Microstructure evolution and magnetic properties of overquenched Pr8Fe86B6ribbons during annealing

Microstructure evolution and magnetic properties of overquenched Pr8Fe86B6 ribbons during annealing have been investigated. The results showed that, i n as-quenched state, the microstructure of the ribbons consists of a mixtur e of amorphous phase (Am) + Pr2Fe14B + a-Fe, Am + a-Fe and only amorphous p hase, respectively, for the wheel speed of 22, 26, and 30 m/s. Depending on the overquenched precursor, the microstructure evolution of the ribbons du ring annealing can be classified into: (1) Am + Pr2Fe14B + a-Fe --> Pr2Fe14 B + 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 --> Pr2F e14B + 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 st rongly dependent upon the initial quenching rate of the unannealed precurso r. A fairly significant drop in both H-ci and Br was observed with the incr ease of quenching rate of the precursors. (C) 1999 American Institute of Ph ysics. [S0021-8979(99)59908-8].