Microstructure evolution and magnetic properties of overquenched Pr8Fe86B6 ribbons during annealing

Microstructure evolution and magnetic properties of overquenched Pr8Fe86B6 ribbons during annealing have been investigated. The results showed that, in as-quenched state, the microstructure of the ribbons consists of a mixture of amorphous phase (Am) + Pr2Fe14B + a-Fe, Am + a-Fe and only amorphous phase, respectively, for the wheel speed of 22, 26, and 30 m/s. Depending on the overquenched precursor, the microstructure evolution of the ribbons during annealing can be classified into: (1) Am + Pr2Fe14B + a-Fe --> Pr2Fe14B + a-Fe; (2) Am + alpha-Fe --> Am' + a-Fe --> a-Fe + Pr2Fe23B3 + Pr2Fe14B --> Pr2Fe14B + a-Fe; and (3) Am --> Am + a-Fe --> Pr2Fe23B3 + a-Fe --> Pr2Fe14B + a-Fe. In all cases, the microstructure of optimally annealed ribbon samples consist of magnetically hard Pr2Fe14B and soft magnetic a-Fe phases. The magnetic properties achieved by optimal annealing were found to be strongly dependent upon the initial quenching rate of the unannealed precursor. A fairly significant drop in both H-ci and Br was observed with the increase of quenching rate of the precursors. (C) 1999 American Institute of Physics. [S0021-8979(99)59908-8].