Phase transformations and magnetic properties of melt-spun Pr7Fe88B5 ribbons during annealing

X-ray diffraction and Mossbauer measurements indicate that melt spinning at different wheel velocities causes as-quenched Pr7Fe88B5 ribbons to have distinctive structures. 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) + Pr2Fe14B+ alpha-Fe->Pr2Fe14B+alpha-Fe; (2) Am+alpha-Fe->Am'+alpha-Fe->alpha-Fe+1:7 phase+Pr2Fe14B->Pr2Fe14B+alpha-Fe; and (3) Am->Am'+alpha-Fe->alpha->1:7 phase +alpha-Fe-> Pr2Fe14B + alpha-Fe. However, for all the ribbon samples, the microstructures after optimal annealing with respect to magnetic properties were found to only consist of two magnetic phases: Pr2Fe14B and alpha-Fe. The optimum magnetic properties of H-ef and J(r), and the squareness of the demagnetization curves of the annealed ribbons deteriorate drastically with increasing quenching rate of unannealed precursors. Taking into account TEM results, the values of alpha(ex) and N-eff derived from the temperature dependence of the coercivity, these deteriorated effects can be attributed to the formation of a coarser and more irregular microstructure during annealing in the samples initially melt spun with higher wheel speeds. (C) 2000 Elsevier Science S.A. All rights reserved.