Effects of quenching rate on the phase transformation and magnetic properties of melt-spun Pr8Fe86B6 ribbons during annealing

Phase transformations and magnetic properties of overquenched Pr 8 Fe 86 B 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 2 Fe 14 B+α-Fe→Pr 2 Fe 14 B+α-Fe; (2) Am+α-Fe→ Am′+α-Fe→α-Fe+Pr 2 Fe 23 B 3+ Pr 2 Fe 14 B→Pr 2 Fe 14 B+α-Fe; and (3) Am→Am′+α-Fe→Pr 2 Fe 23 B 3 +α-Fe→ Pr 2 Fe 14 B+α-Fe. However, for all the ribbons, the microstructure after optimal annealing was found to consist only of two magnetic phases: Pr 2 Fe 14 B 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.