# Shape Memory (DD-616) > Shape-memory alloys can be bent out of shape when cool and then spring back when warmed. The best known is nitinol, an alloy of roughly equal parts nickel and titanium. Its name comes from Nickel, Titanium and the Naval Ordnance Laboratory, where William J. Buehler's team discovered it in 1959. According to ASME, the team was looking for materials for tools to dismantle mines. Wikipedia recounts that, at a 1961 meeting, someone heated a folded strip of it with a pipe lighter and it snapped back to its original shape. The effect comes from a reversible change in crystal structure. Below its transformation temperature, nitinol is in a monoclinic martensite phase that deforms easily and holds its new shape. Heated above that temperature, it converts to a cubic austenite phase and returns to its original form. The remembered shape isn't automatic. ASME says the part must be shape-set by clamping it in a mold, heat-treating it at about 500 °C and cooling it quickly. Composition and heat treatment adjust the transformation temperature. Above that temperature, nitinol is also superelastic, recovering from strains of about 8%. The shape-memory effect was first seen in a gold–cadmium alloy in 1932 by the Swedish chemist Arne Ölander. Shape-memory polymers became commercially available in the late 1990s. Nitinol is now used in stents, orthodontic wires and eyeglass frames. The footage shows a bent wire and a coil over a red heating element, straightening and contracting as they warm. - Department: Science (also Tech) - First published: 2025-07-17 - Last updated: 2025-07-17 - Permalink: https://detailenjoyer.com/case/shape-memory/ - Author: Marcus Easterling (DetailEnjoyer), The Detail Department ## On the timeline - **1932**: Arne Ölander first sees the shape-memory effect in a gold–cadmium alloy - **1959**: Nitinol is discovered at the US Naval Ordnance Laboratory Full timeline: https://detailenjoyer.com/timeline/?case=shape-memory ## Quick facts - **DD-616-F01**: Nitinol is named for nickel, titanium and the Naval Ordnance Laboratory. _(Write-up · Exhibit C (Navy origins))_ - **DD-616-F02**: A nitinol part's remembered shape is set by heat-treating it at about 500 °C. _(Write-up · Exhibit B (Shape-setting))_ ## Write-up Bend it, heat it, it's back. Nitinol springs back to a shape set at about 500 °C. 1. **Two crystal forms**: Cool nitinol is soft martensite; heated, it becomes austenite and returns to its set shape. 2. **Shape-setting**: The remembered shape is set by clamping the part and heat-treating it at about 500 °C. 3. **Navy origins**: Nitinol was discovered in 1959 at the Naval Ordnance Laboratory, which gives it its name. 4. **First seen**: The shape-memory effect was first seen in a gold–cadmium alloy in 1932. 5. **In use**: Nitinol is used in stents, orthodontic wires and eyeglass frames. ## Related case files - [Smart Plastics (DD-057)](https://detailenjoyer.com/case/smart-plastics/): Another material whose properties are programmed - [Point Curie (DD-669)](https://detailenjoyer.com/case/point-curie/): Another temperature-triggered change in a metal ## Reel vs. record - **Note**: 'A preset shape': the shape has to be set first, by holding the part in a mold while it's heat-treated at about 500 °C and then cooled quickly (ASME). - **Note**: 'Heating triggers a change': it happens above the alloy's transformation temperature, which is tuned by composition and heat treatment. For some devices that's near body temperature. - **Note**: 'Low-temperature martensitic phase where it's easily deformed': correct. Martensite is monoclinic and austenite is cubic. - **Note**: 'Shape memory alloys and shape memory polymers': nitinol is the most common alloy. Copper-based alloys exist too, and shape-memory polymers became commercially available in the late 1990s. - **Missing**: nitinol was discovered in 1959 at the US Naval Ordnance Laboratory. The shape-memory effect itself was first seen in a gold–cadmium alloy in 1932. ## Video: Shape Memory: Bend It, Heat It, It's Back - Duration: 25.4 s · Published: 2025-07-17 - File: https://detailenjoyer.com/assets/videos/DMObMsftlFw.mp4 ### Transcript DID YOU KNOW? Heat-based shape memory refers to the property of certain materials, especially shape memory alloys and shape memory polymers to "remember" and return to a preset shape when heated. The science behind this relies on thermally induced phase transitions. For alloys like nitinol, or nickel titanium, heating triggers a change from the low-temperature martensitic phase where it's easily deformed, to the high-temperature austenitic phase, where it returns to its memorized shape. NOW YOU KNOW! ## Questions & answers **Q: What does the 'NOL' in nitinol stand for?** A: Where it was discovered. _(Source: Write-up · Exhibit A (Two crystal forms) · ASME: Metal That Remembers Its Shape; Wikipedia: Nickel titanium)_ **Q: Which phase of nitinol is easily deformed?** A: The low-temperature phase. _(Source: Write-up · Exhibit A (Two crystal forms) · ASME: Metal That Remembers Its Shape; Wikipedia: Nickel titanium)_ **Q: About what temperature is used to shape-set nitinol?** A: Then it's cooled quickly. _(Source: Write-up · Exhibit B (Shape-setting) · ASME: Metal That Remembers Its Shape)_ **Q: In what year was nitinol discovered?** A: At the Naval Ordnance Laboratory. _(Source: Write-up · Exhibit A (Two crystal forms) · ASME: Metal That Remembers Its Shape; Wikipedia: Nickel titanium)_ **Q: Which is a common medical use of nitinol?** A: It can be compressed into a catheter and spring open. _(Source: Write-up · Exhibit A (Two crystal forms) · ASME: Metal That Remembers Its Shape; Wikipedia: Nickel titanium)_ ## Sources - [ASME: Metal That Remembers Its Shape](https://www.asme.org/topics-resources/content/metal-that-remembers-its-shape) - [Wikipedia: Nickel titanium](https://en.wikipedia.org/wiki/Nickel_titanium) - [Wikipedia: Shape-memory alloy](https://en.wikipedia.org/wiki/Shape-memory_alloy) ## Cite this case file Easterling, Marcus. "Shape Memory." The Detail Department (DetailEnjoyer), case file DD-616, last updated Jul 17, 2025. https://detailenjoyer.com/case/shape-memory/