Characteristics & Properties of Nitinol
Nitinol has the unique ability to adapt to particular strains and is compatible to the human body. Therefore, it has many uses in the medical field. Nitinol displays a cubic crystal structure, known as austenite (also called the parent phase). At lower temperatures, it spontaneously becomes a complex “monoclinic,” crystal structure called martensite. The temperature at where austenite turns to martensite, also known as the transition temp, is commonly called the Ms temperature. Further, the temperature at its full formation is known to be the Mftemperature. These two features of Nitinol’s structural structure — shape-memory and superelasticity — allow it to show a reversible reaction to applied stress. This is directly caused by the phase change between the martensitic and austenitic phases.
These two aspects are critical to Nitinol’s properties. The transition is “reversible”, meaning that heating above transition temperature will return the crystal structure back to the simpler austenite. The second important point is that two-way conversion occurs instantly.
Martensite crystals have the unique ability to undergo finite atomic bond breaking without causing any damage. Twins are a form of permanent deformation. It involves the rearrangement and reorganization of atomic planes. This can withstand around 6-8% of strain.
Martensite can be converted to austenite using heating. The original austenite structure will still be intact, regardless of the deformation of the martensite phases. This is why the term “shape memories” refers to how the high-temperature austenite phases are “remembered”, even if they deform severely at lower temperatures.
The Nitinol medical devices, such as stents, can be made at the body temperature, deformed at another temperature and then placed into an artery. Once it is back at its normal temperature, it will return to its original size. The device will also fully recover after being bent to extremely high strain rates (upto 7%).
Super-elastic effect of Nitinol Wire
This “hyperelasticity” allows you to use a Nitinol-wire device that has been bent, shaped or placed in your body. The tube can accommodate small grasping or biopsy tools that are smaller than those made from standard alloys. Nitinol is a lightweight alloy with unique properties that makes it ideal for biomedical applications. This includes heart valve tools, nails, bone anchors (complex diaphragm defects devices), and many other implants.
But heat-treating Nitinol to achieve the desired transition temperature is delicate. Temperature and age control the precipitation process of various Ni-rich phases. This is done to regulate the nickel content within the lattice. Aging decreases the nickel matrix, increasing the transition temperature. It is crucial to combine heat treatment with cold work in order to control the properties and alloy Nitinol.
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