Shape memory alloys

Typically, when a metal or alloy is subjected to mechanical stress in excess of its yield strength, it undergoes plastic deformation which remains after the stress has ceased. This deformation then evolves little or not at all during subsequent heat treatments. Memory alloys seem to escape this behavior, familiar to metallurgists and mechanics alike: a sample of such an alloy, apparently plastically deformed at a given temperature, can recover its initial shape in its entirety simply by heating. This deformation can reach 8% in tension. This phenomenon is known as shape memory, and is associated with a reversible martensitic-type structural transformation that takes place between the temperature at which the sample was deformed and that at which it was reheated to regain its shape. This martensitic transformation is also responsible for other unusual thermoelastic properties, such as superelasticity. Describing these properties requires a basic understanding of martensitic transformations.