Crystallization behavior of mechanically milled nanocomposite Pr-Fe-B alloys

In the past few years, research studies have proposed the high energy mechanical milling as an alternative route to the melt spinning for the development of nanocomposite two-phase 2:14:1 / bcc Fe magnets. The current work presents our results on Pr$_{9}$Fe$_{85}$B$_{6}$ and Pr$_{9}$Dy$_{1}$Fe$_{76}$Co$_{8}$Si$_{1}$B$_{5}$ mechanically milled powders with emphasis on the microstructure peculiarities, phase transformations upon annealing and their relation with the magnetic properties. Calorimetric investigations in as-milled powders show a main exothermic peak occurring at relatively low temperatures (350$^{\circ}$C) that has been associated with a stress-relief effect. HRTEM micrographs for as-milled powders give evidence of coexistence of bcc Fe or (Fe,Co) phase (d$_{g}\cong $10-15 nm), and an amorphous phase. The amorphous phase is still observed in powders annealed at 400$^{\circ}$C and coexists with slightly enlarged bcc Fe or (Fe,Co) grains (d$_{g}\cong $15-20 nm) and with a newly precipitated 1:7-type phase. A high degree of strain is present in the nanograins. The microstructure becomes free of internal stresses and completely crystallizes at 650$^{\circ}$C. The extrinsic magnetic properties are strongly related to the powder microstructure.