The microstructures of (Nd,Dy)-(Fe,Co,M)-B [M = Nb, (Nb + Ga), (Nb + Si)] sintered magnets have been studied by means of X-ray diffraction, metallography, EPMA and TEM techniques. It was found that in addition to the (Nd, Dy)_2 (Fe, Co)_(14) B, Nd-rich and R_(1+ε)Fe_4B_4 phases found in ordinary Nd-Fe-B magnets, all these magnets contained a (Fe, Co)NbB phase with CoNbB-type orthorhombic structure and Nb-containing fine precipitates dispersed in the magnetically hard matrix. However, structural differences have been found when the substituting element M changes. For M = Nb or (Nb + Ga) a Laves phase Fe_2Nb with MgZn_2 structure was formed. DIslocations around the Fe_2Nb precipitate were discernible in the magnet M = Nb + Ga. The amount of Fe_2Nb Laves phase in the magnet M = Nb depends largely on heat treatment and maximum coercivity was obtained when the Fe_2Nb phase does not appear. For M = Nb + Al or Nb + Si, no Fe_2Nb phase was formed, instead (Nd, Dy)_2 (Fe, Co, Nb, B, M)_7 phases with Th_2 Co_7- and Gd_2Co_7-type structures changerespectively. Magnetic measurements showed that the intrinsic coercivity of magnet with M = (Nb + Ga), (Nb + Al) or (Nb + Si) is higher than that of the magnet with M = Nb under optimum heat treatment. The coercivity mechanisms of these magnets have been discussed in terms of microstructures.
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