because of its high destructive resistance. Therefore, it is often used to make mobile phone antennas and fire inspection valves. In case of fire, when the local temperature rises, the valve will automatically close to prevent dangerous gas from entering. The advantage of this special structural design is that it can check the operation of the valve and then reset to a safe state; This kind of fire detector valve is used in semiconductor manufacturing, and toxic gas is used in the diffusion process of semiconductor manufacturing; This fire inspection valve can also be used in chemical plants and petroleum plants.
(5) Other applications
The potential of TiNi shape memory alloy as sound insulation material and earthquake damage detection and control has been shown in the fields of engineering and architecture. Its application in bridges and buildings has been tested, so its application as sound insulation material and damage detection control has become a new application field.
With the appearance, development and utilization of thin film shape memory alloy materials, shape memory alloys have been highly valued in intelligent material systems and have a broader application prospect.