Effects of as-quenched structures on the phase transformations and magnetic properties of melt-spun Pr7Fe88B5 ribbons

Phase transformations and magnetic properties of overquenched Pr7Fe88B5 rib bons during annealing have been investigated. X-ray diffraction and Mossbau er measurement indicate that melt spinning at different wheel velocities ca used the as-quenched ribbons to have distinctive structures. Depending on t heir as-quenched structure, the phase transformation of the ribbons during annealing may take place in one of the following sequences: (1) amorphous p hase (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 --> 1:7 phase + alpha-Fe --> Pr2Fe14B alpha-Fe. In all cases, the microstructure of the ribbons after optimal an nealing was found to only consist of two magnetic phases: Pr2Fe14B and alph a-Fe. However, with increasing initial quenching rate, the microstructure o f optimally heat treated ribbons becomes coarser and more irregular, and th e magnetic properties of them deteriorated drastically. The delta M plots, irreversible susceptibility, and the temperature dependence of coercivity o f the annealed ribbons were measured and analyzed. It was found that the co arser-grained and more irregular microstructures, as mentioned above, would lead to the weakening of the exchange coupling effect between the hard and soft phase, the inhomogeneous nucleation of reverse domain, a lower critic al nucleation field, and a decrease in alpha(ex) and N-eff. These may be th e reasons that the optimal magnetic properties of annealed ribbons reduce s ignificantly with increasing initial quenching rate. (C) 1999 American Inst itute of Physics. [S0021-8979(99)04124-9].