The effect of heat treatment on the microstructure and magnetic properties of sintered magnets produced from NdFeB based alloys with and without Cu substitution

Abstract

The effects of high-temperature homogenization and low-temperature annealing treatments on the microstructure and magnetic properties of sintered magnets produced from an Nd 7Fe 76.5B 5Cu 1.5-type alloy (A) have been investigated. As a comparison with this investigation, the variations in the magnetic properties of sintered magnets manufactured from a Cu-free Nd 17Fe 79B 4 type alloy (B) and given similar heat treatments have also been studied. The sintered magnets of this alloy behaved differently from those of alloy A with heat treatment. In the cast condition, the microstructure of alloy A consisted of four phases: a 2:14:1 matrix, two Cu-containing Nd-rich phases and free Fe. The as-sintered magnet of alloy A showed a microstructure consisting of a Cu-containing Nd-rich eutectic and a Cu containing Nd 1Fe 2-type phase, both of which were distributed between the 2:14:1 grains. The homogenized and annealed magnet of alloy A exhibited a microstructure similar to that of the as-sintered magnet, but with a more uniform distribution of the 1:2 type phase a more uniform grain size and a substructure were observed. The remarkable improvements in intrinsic coercivity of the sintered magnets of alloy A by heat treatment have been related to these observations. The sintered magnets made from alloy A exhibited much better permanent magnetic properties than those made from alloy B, and this was attributed mainly to the presence of soft magnetic phase(s) in the microstructure of the latter magnets. The substitution of ca. 1.5 at.% Cu in the Nd 7Fe bal.B 4−5 alloys with low B content was found to be very effective in developing good magnetic properties in these alloys.