Processing-driven chemical ordering and its effect on magnetic properties in a high entropy alloy

The influence of chemical ordering and its length scale on the magnetic behaviour of the Al_0.2Ti_0.3Co_1.5CrFeNi_1.5 high entropy alloy (HEA) was investigated across three distinct microstructural conditions, including a solution annealed state, an annealed condition at 750 °C, and a cold rolled plus annealed condition. These processing routes produced changes in the volume fraction, size, and morphology of L1₂ ordered precipitates along with the formation of a minor L2₁ phase in the annealed states. Magnetic measurements performed between 2 K and 300 K revealed clear differences in saturation magnetization, coercivity, and magnetic transition temperatures across the three conditions. The solution annealed condition exhibited a single magnetic transition near 48 K and low coercivity, consistent with a fine dispersion of coherent L1₂ precipitates. In the annealed and cold rolled conditions, coarsening and morphological evolution of the L1₂ phase led to the emergence of a second magnetic transition at lower temperatures, attributed to partial magnetic decoupling of the precipitates. Coercivity increased significantly in these conditions due to enhanced domain wall pinning, while residual hysteresis at 300 K is attributed to a combination of minor L2₁ phase at grain boundaries and microstructural features in the cold-worked condition. These results demonstrate that chemical ordering and its structural evolution play a central role in governing low temperature magnetic behaviour in this alloy system. The findings contribute to the broader understanding of multifunctional HEAs that combine tunable magnetic properties with excellent mechanical performance.