10 Materials Scientists Developed That Are Now in Everyday Products

7. Shape Memory Alloys - Materials That Remember Their Form

Shape memory alloys (SMAs) represent a fascinating class of smart materials that can "remember" and return to predetermined shapes when heated, demonstrating how materials scientists can engineer atomic-scale phase transformations to create macroscopic mechanical responses that enable entirely new categories of applications and devices. The phenomenon underlying shape memory behavior involves a reversible solid-state phase transformation between two crystal structures—martensite and austenite—that occurs at specific temperatures, allowing the material to be deformed in its low-temperature martensitic phase and then recover its original shape when heated above its transformation temperature. The most widely used shape memory alloy, nitinol (nickel-titanium), was discovered accidentally at the Naval Ordnance Laboratory in 1959, but understanding and controlling its properties required decades of research into the complex relationships between composition, processing, and thermomechanical behavior that determine transformation temperatures and mechanical properties. Materials scientists have learned to precisely control SMA properties through careful adjustment of alloy composition, heat treatment procedures, and thermomechanical processing, enabling the development of materials with transformation temperatures ranging from below room temperature to several hundred degrees Celsius, along with different levels of recovery stress and strain. Shape memory alloys now appear in numerous everyday applications that take advantage of their unique ability to generate motion and force in response to temperature changes, including eyeglass frames that return to their original shape after bending, orthodontic wires that apply constant gentle pressure as they warm to body temperature, and actuators in automotive and aerospace systems. The biomedical field has particularly embraced SMAs for applications like cardiovascular stents that can be inserted in a compressed state and then expand to their functional shape at body temperature, while researchers continue developing new alloy compositions and processing techniques to expand the range of applications and improve the reliability and performance of shape memory devices.