In 1961, scientists heated a twisted nickel-titanium strip and uncovered Nitinol’s hidden memory

During the early 1960s, a group of scientists gathered near a laboratory workbench to observe a bent metal. After applying heat to the folded strip, the scientists observed an unbelievable process - it returned to its original shape on heating, a rapid demonstration that surprised observers. Of course, it was not a magic trick. They had witnessed the shape-memory effect in Nitinol -a new type of nickel-titanium alloy that would fundamentally change our understanding of solids.Most conventional metals deform plastically when bent beyond their elastic limit and do not spontaneously return to their original shape. However, Nitinol has a certain built-in memory that enables a substance to bend, twist, or even crush while cool, but to revert to its previous form after a sudden heat impulse triggers internal changes.A laboratory name became a metal nameAlthough the word seems to be made up specifically to market the product, the name of the metal Nitinol is, in fact, an abbreviation for Nickel Titanium Naval Ordnance Laboratory. In other words, the material is named after the very government laboratory where it was developed at the US Naval Ordnance Laboratory.According to the historical review provided by Springer Nature, the development of Nitinol happened during the early 1960s. At that time, the field of shape memory alloys had been recently introduced to the worldwide scientific community. In fact, according to the paper, the remarkable achievement took place from 1961 to 1962, when the scientists working in the US laboratory managed to discover an extraordinary process of recovery of the alloy. As one can see, the recovery takes place due to an atomic rearrangement when the material changes temperature conditions and becomes stable again. Why does heating make the alloy recover?As has been stated above, the effect can be made predictable when the alloy is properly processed. To put it differently, there are two different phases of an alloy that depend on the temperature of the material. Thus, at lower temperatures, Nitinol is in a martensitic phase that is easily deformed (often reversibly by twinning). When heated above a transformation temperature, it converts to the austenitic phase, which recovers the original shape and is stiffer.As pointed out in the 2024 scientific review on Heliyon, the shape-memory effect described is a fully reversible functional property. According to the review, when the alloy is bent at room temperature, it does not lose the intrinsic pattern of atomic organisation. Since the structure itself is preserved, the material is capable of restoring itself once again to its former position with the help of external thermal energy.From the military laboratory to household appliancesOnce a simple discovery made in a military laboratory, the effect became part of a commercial industry. The engineers immediately understood that there was enormous potential in developing a material that is capable of generating a force upon being heated.Nowadays, the material has left the scope of laboratory experiments. Because the material's response is predictable and measurable, it has found its place in industrial and medical applications. For instance, it is highly appreciated for its high actuation strain and biocompatibility. As a result, the alloy can be used to design various devices such as smart archwires used for dental orthodontics or flexible stents used to clear blocked blood vessels.It is fascinating, even after decades, to see the visual transformation of the bent metallic strip. However, Nitinol has already been proven by engineers for decades to be more than just a curious phenomenon from the past. With their discovery of a metal that retains its memory, the researchers created a whole new area of research.