Thermodynamic Investigation of Fe–Mn–Al–Ni–Ti Shape Memory Alloys

• Applicant: Dr.-Ing. Mario J. Kriegel
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2016 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 314279646

Fe–Mn based shape memory alloys (SMAs) have attracted considerable attention in recent years due to two unique features: an unusual transformation between a bcc-type parent phase (bcc, Strukturbericht designation A2) and a martensitic fcc-type phase (fcc, A1), and a small temperature dependence of the superelastic behavior due to an extremely low slope for the Clausius-Clapeyron relationship (0.53 MPa/°C). A thermoelastic shape memory behavior of this alloy can be ensured only by formation of ordered, coherent and nano-sized precipitates (B2) inside the parent matrix. These precipitates play a key role because their chemical composition, size and spatial distribution strongly affects the shape memory properties of this alloy. Therefore, the microstructure of the alloy needs to be adjusted in a target-oriented fashion to further improve the fracture behavior of Fe–Mn–Al–Ni SMAs. The alloying of small amounts of Ti (1.5 at.%) reduces the quenching sensitivity and accelerates grain growth, which shows the tremendous potential of the Fe–Mn–Al–Ni–Ti-alloy system. However, the effect of Ti additions to the coherent nano-sized β precipitates is not understood yet. The aim of the proposed research activities is to provide a profound thermodynamic database and thermodynamic models, which are based on Gibbs energy descriptions of the phases to systematically study Fe-based shape memory alloys. The understanding of the correlation between the thermodynamics in the quinary Fe–Mn–Al–Ni–Ti system and the shape memory properties is the central part of the proposed project. Moreover, information on the various phase transformations (microstructural changes) and their contribution to the shape memory effect needs to be known. Therefore, the methods of computational thermodynamics based on the CalPhaD approach will be combined with microstructural investigations as well as thermal analysis methods to understand the contribution of the respective phase stabilities to the performance of these materials. The proposed investigations will contribute to the basic knowledge regarding the role of Gibbs energy contributions of the phases to the superelasticity of Fe–Mn based SMAs and will help to understand the unique features of this materials system. The obtained results will be important not only from the point of view of the basic research, but will also allow to tailor the functional and mechanical properties via targeted alloy design.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr.-Ing. Thomas Niendorf