Pseudoelasticity

Pseudoelasticity, sometimes called superelasticity, is an elastic (reversible) response to an applied stress, caused by a phase transformation between the austenitic and martensitic phases of a crystal. It is exhibited in shape-memory alloys. Pseudoelasticity is from the reversible motion of domain boundaries during the phase transformation, rather than just bond stretching or the introduction of defects in the crystal lattice (thus it is not true superelasticity but rather pseudoelasticity). Even if the domain boundaries do become pinned, they may be reversed through heating. Thus, a pseudoelastic material may return to its previous shape (hence, shape memory) after the removal of even relatively high applied strains. One special case of pseudoelasticity is called the Bain Correspondence. This involves the austenite/martensite phase transformation between a face-centered crystal lattice (FCC) and a body-centered tetragonal crystal structure (BCT). Superelastic alloys belong to the larger family of shape-memory alloys. When mechanically loaded, a superelastic alloy deforms reversibly to very high strains (up to 10%) by the creation of a stress-induced phase. When the load is removed, the new phase becomes unstable and the material regains its original shape. Unlike shape-memory alloys, no change in temperature is needed for the alloy to recover its initial shape. Superelastic devices take advantage of their large, reversible deformation and include antennas, eyeglass frames, and biomedical stents. Nickel titanium (Nitinol) is an example of an alloy exhibiting superelasticity. Recently, there have been interests of discovering materials exhibiting superelasticity in nanoscale for MEMS (Microelectromechanical systems) application. The ability to control the martensitic phase transformation has already been reported. But the behavior of superelasticity has been observed to have size effects in nanoscale. Qualitatively speaking, superelasticity is the reversible deformation by phase transformation.