Optimizing the microstructure of sintered Nd-Fe-B magnets via the application of scandium hydride

Abstract

This paper is focused on optimizing the microstructure of near-stoichiometric Nd₂Fe₁₄B-based magnet by grain boundary structuring via the application of ScH_x (0.5 and 1 wt.%) added to the matrix powder mixture at the fine milling stage. Scandium is assumed can acts as component, which restricts the Nd₂Fe₁₄B-phase grain growth and by analogy with titanium, niobium, molybdenum and vanadium alloying for Nd-Fe-B compositions, Sc improves the corrosion resistance and technological stability of magnets. The base alloy having the composition (wt.%) Nd-24.0, Pr-6.5, Dy-0.5, B-1.0, Al-0.2, Nb-0.5, Fe-balance was prepared by strip-casting technique and subjected to hydrogen decrepitation during heating to 270 °C in a hydrogen flow at a pressure of 0.1 MPa and subsequent 1 h dwell at this temperature. The scandium hydride was prepared by direct reaction of Sc with hydrogen using a Sieverts apparatus. The microstructure, phase composition and distributions of REM and Sc for the prepared magnets were investigated by SEM/EDX method.

It was found that the total REM content in the main magnetic (Nd, Pr, Dy)₂Fe₁₄B phase of both magnets was~11.8 at % and as well as the scandium content was nearly identical. Sc mainly was observed as individual phases. The Sc content in them for magnets prepared with 0.5 and 1.0 wt.%ScH_x is 15 and 38 at.%, respectively. In this case, the chemical composition of the phase in the magnet prepared with 0.5 wt % ScHx corresponds to that of Laves phase, whereas for the magnet with 1.0 wt % ScH_x, the iron content in the phase is less than 10 at %. The R-rich phase was found is enrich in Nd and Pr. The further experiments with ScH_x should be performed in varying boron content in the magnets.