Synthesis and characterization of bulk amorphous/nanocrystalline soft magnetic materials

Karataş, Mediha Merve
The aim of the study is to reach non-equilibrium cooling conditions and produce bulk glassy (Fe36Co36B19.2Si4.8Nb4)99.25Cu0.75 rod which has a good soft magnetic property and high electrical resistivity and to investigate its crystallization kinetics. The bulk metallic glass formation was achieved by arc melting and suction casting and it was confirmed by scanning electron microscopy (SEM), x-ray diffraction (XRD) and thermal analysis techniques. After characterizations, by isochronal and isothermal differential scanning calorimeter (DSC) analyses, crystallization kinetics of the alloy was investigated. From isochronal DSC analyses, activation energies for glass transition and crystallization events were determined by using different analytical methods such as Kissinger and Ozawa. The critical cooling rate was calculated from two approaches called Barandiaran and Colmenero and Liu et al. and results were compared. Numerous glass forming ability parameters such as Trg, ΔTx, and γ were estimated for the samples. High majority of parameters showed that the alloy is a good bulk glass former. The isothermal crystallization kinetics of the alloy was studied at temperatures chosen in above the first crystallization temperature. There were no crystallization signals detected in the isothermal DSC scans except for 585 ºC at 5 min holding. Although SEM showed featureless matrix, α-(FeCo) crystallites were distinguished in the XRD pattern and size of these crystallites was estimated by Scherrer equation. The magnetic and mechanical properties of the as cast and annealed alloys were compared. According to the obtained results, annealed alloys have higher saturation magnetization, lower coercive force and higher microhardness values than as cast alloys due to the presence of α-(FeCo) nanocrystalline structures within amorphous matrix.