Rare-earth based alloys for hard-magnetic applications: Temperature and pressure dependent phase stabilities
Synthesis and Properties of Functional Materials
Final Report Abstract
Hard-magnetic materials play a central role in current European efforts to develop sustainable energy concepts. The current trend goes towards systems with a high magnetic energy density to increase efficiency and to reduce size, weight and production costs of devices. For this Nd2Fe14B is currently the system of choice, but the high price for the involved rare-earth elements such as Dy or Tb, but also Nd, calls for alternative material systems. Ce is a rather abundant rare-earth element, which when used in compounds could lead to rare earth balance magnets. Ce-based hard-magnetic materials are, however, difficult to produce, since the magnetically interesting ternary phases are often competing with binary phases such as CeFe2 in the resulting microstructures. A main goal of the RE-MAP research project was therefore to achieve a fundamental understanding for the physical reasons behind the thermodynamic stability of RE-based alloys, with a focus on CeFe11Ti, and to develop from there efficient routes to synthesize Ce-containing intermetallics. Using a variety of theoretical and experimental methods, we were aiming at phase diagrams for these materials. Three degrees of freedom had been in the focus of the investigations: (i) the temperature, is a key control parameter in the production process, but also for many applications; (ii) the pressure can have a strong impact on meta-stabilities as well as magnetic properties, and (iii) the chemical composition can be varied such that physical trends for phase stabilities become more apparent and that intrinsic and extrinsic magnetic properties are further optimized. To this end, density-functional theory-based techniques have been used at Max-Planck-Institut für Eisenforschung to derive free energies and all relevant entropy contributions from first principles. The theoretical challenge of strong electronic correlations has been addressed by Ecole Polytechnique with a particular focus of chemical trends using dynamic mean-field theory (DMFT). Advanced synthesis and comprehensive characterization have been performed at the Technische Universität Darmstadt. Within this joint approach we have achieved the desired complete understanding of the phase stability in the Ce-Fe-Ti ternary system and the impact of temperature, pressure and chemistry. Out of these three degrees of freedom, the pressure dependence turned out to have the least decisive impact. The observed modifications of ab initio phonon spectra, for example, turned out to be too small to be observable in neutron scattering experiments. Hydrostatic pressure experiments for ternary Ce1+xFe11Ti samples have also resulted in a small reduction of the magneto‐crystalline anisotropy field. The focus of the project was, therefore, more on the temperature dependence. To this end, the free energies of the unary element Ce and a large set of binary and ternary intermetallics in the Ce-Fe-Ti system have been systematically determined. In order to assess the quality of the derived free energies a comparison with experimental data has been performed. The ab initio calculations predict that the desired hard magnetic phase of the 1:12-type Ce(Fe, Ti)12 is only be stable if the Ti content is approximately equal to the Ce content and the annealing temperature is above 700 K. These predictions turned out to be in overall perfect agreement with experimental phase diagrams obtained from the high-throughput synthesis method [reactive crucible melting (RCM)]. The RCM yields additional information about solubility ranges in the different phases. Based on this success, we have further explored the chemical degree of freedom X on the stability of these phases in the Ce‐Fe‐Ti system by additions of 3d and 4d‐elements. Starting with the additions Ga and Cu, various decomposition scenarios have been considered, including the coexistence of the 1:12 phase and the competing Laves phases. We have then combined the highly accurate free energy calculations with an efficient screening technique to determine the critical annealing temperature for the formation of Ce(Fe,X)11Ti. According to these calculations promising candidates to promote the stability of the hard‐magnetic phase have been identified. The comparison with suction casting and RCM experiments for Ce‐Fe‐Ti‐X (X= Cu, Ga, Co and Cr) has highlighted the relevance of additional quaternary and binary phases, the impact of which has subsequently been analyzed theoretically. We are convinced that the developed theoretical and experimental analysis of complex ternary alloys performed in this project, will have an impact on many other material systems, where the thermodynamic phase stability is critical for using promising materials in applications.
Publications
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Ab initio phase stabilities of Ce-based hard magnetic materials (Doctoral thesis, Ruhr University Bochum, 2019
H. Sözen
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Ab initio phase stabilities of Ce-based hard magnetic materials and comparison with experimental phase diagrams, Phys. Rev. Materials 3, (2019) 084407
H. İ. Sözen, S. Ener, F. Maccari, K. P. Skokov, O. Gutfleisch, F. Körmann, J. Neugebauer, and T. Hickel
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Permanent Magnets – Research for Sustainability, MPIE Newsletter, 2/2019
H. Sözen, T. Hickel
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Impact of magnetism on the phase stability of rare-earth based hard magnetic materials, Calphad 68, (2020) 101731 (Best Paper Award of the CALPHAD journal in the year 2020)
H. İ. Sözen, T. Hickel, J. Neugebauer
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Correlating changes of the unit cell parameters and microstructure with magnetic properties in the CeFe11Ti compound, J. Alloys Compd. 867 (2021) 158805
F. Maccari, S. Ener, D. Koch, I. Dirba, K. P. Skokov, E. Bruder, L. Schäfer, O. Gutfleisch
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Intrinsically weak magnetic anisotropy of cerium in potential hard-magnetic intermetallics, npj Quantum Materials 6, (2021), 2
A. Galler, S. Ener, F. Maccari, I. Dirba, K. P. Skokov, O. Gutfleisch, S. Biermann and L. V. Pourovskii
