Shape memory alloy (SMA) has been widely used in clinical medical field, such as artificial skeleton, fixed compression device for injured bone, dental orthodontic device, various intraluminal stents, embolic devices, cardiac repairs, thrombus filters, interventional guides and surgical sutures, etc. SMA is playing an irreplaceable role in modern medical treatment. Memory alloys are also closely related to our daily lives.
Shape memory effect exists in shape memory alloys. Take the spring made of shape memory alloys as an example. When the spring is put in hot water, the length of the spring immediately extends, and then in cold water, it will immediately restore to its original state. Shape memory alloy springs can be used to control the water temperature of bathroom water pipes: when the hot water temperature is too high, the water supply pipes can be adjusted or closed to avoid scalding through the "memory" function. It can also be made into a fire alarm device and a security device for electrical equipment. When a fire occurs, the spring made of memory alloy deforms and starts the fire alarm device to achieve the purpose of alarming. Springs made of memory alloy can also be placed in the heating valve to maintain the temperature of the heating room. When the temperature is too low or too high, the heating valve can be opened or closed automatically. Shape memory effect of shape memory alloys is also widely used in temperature sensor triggers.
Another important property of shape memory alloys is pseudoelasticity (also known as superelasticity), which shows that under the action of external forces, shape memory alloys have much greater deformation recovery ability than ordinary metals, that is, the large strain generated during loading process will recover with unloading. This performance has been widely used in medicine, building shock absorption and daily life. For example, the aforementioned artificial skeleton, traumatic bone fixation compressor, dental orthodontics and so on. The spectacle frames made of shape memory alloys can withstand much larger deformation than ordinary materials without damage (instead of using shape memory effect, they are heated and restored after deformation).
In the 1970s, a kind of alloy with shape function of "memory" appeared in the world material science. Memory alloy is a special kind of metal strip. It is very easy to bend. We put it in a glass jar filled with hot water, and the metal strip rushes forward. When it is put in cold water, the metal strip is restored to its original state. In a glass jar with cold water, stretch a spring and put it in hot water. The spring automatically closes again. In cold water, the spring restores its original shape, while in hot water, it shrinks. The spring can be stretched and contracted indefinitely, and then pulled apart. These are made of a memory smart metal. Its microstructure has two relatively stable states. At high temperatures, the alloy can be changed into any shape you want. At lower temperatures, the alloy can be stretched. But if it is reheated, it will remember its original shape and change back.
This material is called memory metal. It is mainly nickel-titanium alloy material. For example, a spiral superalloy is in spiral shape after high temperature annealing.
