Memory alloy Memory alloy is a martensitic phase change alloy with regular atomic arrangement and volume less than 0.5%. This alloy will deform under the action of external force, and when the external force is removed, it can return to its original state at a certain temperature. It is called "memory alloy" because it has more than one million restoration functions. Of course, it is impossible to think and remember like a human brain. More accurately, it should be called "the alloy of memory shape". In addition, memory alloys have the advantages of non-magnetism, wear resistance, corrosion resistance and non-toxicity, so they are widely used. Scientists have now discovered dozens of alloys with different memory functions, such as Ti-Ni alloy, Au-Cd alloy, Cu-Zn alloy and so on. What is memory alloy 65438+In 1970s, 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. The classification and application of shape memory alloys can be divided into three types: (1) One-way memory effect Shape memory alloys deform at a lower temperature and can be restored to their original state 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 high-temperature phase state when heated and low-temperature phase state when cooled, which is called two-way memory effect. (3) The whole-course memory effect is called the whole-course memory effect, which restores the shape of the high-temperature phase when heating, and becomes the shape of the low-temperature phase with the same shape but opposite orientation when cooling. 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. Prospect of high-tech application: 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 observed carefully and experimented repeatedly, and the results confirmed that these wires really existed? "Remembering" 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. For example, mechanical fastening pins and pipe joints, welding of fire alarms, connectors and integrated circuits on electronic instruments and equipment, artificial heart valves, massage sticks, cranioplasty, orthodontics and jaw restoration in medical treatment, etc. It will also play a magical role in communication satellites, color TVs, thermostats and toys, and will also become a new material for modern navigation, aviation, aerospace, transportation, textiles and other fronts. Memory alloy has been used in pipeline assembly and automatic control. Bushing made of memory alloy can replace welding. The method is to make the inside of the pipe end fully expand by about 4% at low temperature, and put them together when assembling. Once heated, the sleeve shrinks back to its original shape and forms a tight bond. There are 6,543,800 such joints used in the hydraulic system of American Navy aircraft, and there has been no oil leakage or damage for many years. It is very convenient to repair damaged pipelines of ships and submarine oil fields with memory alloy fittings. In some inconvenient parts, the pin is made of memory alloy, put into the hole and heated, and the tail automatically curls separately to form a single-sided assembly. Memory alloy is especially suitable for thermal machinery and constant temperature automatic control. The ventilation window can be opened in sunny days and closed automatically when the room temperature drops at night. There are also many design schemes of memory alloy heat engine, which can work between two kinds of media with small temperature difference, opening up new ways for the utilization of industrial cooling water, nuclear reactor waste heat, ocean temperature difference and solar energy. At present, the common problem is low efficiency, only 4% ~ 6%, which needs to be further improved. The application of memory alloy in medical treatment is also very eye-catching. For example, the bone plate used for fracture can not only fix two broken bones, but also produce compressive force in the process of restoring to the original state, forcing the broken bones to join together. Dental orthodontic wires, long clips for ligating cerebral aneurysms and vas deferens, and stents for straightening the spine are all started by body temperature after being implanted in the human body. Thrombotic filter is also a new product of memory alloy. After the straightened filter is implanted in a vein, it will gradually return to a network, thus preventing 95% of blood clots from flowing to the heart and lungs. Artificial heart is an organ with more complicated structure. Muscle fibers made of memory alloy can imitate the contraction movement of ventricle by cooperating with elastic membrane ventricle. Now the pumping has been successful. Because memory alloy is a kind of "living alloy", various automatic control devices can be designed by using its shape change at a certain temperature, and its application field is expanding constantly. Shape memory alloy was first used in pipe joints and fasteners. Machining a sleeve with shape memory alloy, the inner diameter of which is 4% smaller than the outer diameter of the pipe to be connected, and then expanding the sleeve by about 8% at the temperature of liquid nitrogen. When assembling, take the sleeve out of liquid nitrogen and insert it into the pipeline to be connected from both ends. When the temperature rises to normal temperature, the casing contracts to form a tight seal. This connection method can prevent leakage through close contact, which is far superior to welding, and is especially suitable for dangerous occasions such as aviation, aerospace, nuclear industry and submarine oil pipelines. The most encouraging application of memory alloys is in the field of aerospace technology. 1On July 20th, 969, the lunar module Apollo 1 1 landed on the moon, realizing the dream of landing on the moon for the first time. After the astronauts landed on the moon, they placed a hemispherical antenna with a diameter of several meters on the moon to send and receive information to the earth. An antenna several meters in size was put into a small lunar module and sent into space. The antenna was made of memory alloy that was just invented at that time. It is made of extremely thin memory alloy material according to the predetermined requirements under normal conditions, and then it is cooled and pressed into a ball and put into the lunar module to take it to the sky. After being placed on the surface of the moon, the temperature rises under the sunlight. When it reaches the transition temperature, the antenna will "remember" its original appearance and become a huge hemisphere. At present, shape memory effect and superelasticity have been widely used in various fields of medicine and life. Such as making thrombus filters, spinal orthopedic rods, bone plates, artificial joints, women's bras, artificial hearts and so on. But also can be widely used in various automatic control devices. Shape memory films and filaments may become ideal materials for future subminiature manipulators and robots. Especially its light weight, high strength and corrosion resistance make it popular in various fields. References:

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