How Does Nitinol Work? All About Nitinol Shape Memory and Superelasticity

A shape memory alloy such as nickel-titanium undergoes a phase change in its crystal structure when cooled from a relatively high temperature form (austenite) to a weaker low temperature form (martensite). This inherent phase change is the foundation of the unique properties of these alloys, especially shape memory and superelasticity.

When the shape memory alloy is in its martensitic form, it easily transforms into a new shape. However, when the alloy is heated at its transition temperature, it returns to austenite and recovers its previous shape with great force. Shape memory

The temperature at which the alloy memorizes its high temperature form during heating can be adjusted by slight changes in alloy composition and heat treatment. For example, in a nickel-titanium alloy, it can be changed from + 100 ° C. or more to -100 ° C. or less. The shape recovery process is done only within a few degrees and you can control the start or end of a transition to some extent or within 2 if necessary.

These unique alloys also exhibit superelastic behavior when deformed at temperatures slightly higher than their transition temperature. This effect is caused by the formation of martensite caused by stress above its normal temperature. Since it is formed at room temperature or higher, when stress is removed, martensite immediately returns to undeformed austenite. This process gives these alloys very elastic "rubbery" elasticity.

Nitinol combines two closely related unique functions of shape memory and superelasticity. Shape memory deforms Nitinol at a temperature and recovers its original shape when heated to its transition temperature. This effect is caused by the material changing from its low temperature monoclinic martensitic structure to a high temperature cubic austenite structure. Our finished Nitinol wire has a transition temperature range of -15 ° C to + 22 ° C depending on the Nitinol rating used.

Nitinol alloys have two closely related unique properties of shape memory effect (SME) and superelasticity (SE; pseudoelasticity, also known as PE). Shape memory is the ability to restore the original undeformed shape when Nitinol deforms at a certain temperature and is heated beyond its "transition temperature". The superelasticity takes place in a narrow temperature range just above its transition temperature, in this case there is no need to heat it to recover the undeformed shape, the material shows 10 to 30 times more elasticity than the usual metal .

Features of NiTi or Nitinol include shape memory effect, super elasticity and high damping capacity. By changing composition, machining and heat treatment these technical properties of the NiTi shape memory alloys can be changed drastically to suit Johnson Matthey's specific customer application. In the nickel-titanium shape memory alloy of Johnson Matthey Medical Components Components, the temperature of the SMA memory of its high temperature form can be adjusted by heat treatment, the composition of the alloy varies from 0 ° C to + 105 ° C. Shape recovery occurs within a few degrees, and start / end points of shape recovery can be controlled within a few degrees Celsius

Martensite deformation temperature (Md) - the maximum temperature at which martensite is formed from the austenite phase according to the applied stress. At temperatures above Md, Nitinol shape memory alloys do not exhibit superelasticity and exhibit typical elastoplasticity during loading.