Abstract
We report on microstructure, hard magnetic properties and thermal stability of nanocrystalline (Nd0.85Ce0.15)15(Fe1-xCox)78B7 (x = 0 – 1) hot-compacted permanent magnets. At lower Co concentrations (x < 0.3), the substitution of Fe by Co changes the composition of grain boundary phase from non-magnetic (Nd,Ce)2Fe1 to magnetic (Nd,Ce)1(Fe,Co)2, leading to the degrading of the coercivity. At x ≥ 0.6, the grain boundary phase is no longer observed between the nanocrystalline grains, resulting in strong magnetic coupling of constituent phases. With the further increase of Co concentration (x>0.6), additional RE(Fe,Co)2, RE(Fe,Co)4B and RECo5 phases are gradually formed in the magnets. A new type of microstructure with nanograins of RE2(Fe,Co)14B phase, RE(Fe,Co)2 phase and RE(Fe,Co)4B phase, with partial or without grain boundary phase shows good thermal stability (x = 0.4 and 0.6). In (Nd0.85Ce0.15)15(Fe0.4Co0.6)78B7 magnets, the absolute values of temperature coefficient of coercivity |β| can reach 0.3 %/K within the temperature range of 300 K - 500 K, and it exceeds 0.23 %/K for the temperature range of 300 K - 650 K, indicating a promising method for designing highly thermal stable permanent magnets.
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