Dependence of structures and performances of soft-magnetic FeCo-based metal glasses on cryogenic treatments

Abstract

The amorphous structures, thermal stability, soft-magnetic properties and microhardness have been explored for quaternary quenched (Fe0.8Co0.2)75B21Ta4 (Fe0.8) and (Fe0.5Co0.5)75B21Ta4 (Fe0.5) metallic glasses (MGs) after deep (77 K), moderate (173 K) and shallow (253 K) cryogenic treatments (CTs). Compared to Fe0.5, the as-spun Fe0.8 exhibits significantly higher thermal stability and Curie temperature (686 K), which could be explained by the Mössbauer spectra results of higher A23 and lower hyperfine magnetic field as well as isomer shift. But the as-spun Fe0.5 shows a more prominent magnetic rejuvenation trend. Additionally, the observation of magneto-optic Kerr microscope displays that magnetic domains around surface defects exhibit a trend from being easy to magnetize to becoming difficult with increasing reverse magnetic field intensity.The cryo-treated Fe0.8 and Fe0.5 ribbons maintaining a completely amorphous structure reveal the improved soft-magnetic property and microhardness, and the performance optimization of Fe0.5 is more sensitive to CTs. Appropriate CTs distinctly decrease coercivity (Hc) of two tested MGs, and moderate CT reduces the Hc of Fe0.5 by 24.1%, achieving commendable magnetic softening. The Hc variation can be rationalized by the evolution of free volume in the amorphous matrix involving freezing, partial annihilation and imbalance under different CTs. Moreover, shallow CT significantly expands the heat-treatment processing window of MGs.

Keywords

• Metallic glass
• Soft-magnetic properties
• Shallow/moderate/deep-cryogenic treatment
• Microhardness
• Thermal stability