In 1963, the U.S. Naval Ordnance Research Laboratory needed some Ti-Ni memory alloys in an experiment. The wires they brought back were bent. In order to use it conveniently, the bent and curved filaments are stretched out one by one and then used.

In subsequent experiments, a strange phenomenon emerged: when the temperature rose to a certain value, these wires, which had been pulled straight, suddenly and magically quickly returned to the original curved shape, and no difference from the original shape. After repeated tests, the results are exactly the same. As long as the straightened alloy wire reaches a certain temperature, it will immediately return to the original bending pattern. It's as if people changed their shape when they were "frozen" unconscious in the past. When the temperature rose to a certain value, they suddenly "woke up" and "remembered" their original appearance, so they recklessly restored their "true face".

The crystal structure of nickel-titanium alloy is different from that of nickel-titanium alloy below 40 and above 40 C. However, when the temperature changes above 40 C, the alloy will shrink or expand, which will change its morphology. Here, the "metamorphic temperature" of nickel-titanium memory alloys is 40 degrees centigrade. Various alloys have their own metamorphic temperatures.

[Application of Nickel-Titanium Memory Alloy]

Memory alloys have been used in pipeline bonding and automation control. Casing made of memory alloys can replace welding. The method is to enlarge the end of the pipe by about 4% at low temperature. When assembled, the sleeve is socketed together. Once heated, the shrinkage of the sleeve restores to its original shape and forms a tight joint. The hydraulic system of U.S. Navy aircraft uses 100,000 of these connectors, and for many years there has been no oil leakage or damage. It is very convenient to repair damaged ship and submarine oil field pipeline with memory alloy fittings. In some parts where construction is inconvenient, memory alloy pins are made and heated into holes. The tail ends of pins are automatically separated and curled to form one-sided assemblies.

Memory alloy is especially suitable for thermo-mechanical and constant temperature automatic control. It has been made into room temperature automatic opening and closing arms. It can open ventilation windows in sunshine day and shut down automatically when room temperature drops at night. There are many design schemes for memory alloy heat engines, which can work between two media with low temperature difference, thus opening up a new way to utilize industrial cooling water, waste heat of nuclear reactor, ocean temperature difference and solar energy. At present, the common problem is that the efficiency is not high, only 4%-6%, which needs further improvement.

Memory alloys have also attracted considerable attention in medical applications. For example, the plate used for bone grafting can not only fix two broken bones, but also produce compressive force in the process of restoring the original shape, forcing the broken bones to join together. Dental orthodontic wires, long clips for ligating cerebral aneurysms and vas deferens, and supporting plates for spinal straightening are all activated by body temperature after implantation. Thrombofilter is also a new memory alloy product. After the straightened filter is implanted into the vein, it gradually restores to a net shape, thus preventing 95% of the clots from flowing to the heart and lungs.

Artificial heart is a kind of organ with more complex structure. The muscle fibers made of memory alloy can imitate the contraction of ventricle by matching with the elastomeric membrane ventricle. Now pumping water has been successful.

Because memory alloy is a kind of "living alloy", various kinds of automatic control devices can be designed by using its shape change at a certain temperature, and its use is expanding.