Ce and La as substitutes for Nd in Nd2Fe14B-based melt-spun alloys and hot-deformed magnets: A comparison of structural and magnetic properties
Ce and La as very cheap rare-earth elements were used to substitute Nd in nanocrystalline melt-spun ribbons of nominal compositions (Nd1−xREx)13.6FebalCo6.6Ga0.6B5.6 (x = 0, 0.1, 0.2, EL 1 for RE = Ce) and (x = 0, 0.1, 0.2, EL 0.5 for RE = La). Ce substitution gradually decreased the Nd2Fe14B lattice constants and produced CeFe2 segregation from x = 0.7. La substitution led to lattice expansion along the c-axis and induced segregation of a-Fe and Nd2Fe17 at x = 0.5. Grain coarsening was observed in the Ce-substituted samples while La was found to suppress grain growth. Cerium worsened the magnetic properties of as-spun powders after an initial improvement in (Nd0.9Ce0.1)13.6FebalCo6.6Ga0.6B5.6 alloy which showed a coercivity (µ0Hc) of 1.54 T and a remanence (Br) of 0.81 T. Coercivity dropped with increasing La concentration but remanence increased from 0.73 T in the base composition to 0.88 T at x = 0.3. The Curie temperatures (TC) showed a slight decrease in both cases until x = 0.4. It then dropped abruptly for increasing Ce fractions and increased at x = 0.5 La. For x = 0.2 and 0.3 Ce and x = 0.2 La fractions, the melt-spun samples were further processed by hot-pressing and hot-deformation. The hot-pressed (Nd0.8La0.2)13.6FebalCo6.6Ga0.6B5.6 alloy measured lower coercivity but increased remanence comparing to the Ce-substituted alloys. However, this composition responded poorly to hot-deformation, severe cracking being induced in the process. Due to enhanced hot-workability, best magnetic properties were obtained after deformation for the (Nd0.7Ce0.3)13.6FebalCo6.6Ga0.6B5.6 alloy (µ0Hc = 1.09 T, Br = 0.97 T and energy product (BH)max = 170 kJ/m3).