They have two common characteristics:
1, large bending, high plasticity.
2. Restore the shape before above the memory temperature.
In my impression, the earliest discovered memory alloy seems to be 50% Ti+50% Ni.
Generally, memory metal is greatly influenced by temperature, and it is a state at a certain temperature. When the temperature changes, the state changes, but when the temperature returns to the original, the state returns to the original.
The phase transition of this metal is reversible. It is affected by temperature changes and will return to its original shape above a certain temperature. The solar panels of artificial satellites can be made of it.
Mysterious shape memory alloy
At a seminar on new materials, a professor was holding a glass bottle full of water with a beautiful paper butterfly on it. Without saying a word, he calmly took out his lighter and heated the bottle. Soon, the butterfly's wings fanned. This experiment aroused great interest of the participants. It turns out that there is a so-called "shape memory" alloy wire under the butterfly, which will suddenly extend or shorten with the rise and fall of water temperature. The so-called shape memory effect refers to the phenomenon that the alloy can still return to its original shape after deformation under certain conditions.
Shape memory effect comes from thermoelastic martensite transformation. Martensite transformation, as a quenching strengthening method of steel, has existed since ancient times, that is, steel is heated above a certain critical temperature for a period of time, and then cooled rapidly, for example, it is directly inserted into cold water (called quenching), when steel is transformed into a structure called martensite and hardened. This martensitic transformation has special properties. Once martensite is formed at a certain temperature, it will not grow with time. In order to increase the amount of martensite, the temperature must be further lowered to produce new martensite. Later, another so-called thermoelastic martensitic transformation was found in some alloys, which can continue to grow with the decrease of temperature once it is produced. On the contrary, when the temperature rises, the grown martensite can shrink again until it returns to its original state, that is, martensite can grow or shrink reversibly with the change of temperature. Because the volume of martensite is generally larger than the original state, and martensite transformation is accompanied by regular shear in the crystal, thermoelastic martensite transformation is accompanied by shape change.
As early as 195 1, Americans accidentally discovered that gold-cadmium alloy has shape memory characteristics in an experiment, which did not attract attention at that time. This effect was found in 1953 indium-thallium alloy and 1963 nickel-titanium alloy with shape memory characteristics, which set off a wave of research on these alloys and produced many practical applications. Later, it was found that the rivet has bidirectional memory effect, that is, when the rivet is made of bidirectional memory alloy, it will straighten again after cooling again. As mentioned at the beginning of this section, the alloy wire under the butterfly is a bidirectional memory alloy, which can be extended or shortened back and forth with the change of temperature, making the butterfly wings fly up and down.
Shape memory alloy not only has important theoretical significance, but also has industrial application value. It is reported for the first time that the lunar antenna is made of shape memory alloy. The moon antenna is too wide for the rocket to hold. So, how to send such an antenna into space and the moon? Shape memory alloy miraculously solved this problem. The lunar antenna is made of NiTi alloy wire with martensitic transformation temperature above, and then pressed into small balls at a temperature lower than Mf and loaded into a launch vehicle. When it was launched to the surface of the moon, it was restored to its original shape by solar heating and unfolded on the moon to become a normal working moon antenna. It has great advantages to make plugs, sockets or pipe joints with shape memory alloys. Firstly, the shape memory alloy is made into a pipe joint slightly smaller than the stainless steel pipe to be connected, cooled to below Mf temperature, expanded to slightly larger than the connecting pipe, and then smoothly sleeved on the connecting pipe. Finally, when the temperature is raised to above Af (that is, the service temperature), the joint will automatically shrink, so that the two pipes are firmly connected. U.S. Air Force F- 14 aircraft use this connector to connect the pipelines of the oil pressure system and the water pressure system. It is said that there are nearly 300,000 joints without accidents, and shape memory alloy pipe joints are also widely used in submarines and warships of the navy, because the pipelines are arranged so densely in these occasions that it is impossible to connect the pipelines by ordinary methods.
Shape memory alloy has far-reaching significance in the utilization of low-quality energy. At present, shape memory alloy has been successfully used as orthodontic appliance in medicine. Shape memory alloy wire is made into normal shape above Ms temperature, and then deformed below Mf temperature and sheathed on abnormally deformed teeth. When the temperature rises to the temperature of the oral cavity, the orthodontic appliance automatically changes into a normal shape to correct the deformed teeth. Can also be used to correct scoliosis. Venous filter is to pull the sieve filter into a straight line at low temperature and send it into the vein. After being heated by body temperature, it becomes sieve-shaped, which plays the role of filtration and coagulation. At present, artificial kidney micropump, contractible artificial muscle and artificial heart are being trial-produced with shape memory alloy.
The invention and application of shape memory alloy have broadened people's understanding of the characteristics and functions of metal materials, and opened the mysterious door called intelligent or intelligent materials.