Shape memory alloys are metals that when deformed, can spring back into its original shape once released.

• A. Shape memory alloys
• B. Superelastic alloys
• C. Nickel titanium (Nitinol)
• D. Thermoelastic alloys

Answer: (A)

Shape memory materials recover their original shape because a reversible phase change drives the return. In these metals, deformation occurs in one phase (martensite) at lower temperatures, and when the material is heated or the stress conditions are right, it transforms back to the other phase (austenite). This transformation pulls the material back to the shape it had before deformation, which is the hallmark of this class. Nickel-titanium, or Nitinol, is a classic example you might have heard of, showing this behavior in real life applications. The broad term that describes this recovery property is shape memory alloys, since it covers both the memory-shape behavior and related recoveries that can occur under different temperatures and loading conditions. Superelastic alloys describe a closely related behavior under specific conditions, but the general label for materials that exhibit this recoverable shape is shape memory alloys.