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 di stinctive structures. Depending on their as-quenched structures. the phase transformations of the ribbons during annealing may take place in one of th e following sequences: (1) amorphous phase (Am) + Pr2Fe14B+ alpha-Fe->Pr2Fe 14B+alpha-Fe; (2) Am+alpha-Fe->Am'+alpha-Fe->alpha-Fe+1:7 phase+Pr2Fe14B->P r2Fe14B+alpha-Fe; and (3) Am->Am'+alpha-Fe->alpha->1:7 phase +alpha-Fe-> Pr 2Fe14B + 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 mag netic properties of H-ef and J(r), and the squareness of the demagnetizatio n curves of the annealed ribbons deteriorate drastically with increasing qu enching rate of unannealed precursors. Taking into account TEM results, the values of alpha(ex) and N-eff derived from the temperature dependence of t he coercivity, these deteriorated effects can be attributed to the formatio n of a coarser and more irregular microstructure during annealing in the sa mples initially melt spun with higher wheel speeds. (C) 2000 Elsevier Scien ce S.A. All rights reserved.