Spinodal decomposition for high-entropy soft magnetic alloys design

In this project, we work on a generic solution to design advanced high-entropy alloys (HEAs) with enhanced magnetic properties. By overturning the concept of stabilizing solid solutions in HEAs, we propose to render the massive solid solutions metastable and trigger spinodal decomposition. The motivation for starting from the HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behaviour using multiple components.

Magnetic properties of the Fe15Co15Ni20Mn20Cu30 HEA samples in different processing conditions. a, b) Temperature dependence of magnetization of the homogenized a) and 600 oC / 240 h annealed b) HEA samples after zero-field cooling in applied magnetic fields of 50-0.05 T and after a 0.5 T field cooling in an applied magnetic field of 0.01 T. c) Hysteresis loops investigated up to 2 T of the HEA with different annealing time at 300 K. d) Experimental Curie temperatures as a function of annealing time. The morphological evolution of the alloy’s nanostructure as a function of annealing time is shown in terms of APT reconstructions of volume portions with dimensions of 40×40×200 nm3. The APT reconstructions also show 50 at. % iso-composition surfaces of Cu. e) DFT calculated Curie temperatures as a function of annealing time. The blue shaded area indicates the impact of strain induced volume dilatation on the Curie temperature, i.e., the difference between the Curie temperature calculated for a hypothetical free-standing unconstrained single-phase bulk-like Fe-Co alloy and that for the experimentally measured strained volume of the same region in the HEA. For the 6 h and 24 h annealed samples, light blue squares and spheres indicate the Curie temperature values calculated for compositions corresponding to three different APT tips while the final value is the arithmetic mean over the three compositions. The oval shapes for these two annealing times mark the fluctuations in the Curie temperatures that occur due to the scatter in local composition among the different APT specimens. — **Magnetic properties of the Fe₁₅Co₁₅Ni₂₀Mn₂₀Cu₃₀ HEA samples in different processing conditions.** a, b) Temperature dependence of magnetization of the homogenized a) and 600 ^oC / 240 h annealed b) HEA samples after zero-field cooling in applied magnetic fields of 50-0.05 T and after a 0.5 T field cooling in an applied magnetic field of 0.01 T. c) Hysteresis loops investigated up to 2 T of the HEA with different annealing time at 300 K. d) Experimental Curie temperatures as a function of annealing time. The morphological evolution of the alloy’s nanostructure as a function of annealing time is shown in terms of APT reconstructions of volume portions with dimensions of 40×40×200 nm³. The APT reconstructions also show 50 at. % iso-composition surfaces of Cu. e) DFT calculated Curie temperatures as a function of annealing time. The blue shaded area indicates the impact of strain induced volume dilatation on the Curie temperature, i.e., the difference between the Curie temperature calculated for a hypothetical free-standing unconstrained single-phase bulk-like Fe-Co alloy and that for the experimentally measured strained volume of the same region in the HEA. For the 6 h and 24 h annealed samples, light blue squares and spheres indicate the Curie temperature values calculated for compositions corresponding to three different APT tips while the final value is the arithmetic mean over the three compositions. The oval shapes for these two annealing times mark the fluctuations in the Curie temperatures that occur due to the scatter in local composition among the different APT specimens.

© Max-Planck-Institut für Eisenforschung GmbH

**Magnetic properties of the Fe₁₅Co₁₅Ni₂₀Mn₂₀Cu₃₀ HEA samples in different processing conditions.** a, b) Temperature dependence of magnetization of the homogenized a) and 600 ^oC / 240 h annealed b) HEA samples after zero-field cooling in applied magnetic fields of 50-0.05 T and after a 0.5 T field cooling in an applied magnetic field of 0.01 T. c) Hysteresis loops investigated up to 2 T of the HEA with different annealing time at 300 K. d) Experimental Curie temperatures as a function of annealing time. The morphological evolution of the alloy’s nanostructure as a function of annealing time is shown in terms of APT reconstructions of volume portions with dimensions of 40×40×200 nm³. The APT reconstructions also show 50 at. % iso-composition surfaces of Cu. e) DFT calculated Curie temperatures as a function of annealing time. The blue shaded area indicates the impact of strain induced volume dilatation on the Curie temperature, i.e., the difference between the Curie temperature calculated for a hypothetical free-standing unconstrained single-phase bulk-like Fe-Co alloy and that for the experimentally measured strained volume of the same region in the HEA. For the 6 h and 24 h annealed samples, light blue squares and spheres indicate the Curie temperature values calculated for compositions corresponding to three different APT tips while the final value is the arithmetic mean over the three compositions. The oval shapes for these two annealing times mark the fluctuations in the Curie temperatures that occur due to the scatter in local composition among the different APT specimens.

© Max-Planck-Institut für Eisenforschung GmbH

Since its first emergence in 2004, the HEA concept has aimed at stabilizing single- or dual-phase multi-element solid solutions through high mixing entropy. Here, we change this strategy and render such massive solid solutions metastable, to trigger spinodal decomposition for improving the alloys’ magnetic properties. The motivation for starting from a HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behavior using multiple components. The key idea is to form Fe-Co enriched regions which have an expanded volume (relative to unconstrained Fe-Co), due to coherency constraints imposed by the surrounding HEA matrix. As demonstrated by theory and experiments, this leads to improved magnetic properties of the decomposed alloy relative to the original solid solution matrix. In a prototype magnetic FeCoNiMnCu HEA, we show that the modulated structures, achieved by spinodal decomposition, lead to an increase of the Curie temperature by 48% and a simultaneous increase of magnetization by 70% at ambient temperature as compared to the homogenized single-phase reference alloy.

Spinodal decomposition for high-entropy soft magnetic alloys design

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