Electrodeposition of Dy and its grain boundary diffusion behavior of sintered NdFeB magnets

Abstract

This paper presents the electrodeposition of a Dy layer onto the surface of sintered NdFeB magnets to serve as a diffusion source. Subsequently, the sintered NdFeB magnets were subjected to grain boundary diffusion treatment through electrochemical deposition at the solution temperature of 45 °C and the solution pH (hydrogenii) of 4.0. The results indicated that the coercivity of the Dy-diffused magnet increased from 1073 kA/m to 1418 kA/m compared to the original magnet after annealing at a primary diffusion heat treatment temperature of 900 °C for 6 h and subjected to the same secondary tempering heat treatment at 500 °C for 2 h, while the remanence remained relatively unaffected. SEM (Scanning Electron Microscopy) revealed that the Dy coating exhibited a compact and flat surface with a small grain size. Energy spectrum analysis confirmed a relatively uniform distribution of Dy elements in the electrodeposition coating, without any significant agglomeration. SEM-EDS (Scanning Electron Microscopy-Energy Dispersive Spectrometer) line scan demonstrated that the diffused Dy elements along the grain boundary phase primarily concentrated in the grain boundary phase itself and the epitaxial layer of the main grains. BSE (Backscattered Electron imaging) revealed the presence of a thin and continuous grain boundary phase structure between the main phases following grain boundary diffusion. XRD (X-ray diffraction) analysis demonstrated a general shift toward higher angles in the main peak of NdFeB after Dy diffusion. The remarkable increase in coercivity of the diffused Dy magnets can be attributed to the optimization of microstructure, grain boundary phase composition, and the formation of the (Dy,Nd)2Fe14B structure. The coercivity mechanism in diffusion Dy magnets was still governed by a nucleation reversal mechanism, as supported by the Kronmüller-Plot curve fitting. Tafel tests have revealed that Dy-diffused magnets have a much higher corrosion resistance with a more positive equilibrium potential, Ecorr, and a lower corrosion current density, Icorr.