Nitinol is a nickel and titanium alloy that contains about 50 percent nickel. Although it was discovered in the early 1960s, Nitinol would take decades to reach the market due to difficulties encountered in processing and manufacturing. In fact, many of Nitinol's uses have been realized for the most part since the mid-1990s, when its usefulness in a wide range of applications resulted from its shape memory properties and super-elastic properties. Shape memory means that Nitinol can change into a new form at one temperature - below normal body temperature, for example - and then return to its original shape when heated beyond its transition temperature.
Characteristics and Properties of Nitinol Nitinol also has a unique ability to adapt to specific strains and is compatible with the human body, so it has many applications in the medical field. At higher temperatures, Nitinol exhibits a cubic crystal structure called austenite (also known as the parent phase). At lower temperatures, it spontaneously transforms into a more complex "monoclinic" crystal structure called martensite. The temperature at which austenite changes to martensite is generally called the transition temperature -- more specifically, martensite begins to form at the so-called Ms temperature, and the temperature at which it is fully formed is called the Mf temperature. These two aspects of its structure -- shape memory and superelasticity -- allow Nitinol to exhibit a reversible response to the applied stress, which itself is caused by the phase transition between the austenitic and martensitic phases of the crystal.
Crucial to the properties of Nitinol are two key aspects of this phase transition. The first is that the transition is "reversible", meaning that heating above the transition temperature will restore the crystal structure to the simpler austenite phase. The second key point is that the two-way conversion is instantaneous.
The crystal structure of martensite has the unique ability to undergo finite deformation with virtually no breaking of atomic bonds. This type of deformation, called twins, consists of a rearrangement of atomic planes without causing permanent deformation. It can withstand about 6-8% of strain in this way.
When martensite is restored to austenite by heating, the original austenite structure will be restored regardless of whether the martensite phase is deformed. Thus, the name "shape memory" refers to the fact that the shape of the high-temperature austenite phase is "remembered", even though the alloy deforms severely at lower temperatures.
So a Nitinol medical device -- a stent, for example -- can be made at body temperature, deformed or folded smaller at another temperature, and inserted into an artery, where it returns to normal temperature and returns to its original size. The phase change also allows the device to fully recover after bending to high strain rates (up to 7%).
Super-elastic Effect of Nitinol Wire This "hyper-elastic" effect allows the use of a Nitinol wire device that has been bent or shaped to allow introduction or use in the body. Tools such as small grasping and biopsy devices can extend from the tube and extend over a much larger area than devices made of standard alloys. Nitinol's weight reduction and unique properties make it particularly attractive for biomedical applications, including heart valve tools, stents, nails, bone anchors, complex diaphragm defect devices, and various implants.
However, heat-treating Nitinol is delicate -- and crucial in fine-tuning the transition temperature. Aging time and temperature control the precipitation of various Ni-rich phases, so as to control the nickel content in the lattice of nickel and titanium. Aging increases the transition temperature by depleting the nickel matrix. The combination of heat treatment and cold working is very important to control the properties of Nitinol alloy.
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