Shape memory alloys (SMA for short) are a new type of functional material that can undergo phase changes under the action of temperature and stress. It has unique shape memory effects, phase change pseudo-elasticity and other characteristics. It is widely used in aerospace, Medical equipment, machinery and electrical appliances.
Traits-Memory
After the shape memory alloy is plastically deformed, it can be restored to the shape before deformation when heated to a certain temperature, that is, it has a shape memory effect (SME).
It is widely known that on July 20, 1969, American astronauts left human footprints on the moon for the first time on the Apollo, and transmitted information between the moon and the Earth through a hemispherical antenna with a diameter of several meters. This behemoth-like antenna is made of a shape memory alloy material.
First make it above the transition temperature according to the predetermined requirements, and then reduce the temperature to press it into a ball and put it in the lunar module to go to the sky. After being placed on the moon, the sunlight reached the transition temperature of the alloy, and the antenna "remembered" its original appearance and became a hemisphere.
Characteristics-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 greater than that of general metals. Recoverable strain.
Shape memory principle
The principle of remembering the shape of the material of the memory alloy is different from that of polymer elastic materials. For example, the rubber ring that we are familiar with. The principle of restoring its shape is the opening and separation of internally crosslinked and entangled rubber macromolecular chains.
The reason why shape memory alloys have the ability to recover from deformation is because of the thermoelastic martensite transformation that occurs inside the material during deformation. The result of the reversible transformation of the low-temperature phase (martensite) produced by thermoelastic martensite transformation to the high-temperature phase (austenite) during heating.
category
It has been found that there are many types of alloys with shape memory effects, which can be divided into three major categories: nickel-titanium, copper, and iron alloys. Currently, the only shape memory alloys that have been put into practical use are Ni-Ti-based alloys and Cu-based alloys.
Recently, Yukiko et al. Of Tohoku University found that Mg-Sc alloy exhibited 4.4% superelasticity at -151 ° C and the shape can be recovered by heating, and the density is only about 2g / cm3, which is compared with the actual NiTi shape memory alloy. One third lower. This lightweight magnesium-rhenium shape memory alloy will have great application potential in areas where weight control is severe (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 with Goodyear, which can be used as a conventional tire in addition to Mars exploration missions. Alternatives used on earth.
The Hubble Space Telescope (HST) uses an arm made of shape memory alloy. After reaching space, the temperature of the sun heats the shape memory arms above a critical temperature, allowing them to return to their original shape and automatically release solar panels.
Shape memory alloys are used to change the wing camber, or the relative angle between the top and bottom of the wing.
Mechanical industry
Micromachines of various shapes
SmartFlex alloy wire
The shape memory alloy oil pipe joint is made of a Ti-Ni-Fe alloy with a phase transition point of about -150 ° C. This tubing joint can be used in fighter aircraft.
Biological Engineering
The memory esophagus can expand into a new esophagus in the throat. When necessary, add ice cubes to the esophagus, and the "esophagus" can be easily removed when it contracts in cold, which improves the quality of life of patients with esophageal cancer who have lost their eating function.
Shape memory alloys mimic human muscles and tendons well, and piezoelectric materials or shape memory alloys can be used to make controlled "muscles."
Nitinol shape memory alloy bone plate.
Orthodontic wire: NiTi shape memory alloy can continuously release more constant correction force than other materials.
Shape memory alloy for hip joint replacement.
A vascular stent was made from a shape memory alloy.