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

摘要

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 alpha-Fe/Co and Fe_2B in a-Fe_(67)Co_(18)B_(14)Si_1 alloy and Fe_2B and FeNi_3 phases in a-Fe_4Co_(65)B_(15)Si_(14)Ni_2 alloy. The crystallization kinetics of the alpha-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 alpha-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_2B formation temperature, the Mossbauer spectra show two sextets associated with the Fe_2B and alpha-Fe/Co phases. For the a-Fe_4Co_(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 12h at 1123 K clearly indicate the presence of FeNi_3 and Fe_2B phases.