Expanding the Repertoire of Shape Memory Alloys

The exceptional properties of many materials often come at the expense of limited performance in other areas. For example, conventional metals and their alloys are strong—they are good at resisting stress (i.e., an applied load)—but they tolerate only a very small amount of strain (i.e., deformation) before they are irreversibly deformed. Rubber can easily return to its original shape, even after large deformations, but is much weaker than conventional metals. However, some metal alloys exhibit “shape memory”; they are strong but can recover from being deformed when heated. This process seems counterintuitive, but these alloys take advantage of solid-to-solid “diffusionless” phase transitions: The atoms rearrange how they pack into crystals in an orderly fashion, and this process changes the material's macroscopic shape. Few other materials possess this combination of strength and flexibility (see the figure), and clever engineering has exploited these properties—for example, in implanted medical devices such as stents. On page 1488 of this issue, Tanaka et al. (1) report on a superelastic alloy that almost doubles the useful range of deformation that can be induced in such alloys.