Phase transformation in iron/cobalt-based amorphous alloys revealed by thermal and magnetic techniques

The crystallization kinetics and the changes in the structure and magnetic properties of some iron/cobalt-based amorphous alloys have been investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and 57 Fe transmission Mossbauer spectroscopy. Heating and isothermal treatments were used to induce mainly nucleation and growth of α-Fe/Co and Fe 2 B in a-Fe 67 Co 18 B 14 Si 1 alloy and Fe 2 B and FeNi 3 phases in a-Fe 4 Co 65 B 15 Si 14 Ni 2 alloy. The crystallization kinetics of the α-Fe/Co phase was studied in the theoretical frame of the Johnson-Mehl-Avrami model. The 57 Fe Mossbauer spectra of the as-received alloys show the expected broad sextet typical of amorphous magnetic alloys. The isothermal treatments conducted at temperatures below the first exothermic peak in the DSC curve of the a-Fe 67 Co 18 B 14 Si 1 alloy (at about 690 K) lead to the crystallization of only the α-Fe/Co phase, as observed by XRD and Mossbauer results. For temperatures just below the second crystallization peak (at about 785 K in the DSC curve), assigned as the Fe 2 B formation temperature, the Mossbauer spectra show two sextets associated with the Fe 2 B and α-Fe/Co phases. For the a-Fe 4 Co 65 B 15 Si 14 Ni 2 alloy, no crystallization is observed by XRD for samples heat-treated at temperatures up to 761 K during 48 h. However, XRD and Mossbauer results in the sample heat-treated for 12 h at 1123 K clearly indicate the presence of FeNi 3 and Fe 2 B phases.