Abstract
Phase transformations and magnetic properties of overquenched Pr 8Fe 86B 6 ribbons during annealing treatment have been investigated. The as-quenched structure of the ribbons was varied by changing the quenching rate during the melt spinning process. It was found that, depending on their as-quenched structures, the phase transformations of the ribbons during annealing may take place in one of the following sequences: (1) Amorphous phase (Am)+Pr 2Fe 14B+α-Fe→Pr 2Fe 14B+α-Fe; (2) Am+α-Fe→ Am′+α-Fe→α-Fe+Pr 2Fe 23B 3+Pr 2Fe 14B→Pr 2Fe 14B+α-Fe; and (3) Am→Am′+α-Fe→Pr 2Fe 23B 3+α-Fe→ Pr 2Fe 14B+α-Fe. However, for all the ribbons, the microstructure after optimal annealing was found to consist only of two magnetic phases: Pr 2Fe 14B and α-Fe. The optimum magnetic properties (intrinsic coercivity H ci and remanent polarization J r), and the squareness of the demagnetization curves of the annealed ribbons deteriorated drastically with increasing quenching rate of unannealed precursors. This deterioration can be attributed to the formation of a coarser and more irregular microstructure during annealing of the samples initially melt spun with higher wheel speeds, which was confirmed by our TEM observations, analysis of the temperature dependence of coercivity and measurement of irreversible susceptibility.
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