Phase separation at different length scales in Galfenol related alloys

• Applicant: Professor Dr. Andreas Leineweber
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 465128335

Global bulk-Gibbs energy minimization yields the “true” equilibrium state at constant pressure and temperature. In any multi-component system this can lead to multiphase equilibria with several, mutually incoherent phases. Phase transformations, however, frequently lead to states of incomplete phase separation. The thermodynamics of such states is typically complicated by microstructural contributions to the overall Gibbs energy, like elastic energies or interface energies. Such states are quite relevant in many structural and functional materials. The proposers have selected binary Fe-base alloys with Al, Ga and Ge as these alloys show different types of heterogeneous states which evolve from body-centered cubic (BCC) solid solution or BCC base ordered intermetallic. These heterogeneities develop due to two types of instabilities shown by many BCC base phases, namely the Bain type elastic instability and the omega-type dynamic instability. The application background of this alloy system is the occurrence of pronounced magnetostriction in particular in Fe-Ga alloys (Galfenols). Structural heterogeneities are made responsible for the large magnetostrictive effects but also for their disappearance at higher Ga contents. The proposed project is, however, not optimization of the magnetostrictive properties in the alloy system by varying composition and heat treatment. Instead by using a multitude of complementary analysis techniques the competing types of decomposition paths occurring in the alloys will be evaluated. Using well-selected heat treatment procedures on the alloys, the analysis techniques are laboratory, neutron and synchrotron diffraction techniques, neutron small-angle scattering but also electron microscopic techniques revealing the spatial arrangement of the phases and compositional heterogeneities. Magnetometry and anelasticity measurements may be added if adequate. Thereby, various types of heterogeneous states as a result of alloy decomposition are expected to develop with varying composition and heat treatment. The effect of these heterogeneities on the magnetostrictive behavior will be evaluated, and the acquired knowledge may then be used by scientific community to apply more educated approaches to optimize magnetostrictive behavior of Galfenols.The project combines the complementary expertise of scientists from Germany and Russia concerned with structure and microstructure research of alloys.

• DFG Programme: Research Grants
• International Connection: Russia
• Partner Organisation: Russian Science Foundation, until 3/2022
• Cooperation Partner: Professor Dr. Anatoly Balagurov, until 3/2022