Synthesis Of High-Entropy-Alloy-Based Magnetic Alloys By Mechanical Alloying

Abstract

The objective of this thesis work was to synthesize high-entropy based magnetic alloys and investigate the phase evolution during the synthesis and thermal treatments. A thorough literature review was performed on magnetic high-entropy alloys. The ternary iron-cobalt-nickel alloys (Fe33.33Co33.33Ni33.33, Ni40Co30Fe30, Fe40Co30Ni30, Co40Fe30Ni30, and Fe46Co34 Ni20) and the quaternary iron-cobalt-nickel-silicon alloys (Fe40Co30Ni30)0.9Si0.1 were synthesized by mechanical alloying, followed by structural characterization of phase evolution by x-ray diffraction (XRD), scanning electron microscopy (SEM) and magnetic characterization by magnetometry. The Fe33.33Co33.33Ni33.33 alloy was formed after mechanical alloying for ~9 hours. The lattice parameter of the face-centered-cubic (f.c.c) alloy was ~0.3591 nm and the crystallite size was ~12 nm. The SEM revealed particle size (D90) ~15.9 µm. The alloy exhibited decent magnetic properties having saturation magnetization (MS) of ~136 ± 3 Am2/kg and coercivity (HC) of ~2.4 kA/m at room temperature. After the thermal treatment, Ms increased by ~14% and Hc decreased by 60%. Among the Ni40Co30Fe30, Fe40Co30Ni30, and Co40Fe30Ni30 alloys, the Fe-rich alloy mechanically alloyed for 12 hours showed superior magnetic properties. The Ms was ~148 ± 3 Am2/kg and the Hc was ~4.3 kA/m. The lattice parameter, crystallite size, and D90 was ~0.3574 nm, ~8 nm, and ~5.5 µm, respectively. Thermal treatment of the Fe-rich alloy improved its magnetic properties. It increased Ms by 10% and decreased Hc by 25%. The Fe46Co34 Ni20 showed the best magnetic properties among all, the Ms was ~167 ± 2 Am2/kg and the Hc was ~3.3 kA/m. Lastly, in the case of (Fe40Co30Ni30)0.9Si0.1 alloy, the Ms decreased by ~10% and the Hc increased by ~40% compared to Fe40Co30Ni30 alloy. A systematic study of magnetic properties of the iron-cobalt-nickel alloys is likely to provide the necessary foundation for the development of high-entropy based magnetic alloys by further alloying additions.