Principles and classification of memory metals

Principle of memory metal:

Memory metals take advantage of the fact that the crystal structure of certain alloys changes with temperature when they are solid. For example, Nitinol alloys have different crystal structures at temperatures above and below 40 degrees. However, if the temperature changes around 40 degrees, the alloy will shrink or expand, and the morphology changes, so 40 degrees is the metamorphosis temperature of nickel-titanium memory alloy. In fact, every alloy has its own temperature. We can shape the alloy at high temperatures to bring it to a stable state, then force it to straighten it at room temperature to bring it to an unstable state, and finally heat it to a temperature that returns it to its stable state shape.

Memory metal characteristics:

(1) Large bending range, high plasticity.

(2) When the temperature reaches above the memory temperature, it can return to its previous shape.

(3) Depending on the alloy type, when the temperature reaches a certain value, the crystal structure inside the material will change, which will further lead to the change of the external morphology.

Classification of memory metals:

(1) One-way memory effect

Memory metals can deform at lower temperatures and can return to their previous shape when heated. This shape memory phenomenon exists only during the heating process, which is called one-way memory effect.

(2) two-way memory effect

Some memory metals revert to a high temperature phase shape when heated and a low temperature phase shape when cooled. This change is called the two-way memory effect.

(3) whole memory effect

This memory metal is special. When heated, it reverts to the shape of the hot phase, while when cooled, it changes into the shape of the cold phase with the same shape but opposite orientation. This is called the whole memory effect.

In terms of the current memory metal market, the successfully developed shape memory metals include copper based shape memory alloy, TiNi based shape memory alloy and iron based shape memory alloy.