In the 1970s, a kind of alloy with "memory" shape function 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. At room temperature, even if it is forced to straighten with great effort, but as long as it is heated to a certain "metamorphic temperature", the alloy seems to remember something and immediately restores to its original spiral form. What's the matter? Do alloys have the same memory as humans?
That's not the case! This is only based on the law that the crystal structure of some alloys changes with temperature when they are solid. For example, the crystal structure of nickel-titanium alloy is different above 40 ℃ and below 40 ℃, but when the temperature changes at above 40 ℃, the alloy will shrink or expand, which makes the shape of Ni-Ti alloy change. Here, the "metamorphic temperature" of nickel-titanium memory alloys is 40 degrees centigrade. Various alloys have their own metamorphic temperatures. The metamorphic temperature of the superalloys mentioned above is very high. At high temperatures it is made into a spiral and is in a stable state. When it is forced to straighten at room temperature, it is in an unstable state, so as long as it is heated to the metamorphic temperature, it will immediately restore to the original stable spiral shape.
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