Memory function of nickel-titanium alloy
Nickel-titanium alloy, also known as memory alloy, is a material with unique physical properties. It deforms under external force, but can return to its original shape when the external force is removed. This "memory" property makes nickel-titanium alloy widely used in many fields. So, why does nickel-titanium alloy have memory function? What is its principle?
1. Shape memory effect
1) Definition:
The shape memory effect refers to the ability of nickel-titanium alloy to remember its original shape and return to that shape under certain conditions.
2) Principle:
(1) Nickel-titanium alloy has two main crystal structure phases: austenite and martensite. Austenite usually appears at higher temperatures and has a more compact and ordered crystal structure; while martensite appears at lower temperatures and has a looser crystal structure.
(2) When nickel-titanium alloy is heated above a certain temperature, the low-temperature martensite phase will transform into the high-temperature austenite phase, and the alloy will return to its original shape. This process is a heat-induced phase transformation process.
(3) When the alloy is cooled and an external force is applied to deform it, it transforms from the austenite phase to the martensite phase and "remembers" the deformed shape in the process. However, once heated again to a high enough temperature, the martensite phase will transform back to the austenite phase and the alloy will return to its original shape.
2. Superelasticity
1) Definition:
Superelasticity refers to the ability of nickel-titanium alloy to produce a strain far greater than the elastic limit of conventional materials under an applied load and automatically recover its shape after unloading.
2) Principle:
(1) The superelasticity of nickel-titanium alloy is also closely related to its crystal structure changes. When an external force acts on the alloy, the austenite phase transforms into the martensite phase, allowing the material to undergo a large deformation.
(2) However, this deformation is within the elastic limit, which means that when the external force is removed, the martensite phase transforms back to the austenite phase, allowing the alloy to return to its original shape.
3. Practical application of memory function
The memory function of nickel-titanium alloy has been widely used in many fields:
1) Medical field: used to make medical devices such as dental correction wires and vascular stents. Dental correction wires use shape memory effect and superelasticity to apply appropriate force to teeth to achieve tooth correction; while vascular stents can be expanded and support blood vessels through shape memory effect after implantation in the body to keep blood vessels unobstructed.
2) Aerospace field: used to make key components of aircraft engines, etc. The superelasticity and shape memory effect of nickel-titanium alloy can improve the reliability and service life of components.
3) Mechanical engineering field: used to make various springs, sensors and other components.
4) Other fields: such as thermosensitive materials in shower mixing valves, Mars rover tires, etc. also use the memory function of nickel-titanium alloy.
The memory function of nickel-titanium alloy comes from its unique phase change characteristics. Through temperature changes, nickel-titanium alloy can switch between different physical forms and "remember" its original shape when the temperature is restored. This property makes nickel-titanium alloy widely used in fields such as medicine, robotics and aerospace, and has become an indispensable part of modern technology.
