EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

High-entropy alloys (HEAs) are a new class of materials exhibiting a superior combination of magnetic, mechanical and electrical properties. FeNiCoAlMn is a promising candidate for a soft magnet, important for applications in electrical systems such as power generation and electromagnets. However, the high entropy effect is partially true, since other thermodynamic properties can also play a role in phase stability, making the alloys vulnerable to phase separation that could affect their magnetic behavior. Here, we report on the first nanoscale investigation of FeNiCo(AlMn)x processed by Laser Metal Deposition, by applying (Scanning) Transmission Electron Microscopy ((S)TEM) combined with Energy-Dispersive X-ray spectroscopy (EDX) and Electron Energy-Loss Spectroscopy (EELS). Experiments were performed on an FEI Titan G2 60-300 kV equipped with a CEOS DCOR probe-corrector, monochromator and Super-X EDX detectors.
A gradual change of the FCC phase towards BCC by increasing x was observed. A thorough analysis employing STEM-EELS revealed a coexistence of L21 and B2 ordered structures in the BCC phase, and ordered nano-percipitates were detected in the FCC phase. Low-loss EELS revealed a plasmon-peak splitting in FCC indicative of two valence electron densities. This finding in combination with intense Moiré fringes and high-defect density indicate that FCC exhibits intense phase separation tendency. However, ordering and phase separation trends did not have a severe impact on the magnetic properties. All samples exhibited good soft-magnetic behavior since they were easily magnetized to the saturated state with coercivity values of <1000 A/m. This finding indicates that the magnetic properties of ordered B2 or Heusler alloys with similar compositions to HEAs, are more associated with the specific chemical environment of the segregated atoms. Finally, in-situ experiments in STEM revealed that ordering is suppressed with increasing temperature.