Origin of hard magnetism in Fe-Co-Ni-Al-Ti-Cu high-entropy alloy: Chemical shape anisotropy

Some non-equiatomic magnetic high-entropy alloys (HEAs), e.g. Fe ₂CoNiAlCu _0.4Ti _0.4 (at.%), have recently been found to yield appealing hard magnetic properties even at as-cast state, showing great potential in developing advanced hard magnets. However, the underlying origin remains unclear, restricting the effective manipulation towards better magnetic performance. By detailed microstructural and chemical investigations, in this work, it was found that the as-cast Fe ₂CoNiAlCu _0.4Ti _0.4 (at.%) alloy has a hierarchically heterogeneous microstructure, where the nano-arrays enriched or depleted with ferromagnetic element Fe exhibit large aspect ratio (defined as chemical shape anisotropy, CSA), penetrating through the cellular nanostructure consisting of A2 (disordered body-centered cubic, bcc) and L2 ₁ (ordered bcc) phases. The aspect ratio and elemental concentration of the nano-arrays can be further manipulated by magnetic annealing (MA), significantly enhancing the coercivity. These results suggest that the nanoscale CSA is responsible for the hard magnetism. Further study showed that the CSA is not strengthened continuously with MA time because over-annealing creates interfacial precipitations of hexagonal ω and face-centered-cubic (fcc) Cu-rich nanoparticles, deteriorating both coercivity and saturation magnetization. As a result, a ∼49% enhancement in coercivity can be achieved by balancing the strengthened CSA and the detrimental interfacial precipitates. Consequently, this study not only uncovers that the nanoscale CSA accounts for the hard magnetism of Fe-Co-Ni-Al-Ti-Cu HEAs, but also provides a feasible approach to manipulate CSA towards better magnetic performance.