Shape Memory Alloys (SMA for short) is a new type of functional material that can undergo phase change under the effect 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, mechanical appliances and other fields.
Characteristics-Memory
After plastic deformation of shape memory alloy, heating to a certain temperature can restore the shape before deformation, that is, it has shape memory effect (SME).
On July 20, 1969, American astronauts used 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 with a diameter of several meters. This giant antenna is made of a shape memory alloy material.
First of all, it should be done according to the predetermined requirements above its transition temperature, and then lower the temperature to compress it into a ball and install it in the lunar module to go to the sky. After being placed on the moon, the temperature of the alloy is reached under the sunlight, and the antenna "remembers" its original appearance and becomes a hemisphere.
Characteristics-Super Elastic
Another unique property of shape memory alloys is that large “pseudo-elasticity” or “superelasticity” occurs at high temperatures (austenitic state), which means that the bearing capacity of this alloy is several times or even tens of times greater than that of ordinary metals. The recoverable strain.
Principle of shape memory
The principle of memory alloys to remember the shape of materials is different from the elastic materials of polymers, such as the rubber bands we are familiar with. The principle of restoring its shape is the opening and separation of internal cross-linking and entangled rubber macromolecular chains.
The reason why the shape memory alloy has the ability to recover from deformation is due to the transformation of the thermoelastic martensite generated inside the material during the deformation process. The result of the reversible transformation of the low-temperature phase (martensite) produced by the thermoelastic martensite transformation to the high-temperature phase (austenite) when heated.
Types of shape memory alloys
It has been found that there are many types of alloys with shape memory effect, which can be divided into three categories of nickel-titanium, copper and iron alloys. At present, only shape memory alloys that have been put into practical use are Ni-Ti alloys and Cu alloys.
Recently, Yukiko et al. of Tohoku University, Japan found that Mg-Sc alloy exhibits 4.4% superelasticity at -151°C and can recover shape after heating, with a density of only about 2g/cm3, compared with the actual nickel-titanium shape memory alloy. Lower third. This 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 Goodyear (Goodyear). This tire can be used as a traditional tire in addition to the Mars exploration mission. Alternatives are used on the 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, returning them to their original shape and automatically releasing the solar panels.
A shape memory alloy is used to change the camber angle of the wing, 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 Ti-Ni-Fe alloy with a phase transition point of about -150°C. This kind of oil pipe joint can be applied to fighter aircraft.
Biological Engineering
The memory esophagus can expand into a new esophagus in the larynx. If necessary, add ice cubes to the esophagus, and the "esophagus" can be easily removed when contracted by cold, improving the quality of life of esophageal cancer patients who have lost their eating function.
Shape memory alloys simulate human muscles and tendons very 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 a more constant correction force than other materials.
Shape memory alloy for hip replacement.
Using shape memory alloy to make vascular stent.