The Shape Memory Chapter: The Deadpool of Alloys-NiTiNOL
“The dignity you are receiving has spread around the world in your development of the alloy Nitinol. If it hadn’t been for you, your wife would be treated with stainless steel wire. If you had not sent me the 3-foot piece of Nitinol wire, I could not have applied it to orthodontics. In fact, I’ll be in your debt forever.”
— from an interview with William J. Buehler,14 September 1988.
Once, in the depths of my internet browsing, I stumbled upon a captivating YouTube video showcasing a remarkable material known as NiTiNOL. I was super fascinated when I saw the material return to its original shape when heated even after being crushed and deformed several times, it was like watching Deadpool come back to life even after being killed several times. So I looked up on Google what this metal alloy is and why this Nickel Titanium alloy is so special and in the process, I learned that this material has a fascinating property called Shape memory. It should not be only me who is fascinated by this superhero-Alloy. So, let’s go on a journey through time to uncover the remarkable tale of our unsung real-life superhero, NiTiNOL.
A Moment in Time: The History of NiTiNOL.
It all started in the late 1950s, A metallurgist named William J Buehler at the Naval Ordnance Laboratory was bored when his research on Iron Aluminum alloys was complete. Then, he came across an ongoing materials project which intended to develop a metallic material for the Nose cone of the U.S. Navy Polaris reentry vehicle, a submarine-launched missile. Mr. Buehler was able to connect himself to the project. Initially, he worked on providing data related to the physical and mechanical properties of existing metals and alloys. He soon found no fun in that and immediately started thinking about the possibility of making new alloys that could satisfy the project’s goals.
Having some time on his hands, He worked day and night in the lab. With the help of the book by Max Hansen, Constitution of Binary Alloys, Mr. Buehler selected approximately sixty intermetallic compound alloys and then later on, the number was reduced to twelve after a lot of careful study and observations. One of the twelve alloys was the Equi-atomic Nickel-Aluminum alloy. This alloy showed high ductility and impact resistance than the other eleven alloys.
In 1959, Mr. Buehler decided to concentrate on this Equi-atomic alloy for further deep research by naming it NiTiNOL, is the abbreviation for ‘Nickel Titanium Naval Ordnance Laboratory’.
The Magical Properties:
After the discovery of the alloy, various experiments were conducted with it and many fascinating observations were made with it. Buehler’s fascinating observation of NiTiNOL bars revealed that even minor temperature variations can induce significant changes in the acoustic damping of the material. This captivating behavior can be attributed to the alloy’s unique microstructure and strain characteristics, which are influenced by temperature fluctuations-near room temperature. By all these observations Beuhler was able to sense that this alloy was capable of major atomic movement at a lower temperature regime near room temperature.
But was it all that NiTiNOL had?
The Serendipitous Discovery
Many amazing properties of NiTiNOL were discovered except for shape memory. This is the most important property of NiTiNOL without it NiTiNOL will not be a smart material or a super-material. NiTiNOL without shape memory is similar to Hulk without rage; we all know how boring that is. So when was this property discovered? Was it in a lab? The answer is no. Like all amazing things, the discovery of this property is a fortunate and happy little accident. It is more like the invention of penicillin which was a revolutionary and fortunate accident in the field of medicine.
In 1961, a laboratory meeting was scheduled to review the ongoing projects in Naval Ordnance Laboratory. When the board members examined a strip of crushed NiTiNOL Dr. Davind S Muzzey, a pipe smoker, applied heat using the pipe lighter to the compressed strip. To everyone’s surprise, the NiTiNOL strip stretched and straightened itself. And finally, NiTiNOL’s incredible ability to recall its original shape was found by surprise at a conference, rather than during a planned scientific experiment. This is why accidents are frequently regarded as the greatest innovators of all time, as they may lead to incredible discoveries that no one could have predicted. Discovering the property might spark curiosity and joy, but the real excitement comes from unraveling its mysterious science.
The Martensite transformation
As a fan of this material, I find it incredibly exciting to explore the origins and workings of the shape memory property in alloys like NiTiNOL. This remarkable ability is attributed to the solid-solid phase transformation known as the austenite-martensite phase transformation, which grants the material its extraordinary superpower qualities. This is a temperature-dependent phase change which is also called the martensite transformation. This transformation happens when the temperature varies through a small temperature range called the Transformation temperature range(TTR). Austenite is a stable phase at a relatively higher temperature. In this phase, the crystal structure of the NiTiNOL becomes that of an “ordered” cubic, also sometimes called CsCl structure (Body-centered cubic lattice). When the NiTiNOL is cooled down it goes to a structurally complex Martensite phase. There is almost no shape change visible macroscopically when the material goes from the austenite phase to the martensite phase unless there is no force applied externally. These two transformation phases are named after the people who studied them.
Generally, NiTiNOL is molded to its parent shape when it is in its austenite phase and cooled to the martensite phase. The TTR can be manipulated to occur between -200°C to 110°C by varying the nickel content in the alloy or by adding other metals like copper and niobium. This way the TTR is adjusted based on the requirements. And this has opened a wide and mind-boggling range of application possibilities in engineering and medicine.
Where has it been-From the F-14 to the Mars Rover
This amazing alloy is used in many fields from engineering to medicine and even art. NiTiNOL was first commercially used by The Raychem Corps for making “shrink-to-fit pipe couplers” that can be used for coupling hydraulic-fluid lines in the F-14 fighter jet . NiTiNOL tires are also being developed by NASA for future Mars Rover missions.
The medical field stands as a prominent consumer of Nitinol due to its versatile applications and unique properties such as shape memory.
NiTiNOL made a revolution in cardiovascular surgery.
Blood clots (pulmonary embolisms) killed 200,000 people each year. To solve this problem in 1989 Radiologists Morris Simon from Israel created a mushroom-shaped NiTiNOL blood clot filter that could be used without performing surgery to implant the Blood clot filter which is risky and expensive.
All in All
NiTiNOL stands as a real-life superhero, showcasing incredible properties and an extensive range of applications. Its shape memory effect, super elasticity, and thermal characteristics have revolutionized various fields. From revolutionizing medical procedures to enabling innovative engineering solutions, NiTiNOL has proven its worth as a remarkable alloy. It is versatility and life-saving capabilities make it an awe-inspiring material that continues to serve and save human lives. As we marvel at the wonders of NiTiNOL, we can confidently celebrate its status as a true superhero in the world of materials.
