Introduction, principle and application analysis of shape memory alloy

Shape Memory Alloys, referred to as SMA, is a new type of functional material that can undergo phase change under the action of temperature and stress. It has unique shape memory effect, phase change pseudo-elasticity and other characteristics. It is widely used in aerospace, Medical equipment, machinery and electrical appliances, etc.

Characteristics-memory ability

After the shape memory alloy is plastically deformed, it can be heated to a temperature above a certain temperature, and it can return to the shape before deformation, that is, it has a shape memory effect (SME).

It is well known that on July 20, 1969, American astronauts took the "Apollo" to leave human footprints on the moon for the first time, and transmitted information between the moon and the earth through a hemispherical antenna several meters in diameter. This giant antenna is made of a shape memory alloy material.

First make it above its transition temperature according to the predetermined requirements, then reduce the temperature to crush it into a ball, put it into the lunar module and take it to the sky. After being placed on the moon, it reaches the transformation temperature of the alloy under sunlight, and the antenna "remembers" its original appearance and becomes a hemisphere.

Features-Super Elastic

Another unique property of shape memory alloys is that large "pseudo-elasticity" or "super-elasticity" occurs at high temperatures (austenite state), that is, the bearing capacity of this alloy is several times or even dozens of times larger than that of ordinary metals. The recoverable strain.

Principle of shape memory

The principle of memory alloy remembering the shape of materials is different from that of polymer elastic materials, such as the familiar rubber band. The principle of restoring its shape is the opening and separation of internal cross-linked and entangled rubber macromolecular chains.

The reason why shape memory alloys have the ability to recover from deformation is due to the thermoelastic martensitic transformation generated inside the material during the deformation process. The low temperature phase (martensite) produced by the thermoelastic martensite transformation is the result of the reversible transformation of the high temperature phase (austenite) during heating.

category

It has been found that there are many types of alloys with shape memory effect, which can be divided into three categories: nickel-titanium alloys, copper alloys, and iron alloys. At present, the only practical shape memory alloys are Ni-Ti alloys and Cu alloys.

Recently, Yukiko et al. of Tohoku University in Japan found that Mg-Sc alloy exhibits 4.4% superelasticity at -151°C and can recover its shape when heated, with a density of only about 2g/cm3, which is much higher than actual Nitinol shape memory alloys. Lower one-third. This kind of lightweight magnesium-scandium shape memory alloy will have huge application potential in areas with strict weight control (such as aerospace).

Application of shape memory alloy

Aerospace

In December 2017, NASA released a non-pneumatic tire made of shape memory alloy jointly developed by it and Goodyear. In addition to being used in Mars missions, NASA can also be used as a traditional tire Alternatives are used on earth.

The Hubble Space Telescope (HST) uses arms made of shape memory alloys. After reaching space, the temperature of the sun heats the shape memory arms above the critical temperature, restoring them to their original shape, and automatically releases solar panels.

Use shape memory alloy to change the camber angle of the wing, or the relative angle between the top and bottom of the wing