Microstructural analysis of ultrafine grains in grain-refined Dy-free Nd-Fe-B sintered magnets

Strong permanent magnet availability is critical for electric vehicles to become increasingly prevalent. Although Nd-Fe-B magnets have superior ambient magnetic strength, Dy additions currently are needed for thermal stability, allowing magnet use at motor operation temperatures (150-180C). However, Dy use is not sustainable and must be replaced with an alternate way to provide thermal stability. Ultrafine (<4μm) grain size also has been shown to improve ambient and high temperature magnetic properties of Nd-Fe-B. While high energy milling can refine feedstock powder adequately, full refinement (<2µm) requires optimal microstructure scale, promoting brittle fracture in starting powders. SEM analysis will reveal the grain size and segregation spacing as a function of atomized powder size to guide processing. Achieving a fully refined particle size with single crystal particles is ideal for best alignment and magnetic properties if low temperature sintering maintains ultrafine grain magnets. Funding from DOE-EERE-VTO through Ames Lab contract No. DE-AC02-07CH11358.