Nanostructured shape memory alloys

Nanostructured shape memory alloys

Reversible shape changes of ferroelastic materials showing martensitic phase transformations can be controlled by temperature, stress or magnetic fields. Grain size at a nanoscale can strongly impact the martensitic phase transformation and therefore the shape memory effect and superelasticity. Nanocrystalline and ultrafine grained shape memory materials including NiTi alloys, low-hysteresis NiTiPd alloys, and ferromagnetic high-temperature NiMnGa alloys are processed by methods SPD. The lattice structures of the martensitic phases, self-accommodated martensitic domains, transformation temperatures, as well as the enthalpy and entropy changes upon transformation are studied by systematic experiments. These results are analysed using thermodynamic models considering the total Gibbs free energy of ferroelastic domains confined to small grains.