Correlation of the energy product with evolution of the nanostructure in the Y,Dy,Nd–(Fe, Co)–B magnetic alloy

Abstract

The devitrification behavior of nanocrystalline MRE2(Fe,Co)14B+ZrC (MRE=Nd+Y+Dy) was studied using differential scanning calorimetry (DSC), synchrotron high temperature x-ray diffraction, and analytical transmission electron microscopy (TEM) techniques. Alloy ribbons were melt spun at 25 m/s to obtain an amorphous structure. Optimum hard magnetic properties (Br=7.2 kG, Hc=12.7 kOe and (BH)max=10.8 MG Oe) were obtained in ribbons annealed at 750 °C for 15 min. A reduced annealing temperature of 638 °C and holding time from 0 to 11 min were chosen based on DSC analysis. Large changes in both microstructure and hard magnetic properties were found in a narrow window of annealing time, 4.5–6 min, resulting in a dramatic increase in energy product, remanence and coercivity: 0.96 MG Oe, 5.2 kG, 2.7 kOe to 5.7 MG Oe, 7.2 kG, 8.5 kOe for (BH)max, Br and Hc, respectively. Energy dispersive x-ray spectroscopy and energy filtered TEM analyses indicate that Zr- and C-rich particles (∼5 nm) and thin grain boundary layers (1–2 nm thick) are formed surrounding 2-14-1 hard phase grains when the annealing time is over 6 min. Further annealing resulted in a more distinct hard phase surrounded by a nonmagnetic grain boundary phase ∼1 nm in thickness. The thin grain boundary layer phase starts to disappear with annealing time over 11 min. The partitioning behavior of various elements at different annealing conditions appears to be associated with significant changes in magnetic properties, leading to an improved optimum microstructure.