Abstract text (incl. figure legends and references)

High entropy alloys (HEAs) or complex concentrated alloys (CCAs) are a novel class of material that offers remarkable structural and functional performance in a wide range of applications. Their properties can be tuned by using different combinations of elements and/or processing strategies. Although their mechanical properties have been widely studied, fewer investigations have been performed about their magnetic properties. Here, we perform correlative characterization of the microstructure, chemical composition and magnetic properties of Al0.3CoFeNi and AlCo(Cr)FeNi alloys using transmission electron microscopy (TEM) methods. Off-axis electron holography and the Fresnel mode of Lorentz TEM are used to determine the magnetic states of precipitates, the coercivity mechanism and the widths of magnetic domain walls in the alloy.

A plate-shaped arc-melted ingot of Al0.3CoFeNi was homogenized at 1200 ºC for 1 h and cold-rolled to 90%. It was then either solutionized at 1200 ºC for 5 min or solutionized and annealed at 600 ºC for 50 h . Electron-transparent TEM specimens were prepared using focused ion beam sputtering in a dual beam scanning electron microscope (FEI Helios 400). Microstructural, chemical and magnetic imaging was carried out using probe-corrected and image-corrected TEMs (FEI Titan 80-200 and FEI Titan 60-300 operated at 200 and 300 kV, respectively).

The Al0.3CoFeNi alloy consists of L12-ordered precipitates embedded in an FCC single crystal. Extended annealing at 600 ºC induced lamellar phase separation consisting of FCC+L12 and BCC+B2 regions. High-resolution in situ investigations showed that the magnetization reversal process changes from a nucleation-type mechanism in the FCC+L12 microstructure to a pinning-type mechanism in the lamellar microstructure, with a significant decrease in domain wall width from 171 to 35 nm. At the same time, the magnetic coercivity increased from 2.5 to 160 Oe, while the yield strength and hardness increased by a factor of 2.

The AlCo(Cr)FeNi alloy consists of highly complex hierarchically-decomposed B2 and BCC phases with different dimensions and arrangements [1]. Quantitative measurements of the magnetic phase shift using off-axis electron holography (Fig. 1(a)) and reconstruction of the in-plane magnetization using model-based iterative reconstruction (Fig. 1(b)) were used to reveal the distinct magnetic characteristics of the constituent states, which include magnetic vortices (Fig. 1(c)) and single domains with paramagnetic inclusions [1].

The authors are grateful to N. Balaji, R. Banerjee, J. Caron, V. Chaudhary, S. Dasari, H. Du, R. V. Ramanujan and D. Song for valuable contributions to these results. This work was supported by the European Union"s Horizon 2020 Research and Innovation Programme (Grant No. 856538, project ""3D MAGiC"") and the DFG through CRC/TRR 270 (Project ID 405553726).

Figure 1. (a) Magnetic phase image of the BCC phase in the B2 matrix of an AlCo(Cr)FeNi CCA [1]. Each dark or bright contrast feature is associated with a well-defined magnetic vortex state. (b) Projected in-plane magnetization map of the marked precipitate in (a) generated from the magnetic phase image. (c) Magnetic induction map showing vortices. The contour spacing is 2π/24 radians.

[1] Q. Lan et al., iScience 25 (2022) 104047.

[2] A. Kovács, R.E. Dunin-Borkowski, In Handbook of Magnetic Materials; Brück, E., Ed.; Elsevier: 2018; Vol. 27, 59.