Formation of a hierarchical martensitic microstructure

The martensitic microstructure is decisive for most functionalities of shape memory alloys, including pseudoelastic, pseudoplastic, magnetic shape memory and caloric effects. Here we examine the formation of a hierarchical microstructure by a combination of high resolution ex-situ and in-situ experiments and theoretical models. As model system we select epitaxial Ni-Mn-Ga films as the high fraction of surface to volume makes surface experiments representative. Moreover, the high aspect ratio of a film minimizes mechanical interactions between different regions, making films a statistically relevant ensemble of many nucleation events.

The need to form phase boundaries requires introducing nanotwin boundaries. Interaction energy results in an ordered arrangement, appearing as a modulated phase and forming a-b twin boundaries. Nucleation and growth within the volume requires mesoscopic type I and II twin boundaries. As a consequence of different nucleation sites also macroscopic twin boundaries form. To conclude, the resulting hierarchical microstructure is not result of a global minimization of energy but determined by the most easy transformation path, ending in a metastable configuration containing a well-defined arrangement of many different types of twin boundaries.

This work is supported by DFG through SPP 1599 www.FerroicCooling.de.