Why titanium nickel has memory function?

Nitinol alloy has the ability of "memory", and its memory is very strong. After a long time, it can be repeated thousands of times without error. Its memory ability is to remember its original shape, so people call it "shape memory alloy". It turns out that this alloy has one characteristic transition temperature. Above the transition temperature, it has one structure, while below the transition temperature, it has another structure. Different structures have different performances. For example, a titanium nickel memory alloy is very hard and strong when it is above the transition temperature, but it is very soft and easy to cold work at this temperature. In this way, when we need it to remember any shape, we can make it into that shape, which is its "permanent memory" shape. Below the transition temperature, because it is very soft, we can make it arbitrary to a considerable extent. When we need it to restore to its original shape, just heat it to above the transition temperature