NiTi alloy is composed of two metal elements, nickel and titanium, and has the ability to change its shape and characteristics with changes in temperature or stress. This unique property stems from its internal phase transition and crystal structure changes.
Phase transition mechanism:
NiTi alloy exhibits two different structures at different temperatures: high temperature phase (usually face-centered cubic structure) and low temperature phase (usually body-centered cubic structure). When the alloy is heated to a certain temperature, it will transform from the low temperature phase to the high temperature phase, and return to the original low temperature phase during cooling. This phase transition is the main cause of the shape memory effect.
Crystal structure change:
The crystal structure of nitinol alloy contains many dislocations. When external force acts on the alloy, these dislocations will slip, causing the alloy to deform. Within the elastic limit, these dislocations can return to their original state when the external force disappears, thereby restoring the alloy to its original shape.
Memory effect classification:
One-way memory effect: The alloy deforms at a lower temperature and can restore its shape before deformation after heating.
Two-way memory effect: the alloy can recover to the high-temperature phase shape when heated, and recover to the low-temperature phase shape when cooled.
Full-way memory effect: the alloy can recover to the high-temperature phase shape when heated, and can change to the low-temperature phase shape with the same shape but opposite orientation when cooled.

Application of nickel-titanium alloy
Nitinol alloy is widely used in the aerospace field. For example, in aircraft hydraulic systems, nickel-titanium shape memory alloy joints are widely used. In addition, at the interface of aircraft aerial refueling, the memory effect of nickel-titanium alloy can achieve a tight and oil-free effect, which is much better than traditional welding methods.

The shape memory effect of nickel-titanium alloy also plays an important role in the connection of mechanical parts, pipelines and other applications. Through the heating and cooling process, the expansion and contraction of the casing can be achieved, thereby achieving a tight connection. In addition, nickel-titanium alloy can also be used to make damping energy dissipation devices and vibration isolators to enhance the seismic performance of the structure.

Nickel-titanium alloy has good biocompatibility and shape memory effect, making it an important material in the medical field. For example, nickel-titanium alloy is widely used in orthodontics, plastic surgery, cardiovascular minimally invasive and other aspects. Medical devices such as thrombus filters, surgical sutures, artificial joints, and artificial hearts are also often made of nickel-titanium alloys.

In addition to the above fields, nickel-titanium alloys also show broad application prospects in the electronics industry, chemical industry, new energy and other fields. For example, in smart materials, the shape memory effect of nickel-titanium alloys can be used to manufacture thermosensitive components, artificial muscle wires of robots, etc.

The shape memory effect of nickel-titanium alloys stems from its unique phase change and crystal structure changes, which gives it a wide range of application prospects in many fields. From aerospace to medical fields, from the machinery industry to new energy, nickel-titanium alloys play an irreplaceable role. With the deepening of research and the continuous advancement of technology, it is believed that nickel-titanium alloys will have more innovative applications in the future and make greater contributions to the development of human society.