Permanent magnet materials based on the rare earth-iron-boron tetragonal compounds

Structural and metallographic studies were carried out on the Nd-Fe-B alloy system as well as the Nd-Fe-B tetragonal compound on which record high energy magnets have been developed using a powder metallurgical technique. The study on the new magnet has also been extended to other R-Fe-B componds containing various rare earths (R) and to R-Fe-Co-B alloys. The results are as follows; (1) The sintered Nd-Fe-B magnet is composed of mainly three phases, the Nd2Fe14B matrix phase plus Nd-rich phase and B-rich phase ∼ Nd2Fe7B6) as minor phases. (2) Nd2Fe14B has the space group of P42/mnm. The crystal structure of this phase can be described as a layer structure with alternate stacking sequence of a Nd-rich layer and a sheet formed only by Fe atoms. The sheet of Fe atoms has a structure similar to the σ-phase found in Fe-Cr and Fe-Mo systems. (3) The Nd-rich phase containing more than 95 at.% Nd, 3∼5 at.% Fe and a trace of B has fcc structure with a=0.52 nm. This phase is formed around grain boundaries of the matrix phase. Nd2Fe7B6phase has an one-dimentional incommensurate structure with a=aoand Co, based on a tetragonal structure with ao=0.716 nm and co=0.391 nm. (4) In the as sintered Nd15Fe77B8alloy periodic strain contrasts are observed along grain boundaries, which disappear after annealing at 870K. This may be related to the enhancement of the intrinsic coercivity of the sintered magnet by post sintering heat treatment. (5) Stable R2Fe14B phases are formed by various rare earths except La. Of all the R2Fe14B compounds, Nd2Fe14B has the maximum saturation magnetization as high as 1.57 T. Dy and Tb form R2Fe14B phases with the highest anisotropies. Small additions of these elements greatly enhance the coercive force of the Nd2Fe14B base magnet. (6) Partial replacement of Fe by Co raises the Curie temperature of the Nd2Fe14B compound, which improves the temperature coef- ficient of the remanence of the magnet. But the intrinsic coercive force is decreased by the Co addition.