The martensitic microstructure is decisive for most functionalities of shape memory alloys, including pseudoelastic, pseudoplastic, magnetic shape memory and caloric effects. This microstructure spans several orders of length scale, ranging from atomic- to macro-scale. The twin-within-twins microstructure often appears like modern art and some beautiful micrographs will be shown for the model system Ni-Mn-Ga. Despite its complexity, there are relative simple scientific concepts, which guide the formation of this hierarchical microstructure at each length scale. We will sketch the theoretical models used and present high-resolution in-situ experiments, which confirm our concepts. In addition, we will illustrate the impact of several features of this hierarchical microstructure on the key functional properties of shape memory alloys.
Though this talk will follow the classical path from atomic- towards macroscale, at several decisive points the mesoscale determines the atomic scale. These examples illustrate that it is sometimes difficult, but worth, to distinguish between structure and microstructure.
This work is supported by DFG through SPP 1599 www.FerroicCooling.de.