Principle of memory alloy: it is generally believed that shape recovery memory will occur when the memory alloy changes from a complex diamond crystal structure to a simple cubic crystal structure. When the shape of the memory alloy is restored, a great force is generated. The nitinol alloy is as high as 60 kg / mm2, which is far greater than the force added during the initial deformation. Generally speaking, it can reach ten times of the original deformation, which means that the output energy is much larger than the input energy. Scientists can't explain this. Physicist ROSAR said, "there is nothing wrong with the laws of thermodynamics, but they just don't apply to nitinol?". At present, many scholars think that the reason why the shape of memory alloy can be restored is due to the effect of "memory factor". The "memory factor" is derived from the study of free energy and volume relationship in the phase transition process.