Twins resemble each other and are usually described in materials by a reflection at a twin boundary. Crystalline twins occur in many novel functional materials. For example, the fact that thermomagnetic devices can convert waste heat into electricity or that magnetocaloric devices can cool other systems is because of the formation of crystal twins in many materials. Twinning is also crucial for the hardening of martensitic steels and for the shape memory effect. Hence, the underlying principle of twinning was already studied extensively 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 build a nested structure: tiny twins on the atomic scale give rise to areas that can be identified as higher-order twins, which in turn are part of an even larger twin. This hierarchical "twins-within-twins" structure contains twin boundaries on all length scales: from the atomic to the macroscopic scale.
Although twin formation has already been observed on every single length scale, no comprehensive approach to why and how the twins are nested has yet been developed. Scientists at IFW Dresden, together with colleagues in Prague, have set themselves the task of solving the puzzle 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 jigsaw puzzle" tells one of the co-authors. "First you play with atomic building blocks and then you realize that you can put them together to create a larger building block. Thus, we went on puzzling and the next bigger building block could be put together as well combining the smaller ones from before. We repeated this process until – after five steps – everything fit together.” Once you have understood the construction principle, you no longer see just criss-crossing lines on microscopic images, but can assign different twin boundaries to these lines. "Because of 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 is done simply by twinning repeatedly." Important for science, however, are not the beautiful pictures created by the Ni-Mn-Ga alloy that has been studied as an exemplary system. The novelty is rather that there now is a tool to describe the microstructure of the functional materials, that were introduced at the beginning, from the atomic to the macroscopic length scale with a single parameter. This opens up the possibility to influence these materials even more specifically in order to further improve their functional properties.
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
Dr. Sebastian Fähler
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