HDDR treatment of Ce-substituted Nd2Fe14B-based permanent magnet alloys - phase structure evolution, intergranular processes and magnetic property development

Abstract

Aiming for cost-efficiency through balanced utilization of rare-earth resources, the use of Ce, the cheapest and most abundant of all rare-earth elements, as potential candidate to substitute the expensive and resource-critical Nd in Nd2Fe14B hard magnetic alloys was tested in this work. Emphasis is put on the effects of substitution on the alloys structural properties and the related response to hydrogen treatment, one of the main processing routes in the production of Nd2Fe14B-based permanent magnets. The study follows the influence of Ce substitution on the phase structure and magnetic property development in hydrogen decrepitated and HDDR processed (Nd1-xCex)15Fe79B6 (x = 0, 0.1, …, 0.6) strip-cast alloys. Intergranular CeFe2 segregates in the as-cast alloys from x = 0.3, increases in proportion with x at the expense of the hard magnetic phase and replaces the rare-earth-rich grain boundary phase. Upon hydrogen absorption, CeFe2 transforms in amorphous CeFe2Hx which further decomposes into CeHx and α-Fe. The rare-earth components from CeHx and the rare-earth-rich hydrogenated phase melt upon hydrogen desorption, enabling the redistribution of the intergranular material among the Nd2Fe14B grains upon the desorption and recombination stage of the HDDR treatment, thus facilitating grain decoupling and coercivity development. Within a generally decreasing trend, at x = 0.2 and 0.3 Ce ratios, coercivity (μ0HC) and remanence (Br) of the HDDR powders remained rather stable despite the segregation of CeFe2 phase occurring at x = 0.3. Significant texture and very reasonable magnetic properties were obtained at x = 0.3 (μ0HC = 1.05 T and Br = 0.88 T), attributed to a favorable phase composition with Ce mainly concentrated in the intergranular material.