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twins are similar to each other and are usually described in materials by a reflection at a twin boundary. Crystalline twins occur in many novel functional materials. The fact that, for example, thermomagnetic components can convert waste heat into electricity or magnetocaloric components can cool systems is due to the formation of crystal twins in many materials. Twin formation is also decisive for the hardening of martensitic steels and for the shape memory effect. The underlying principle of twinning is therefore well researched and is part of the basic knowledge of materials science. But the closer you look, the more questions arise. For example, twins do not form randomly and homogeneously in the material, but rather form a nested structure: tiny twins on an atomic scale form areas that form a superior twin, which in turn is part of an even larger twin. This hierarchical "twin-in-twin structure" contains twin boundaries on all length scales: from the atomic to the macroscopic range.

Although twinning has been observed on every single length scale, there has not been a comprehensive approach to why and how twins are nested. Scientists at IFW Dresden, together with colleagues in Prague, have set themselves the task of solving the mystery of hierarchical twins. In the current issue of the journal "Advanced Functional Materials" they report how the formation of hierarchical twin structures can be explained across all length scales. "It was like a puzzle" tells one of the co-authors. "First you play with atomic building blocks and then you realize that you can put together a larger building block from them. Then we went on puzzling and the next bigger building block could be put together from the smaller ones. We repeated the whole thing until after five steps everything fit together. Once you have understood the construction principle, you no longer just see many criss-crossing lines on microscopic images, but can assign different twin borders to these lines. "Through the nested puzzle you get images that are very reminiscent of the Dutch artist Mondrian. To paint them, nature needs only one parameter, the lattice constant of the atomic building blocks. The rest results from the repeated twisting of the grid by itself. "What is important for science, however, is not the beautiful pictures of the Ni-Mn-Ga alloy that has been studied as an example, but the fact that for the first time it is possible to describe the microstructure of the functional materials described at the beginning from the atomic to the macroscopic length scale with a single parameter. This will make it possible to influence these materials even more specifically in the future in order to further improve their functional properties.

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S. Schwabe, R. Niemann, A. Backen, D. Wolf, C. Damm, T. Walter, H. Seiner, O. Heczko, K. Nielsch, S. Fähler, Building Hierarchical Martensite, Adv. Funct. Mater. 2020, 202005715;  doi.org/10.1002/adfm.202005715