Until now both scientific and technological fields have been unable to agree whether the stress-induced martensitic transformation of shape memory alloys (SMAs) exhibits a size dependence similar to that observed in crystal plasticity. Such a size effect could manifest itself as a significant change in the efficiency of the transformation. This would in turn affect, for example, the SMA’s ability to absorb shock energy or damp vibrations.
[Juan et al., doi: 10.1038/NNANO.2009.142] demonstrate that the two phases responsible for shape memory in Cu–Al–Ni alloys are more stable in nanoscale pillars than they are in the bulk. As a result, the pillars show a damping figure of merit that is substantially higher than any previously reported value for a bulk material, making them attractive for damping applications in nanoscale and microscale devices.
The basic mechanism by which shape memory alloys (SMAs) dissipate mechanical energy is through a reversible transformation between a high-temperature phase called austenite and a low-temperature phase called martensite. This transformation, which occurs by means of a rapid shearing of the atomic lattice, can also be induced at constant temperature in the austenite phase by applying a stress to promote the transformation. When the external stress exceeds a particular critical value, martensite nucleates within the austenite to create internal interfaces that then move through the material, driven by the applied stress. If the stress is removed, the material can completely recover its original shape, without any residual deformation, by reverting to austenite. This behaviour is referred to as superelasticity. During superelastic deformation, the internal interfaces between the phases dissipate a large fraction of the available mechanical energy during their formation and motion, giving superelastic materials desirable mechanical damping properties.
Juan and his team successfully demonstrated that the reversible stress-induced martensitic transformation do indeed exhibit a size effect. This effect leads to ultra high damping in micro- and sub- micro metre structures of Cu–Al–Ni SMAs.
The size effect leads to an excellent ultra high damping performance and response times on the order of just a few milliseconds, micro- and nanoscale structures made from Cu–Al–Ni SMAs could offer a practical solution for damping vibrations at the nanoscale, and may pave the way for the development of more precise and reliable MEMS devices.