Key words: shape memory alloy; Characteristics: shape memory effect
I. Introduction
Shape memory alloy material is a special material which integrates sensing and driving. SMA (short for SMA) refers to an alloy with a certain initial shape, which can be plastically deformed and fixed into another shape at low temperature, and then can be restored to its initial shape by heating above a certain critical temperature.
Two. Characteristics of shape memory alloy
1. Shape memory effect: Shape memory alloy has the ability to recover its shape after proper heat treatment, which is called shape memory effect (SME for short). Shape memory effect can be divided into one-way shape memory effect, two-way shape memory effect and full-range shape memory effect according to the recovery situation.
2. Superelastic effect: Shape memory alloy is deformed when it is subjected to external force, and will return to its original state after the external force is removed. This phenomenon is called superelasticity. When the shape and alloy undergo hyperelastic deformation, martensite transformation is induced, and after the external force is removed, martensite reverse transformation occurs.
3. Damping characteristics: Shape memory alloy has good damping characteristics due to the self-compatibility of martensite transformation and various interfaces and interface movements formed in martensite.
4. Resistance characteristics: The research shows that for the Ni-Ti alloy whose initial structure is martensite, there is a linear relationship between resistance and strain during the tensile process; For Ni-Ti alloy whose initial structure is austenite or a mixture of austenite and martensite, when stress-induced martensite transformation occurs, the slope of the curve decreases, and the resistance-strain relationship before and after the transformation remains linear.
Three. Research progress of shape memory alloys
Olander first discovered the shape memory effect in 1932 when he was studying Au-Cd alloy. 1963, Buehler, a weapon laboratory of the US Navy, discovered that titanium-nickel alloy has shape memory effect. This effect was also found in 1964 Cu-Al-Ni alloy. After 1970s, scientists found this effect in 304 austenitic stainless steel and Fe- 18.5Mn.
1969, the Ti-Ni-Fe memory alloy pipe joint produced by Raychem Company of the United States was used to connect the hydraulic pipeline system on the F 14 fighter, which was the first successful application of SMA. 1970, the United States made NiTi memory alloy wire into the antenna of spacecraft. The former Soviet Union began to systematically study shape memory alloys in 1969. Germany began to explore the mechanism and application of shape memory effect in 197 1, and claimed that there were successful products in 1976. Japan also actively carried out research work in this field in the1970s. In 1970s, various countries successively developed Ni-Ti-based, Cu-Al-Ni-based and Cu-Zn-Al-based shape memory alloys. Fe-Mn-Si-based and stainless steel-based iron-based shape memory alloys were developed in 1980s. Since 1990s, high temperature shape memory alloys, wide hysteresis and alloy thin films have become research hotspots. The research and application of shape memory alloys in the United States, Japan and other countries have been relatively mature, and the industrialization of shape memory alloys has been realized earlier.
China began to study shape memory alloys only in the late 1970s, with a late start but a high starting point. In material metallurgy, especially the refining level of practical shape memory alloys, it has been recognized by international academic circles, and there are also some unique achievements in application and development. As a pioneer of nickel and titanium in China, Pell Technology specializes in the research and production of nickel and titanium materials. The engineering team, composed of several doctors and masters in materials science, has been aiming at promoting China's high-end nickel-titanium materials to the world, constantly learning and innovating, improving technology, and gradually narrowing the gap between the industrialization and engineering application of shape memory alloys and foreign countries.
Four. Classification of shape memory alloys
Up to now, the developed shape memory alloys mainly include Ti-Ni base, Cu base and Fe base. In these three categories, according to different requirements and working environment, the amount of some alloying elements is added and adjusted in the matrix, so that a series of alloys are developed in each category and applied to all walks of life to meet various special needs.
Ti-Ni shape memory alloy is the earliest developed, and its shape memory effect is the most stable and mature. It has been used in aerospace industry, automobile industry, electronics industry, medicine and human life. However, due to the high price and processing cost of raw materials nickel and titanium, their application is limited.
Verb (abbreviation of verb) Application of shape memory alloy
Shape memory alloy is widely used in many fields such as machinery, architecture, aerospace, automobile, medical care and so on because of its unique shape memory function, superelasticity, high damping, wear resistance and corrosion resistance.
1. Mechanical engineering. The earliest applications were pipe joints and fasteners. At the joints of aircraft mechanical parts connection, pipeline connection, aerial refueling, etc., the casing with the inner diameter smaller than the outer diameter of the pipeline to be connected is processed with shape memory alloy, and then the casing is expanded 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 temperature is changed by electric heating, so that the interface and alloy are deformed, and the casing contracts to form a tight seal, so that the joint can be accurately dripped (oil) without leakage, which is far better than welding. It is especially suitable for aviation, aerospace, nuclear industry, ships and submarine oil pipelines. In some inconvenient parts, memory alloy is used as a pin, which is put into the hole for heating, and the end is automatically separated and curled to realize fastening. Using the dual function of temperature sensing drive of memory alloy, the robot and manipulator with tiny and compact structure are made.
2. Construction engineering. Shape memory alloy has superelasticity and high damping characteristics. The structure containing shape memory alloy can significantly increase the system damping and reduce the dynamic response of the structure, and can be used to make various forms of damping energy dissipation devices. Shape memory alloy can also be used to design seismic isolators. Its working principle is that the shape memory alloy device will produce large deformation and displacement energy consumption after deformation, reducing the transmission of seismic energy from the lower structure to the upper structure, thus achieving the purpose of protecting the upper structure and improving the seismic performance of the structure.
3. Aerospace industry. In the hydraulic system of American F- 14 aircraft, an average of 800 shape memory alloy joints are used. Since 1970, this kind of pipe joint has been used for hundreds of thousands of times on American navy aircraft, and there is no record of a failure. In addition to pipe joints, shape memory alloys are also used in spacecraft antennas, fasteners, connecting parts, electrical connections and electromechanical actuators.
4. Automobile industry. Used for engine heat-proof fan clutch, exhaust automatic regulating nozzle, diesel engine radiator hole automatic switch, jet engine suction filter shape memory spring, etc.
5. Medical equipment. Shape memory alloy has been widely used in medical field because of its good mechanical properties and biocompatibility. Especially NiTi shape memory alloy, with its excellent superelasticity and shape memory effect, has been successfully applied to oral orthodontics, surgical correction, plastic surgery and minimally invasive interventional therapy of cardiovascular diseases. 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. There are also surgical sutures, orthodontic wires, aneurysm clips, intramedullary nails, artificial joints, contraceptive rings, artificial hearts, artificial kidney micropumps and so on. Has entered the medical clinical trial.
Development trend of intransitive verb shape memory alloy
1. High temperature shape memory alloy. NiTi and Cu-Zn-Al alloy can only be used below 100℃. However, in quite a few cases, such as fire fighting devices, the working temperature of memory alloy elements of automobile engines exceeds 100℃. In nuclear reactor engineering, the working temperature of memory alloy thermal elements is as high as 600℃, so the development of high-temperature shape memory alloys has become a major development direction. High temperature shape memory alloy has a very broad application prospect in high temperature fields such as thermal actuator, relay and nuclear industry.
2. Magnetic shape memory alloy. Magnetic shape memory alloy can output large strain under the action of magnetic field, and at the same time, the working frequency of memory alloy is increased from about1Hz in temperature control state (the highest working frequency of TiNi memory alloy film can reach 100Hz) to over 300Hz in magnetic control state. Using these functional characteristics of magnetic drive memory alloy, the sensing and driving elements made of magnetic drive memory alloy have important application prospects in petroleum, electronics and aerospace industries.
In addition to the above, shape memory alloys with wide hysteresis and narrow hysteresis, shape memory alloy films, high yield limit shape memory alloys, low stress hysteresis shape memory alloys and low temperature pseudo-elastic shape memory alloys are being studied.
(Original from: Baidu Library)