Nd2Fe14B permanent magnets substituted with non-critical light rare earth elements (Ce, La): A review

The growing demand in high performance Nd-Fe-B permanent magnets, driven by emerging key technologies, notably vehicles electrification and wind turbines, raises the question of the availability of critical raw materials, namely the Light Rare Earth Elements (LREEs) Nd and Pr. Co-extracted with these LREEs, Ce and La represent about 70 % of the total amount of rare earths in minerals and, for this reason, could constitute a cost-effective diversification in the raw materials sourcing for magnets. Largely unused and stored, these non-critical elements are progressively incorporated in the permanent magnet industry as a substitution of Nd and Pr, allowing a more balanced utilization of the resources. The restoration of high magnetic performances, after the substitution of Nd by Ce, constitutes, however, a considerable challenge that requires to re-design some steps of the conventional routes for producing dense anisotropic magnets. This objective has motivated a wide range of research activities during the last decade from which different approaches are now emerging: the Hot-Deformation technique followed by an infiltration of eutectic alloys, on the one hand, and, on the other hand, the powder pressing and sintering possibly completed with the Grain Boundary Restructuring or with the Grain Boundary Diffusion Process. The performances of Ce based magnets improved by these routes are analyzed for a wide range of substitution rates and compositions. The strategies specifically deployed in the powder metallurgy route for enhancing the coercivity and the remanence, based on grain boundary engineering, additives alloying and on dual-powder sintering, are discussed in details.