Polygon Empowered Stress-Control (POEMS): A new concept to produce selectively graded shape memory alloys

• Applicant: Professor Dr.-Ing. Hans Jürgen Maier
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Synthesis and Properties of Functional Materials
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 561222279

In single-crystal form, shape memory alloys can exhibit reversible pseudoelastic or pseudoplastic strains of up to 26.5%. However, commercially used polycrystalline alloys typically achieve strains of only up to 8%. Moreover, during cyclic loading, there is often a rapid degradation of the functional properties. This proposed project aims to specifically target this so-called functional fatigue, which currently limits the use of shape memory alloys to niche applications, by employing an entirely new production engineering approach, thus achieving significantly enhanced fatigue resistance. Three approaches have been explored in the literature to overcome functional fatigue: adjusting the chemical composition, grain boundary engineering, and targeted stress-induced ageing in the martensitic phase. The previously pursued approaches, which are associated with significant disadvantages such as a shift in phase transformation temperature, reduction of usable transformation strain, all require development of new materials. By contrast, the proposed project intends to explore a production engineering approach, which is applicable to already existing shape memory alloys. The approach is based on the assumption that by deliberately introducing polygonal structures, it is possible to generate local stress fields that actively influence the selection of nucleation points during the martensitic phase transformation, thus keeping the habit planes cycle-stable in place. Preliminary trials have shown that the phase transformation can be selectively influenced through introduced nanoscale polygonal structures and that these transformations are reversible on the microstructural level. Systematic fundamental research is needed to shed light on the mechanisms that govern the microstructual processes and to develop methods that enable scaling to technically relevant production processes. The proposed venture also enters new territory in this regard. The vision is that this new approach can lead to a paradigm shift in the development of functionally stable and locally graded shape memory alloys.

• DFG Programme: Reinhart Koselleck Projects