Effect of Annealing Temperature on Grain Boundary Structure and Magnetic Properties of Nd–Ce–Gd–Fe–B Sintered Magnet

Abstract

Post-sinter annealing plays an important role in optimizing grain boundary phases of Nd-Fe-B magnets. The sintered magnet is usually processed by two-step annealing, which consists of a high-temperature annealing (first-step annealing) followed by a low-temperature annealing (second-step annealing). In this work, the magnetic properties and microstructure of Nd-Ce-Gd-Fe-B magnets have been investigated by different second-step annealing temperatures. Compared with the magnet annealed at 420 °C, the knee coercivity ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> ) of the 460 °C annealed magnet was increased by 156.7 kA/m, and the squareness factor was improved significantly from 0.85 to 0.98. The volume fraction of RE-rich (RE: rare earth) phases in the magnet has increased after second-step annealing at 460 °C. Microstructural analysis showed that the grain boundary layers in the 460 °C annealed magnets were thicker and evenly distributed around RE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> B grains, which was helpful to decrease the exchange-coupling between RE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> B grains and therefore enhance the coercivity.