Nitinol is a shape memory alloy known for its unique ability to undergo large deformations and return to a pre-programmed form. The name is a portmanteau derived from its constituent elements (Nickel and Titanium) and its place of discovery, the Naval Ordnance Laboratory (Ni-Ti-NOL). It was discovered in 1959 by William J. Buehler and Frederick E. Wang during research into heat and fatigue-resistant materials. This alloy has properties unlike any conventional metal, enabling a wide array of applications in modern engineering.
The Chemical Makeup of Nitinol
Nitinol is an intermetallic compound formed by Nickel (Ni) and Titanium (Ti). The alloy is defined by its equiatomic composition, meaning it contains a nearly equal number of nickel and titanium atoms, typically 50% of each element by atomic percentage. This precise 1:1 atomic ratio (NiTi) is fundamental to the alloy’s unusual behavior.
The manufacturing process requires meticulous control because the precise stoichiometric balance is sensitive. Slight deviations from the equiatomic ratio are intentionally introduced to fine-tune the material’s properties. For instance, a change of just one atomic percent can shift the alloy’s transformation temperature by as much as 100 degrees Celsius, allowing engineers to tailor the material for specific operating temperatures.
Shape Memory Effect
The Shape Memory Effect (SME) allows Nitinol to recover a complex, pre-set shape after being severely deformed. This recovery is driven by a reversible phase transformation that occurs with temperature change. At lower temperatures, the alloy exists in the soft, easily deformed Martensite phase, which has a complex, twinned crystal structure.
The material is “trained” with its permanent shape in the high-temperature Austenite phase, which has a rigid, ordered cubic crystal structure. Once deformed in the low-temperature Martensite phase, the material is locked into the temporary shape. When heated above its transformation temperature, the alloy rapidly reverts from Martensite back to Austenite, returning to its original form. This allows for repeatable shape recovery, often up to 8% recoverable strain.
Superelasticity
Superelasticity allows Nitinol to withstand deformation without permanent damage. Unlike the Shape Memory Effect, superelasticity occurs isothermally, meaning it happens at a constant temperature, often just above the material’s transformation temperature. In this state, the alloy is primarily in the high-temperature Austenite phase.
When mechanical stress is applied, it induces a phase transformation, forcing the Austenite structure to locally transform into the Martensite phase. This stress-induced transformation allows the material to absorb a large amount of strain, up to 8-10%. Once the external stress is released, the Martensite immediately snaps back to the original Austenite phase. This instantaneous recovery allows it to recover its original shape without needing heat.
Biocompatibility and Medical Applications
Nitinol’s excellent biocompatibility makes it highly valued in the medical field. When exposed to the body, Nitinol forms a stable, passive titanium oxide layer on its surface. This inert layer provides exceptional corrosion resistance and prevents the release of nickel ions, ensuring the alloy does not provoke adverse reactions in human tissue.
This combination of properties has made Nitinol indispensable for minimally invasive medical devices. Superelasticity is utilized in orthodontic archwires, applying a light, consistent force for tooth alignment. Both the Shape Memory Effect and Superelasticity are harnessed in vascular stents. The stent can be compressed for delivery through a narrow catheter, and then deployed either by body heat (SME) or by releasing the device to spring open (Superelasticity) once it reaches the target vessel. Nitinol is used in everything from guide wires to surgical tools.