Correlation between microstructure and soft magnetic parameters of Fe-Co-Ni-Al medium-entropy alloys with FCC phase and BCC phase

Abstract The magnetic property and microstructure of as-cast FeCoNiAlx (x = 0.2, 0.4, 0.6, 0.8, 1.0) MEAs prepared by copper mold suction were investigated. For x ≤ 0.6 alloys, the soft magnetic properties monotonously decrease with increasing Al content, while a turning point of improving magnetic property occurs at x = 0.8. XRD result shows that Al addition changes the phase constitution. The alloys (x ≤ 0.4) consist of single FCC phase. FeCoNiAl0.6 (x = 0.6) and FeCoNiAl0.8 (x = 0.8) alloys are composed of FCC phase and BCC phase. FeCoNiAl1.0 (x = 1.0) alloy consists of single BCC phase. Furthermore, more serious lattice distortion is present in FCC phase than that in BCC phase. It results in the deterioration of soft magnetic property by increasing the coercivity (Hc) and the hysteresis loss (Pu), and decreasing the initial permeability (μi) and the maximum permeability (μm). The grain-boundary character distribution of FeCoNiAl0.6 (x = 0.6) alloy and FeCoNiAl0.8 (x = 0.8) alloy was studied by electron backscatter diffraction (EBSD). Less strain concentration is observed in x = 0.8 alloy than that in x = 0.6 alloy because of containing much smaller number of low angle boundaries (LAB) with a misorientation angle between 2° and 5° (5.71%) than 36.6% in x = 0.6 alloy. It leads to an improvement of soft magnetic property by decreasing the Hc, the Pu, and increasing the μi and the μm of x = 0.8 alloy. Energy dispersive spectroscopy (EDS) result shows that, in both FeCoNiAl0.6 (x = 0.6) and FeCoNiAl0.8 (x = 0.8) alloys, the enrichment of Fe and Co elements and the depletion of Ni and Al elements are observed in FCC phase, while Fe and Co elements are depleted and Ni and Al elements are enriched in BCC phase. Moreover, a close value of ferromagnetic elements, (Fe + Co + Ni)%, is obtained in FCC phase (≈85%) and BCC phase (≈78%) in both FeCoNiAl0.6 (x = 0.6) and FeCoNiAl0.8 (x = 0.8) alloys. Combined with results of EDS and magnetic properties of x = 0.6 and x = 0.8 alloys, it can conclude that the saturation magnetization (Ms) of BCC phase is larger than that of FCC phase.