Thermochemical Modeling-Assisted Synthesis of AlxCoCrFeNiMn (0.5 ≤ x ≤ 3) High-Entropy Alloys via Combustion Method for Soft Magnetic Applications

This study reports the synthesis of Al_xCoCrFeNiMn (0.5 ≤ x ≤ 3) alloys via fast, cost and energy-efficient aluminothermic self-propagating high-temperature synthesis (SHS) method. Starting oxides (Co₃O₄, Cr₂O₃, Fe₂O₃, NiO, MnO₂, and Al₂O₃) and reducing agent aluminum (Al) amounts were calculated via thermochemical simulations extensively (FactSage™) together with the adiabatic temperature and gaseous phase compositions. The characterization results demonstrated that Al_xCoCrFeNiMn (0.5 ≤ x ≤ 3) master alloys can be successfully synthesized via thermochemical modeling-assisted SHS method with a substantial composition control by using the optimum amount of Al₂O₃ as a heat suppressant. It was found that when the heat suppressant was higher, adiabatic temperature decreases extensively and large amount of spinel MnAl₂O₄ can form and remain unreacted which results in lower Mn in alloy. Conversely, most of the Mn is lost to evaporation when the heat suppressant is not used. Under optimum synthesis conditions, as Al content increases, the phase structure of the CoCrFeNiMn alloy transforms to BCC (A2, B2) from FCC (A1). The hardness increased substantially above x > 1 in the Al_xCoCrFeNiMn system to the level of 668 HV. Ordered BCC-B2 content is responsible for the hardness increase as well as the reduction in saturation magnetization value (23 emu/g, 147.2 kA/m) for the Al1.5CoCrFeNiMn alloy (6.40 g/cm³). The highest saturation magnetization was found to be 83.7 emu/g (552.4 kA/m) with an observed density of 6.63 g/cm³ and moderate Hc = 43 Oe (3421 A/m) for the AlCoCrFeNiMn alloy. This study shows that Mn containing soft magnetic Al_xCoCrFeNiMn (0.5 ≤ x ≤ 3) high-entropy alloys can be synthesized via thermochemical simulation coupled SHS, cost and time efficient method that can exhibit properties for future next-generation low density requiring applications. The removal of Cr from this system was found to be necessary to further elevate the soft magnetic properties and decrease the overall cost and density because it tends to segregate near the grain boundaries.