Microstructural Studies of High Coercivity Nd 15 Fe 77 B 8 Sintered Magnets
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MICROSTRUCTURAL STUDIES OF HIGH COERCIVITY Ndl 5 Fe77 B8 SINTERED MAGNETS M. H. GHANDEHARI* AND J. FIDLER** * Unocal Corporation, Science and Technology Division, P.O. Box 76, Brea, CA 92621 USA "**Institute of Applied and Technical Physics, Technical University of Vienna, Vienna, Austria ABSTRACT Microstructures of Nd Dy Fe77 B prepared by alloying with Dy, and by using Dy 0 as a sinl'ken adidive, have been determined using electron microprobe and transmission electron microscopy. The results have shown a higher Dy concentration near the grain boundaries of the 2-14-1 phase for magnets doped with Dy 0 , as compared to the Dy-alloyed magnets. A two-step post sintering hea• treatment was also studied for the two systems. The resultant concentration gradient of Dy in the 2-14-1 phase of the oxide-doped magnets is explained by the reaction of Dy 0 with the Nd-rich grain boundary phase and its slow diffusion into thg 4-14-1 phase. Increased Dy concentration near the grain boundary is more effective in improving the coercivity, as domain reversal nucleation originates at or near this region. INTRODUCTION DyFe B displays a much higher single crystal anisotropy than the corresp ndigg Nd compound [1,2]. Therefore, a Dy-substituted sintered Nd Fe B ., which embodies the 2-14-1 phase displays a higher practical copci(tE, as first reported by Sagawa and coworkers [2]. As a low-cost approach in introducing Dy, the reaction of Dy 0 as a sintering additive with Nd5 Fe 7 B powder was investigated by onýRf the authors [3]. This resulted i6 &yO-doped magnets with coercivities similar to those prepared by substilution of Dy for Nd in the alloying step [3]. In this study, we compare the resultant microstructures by scanning transmission electron microscopy (STEM) and by electron microprobe. A two-step post-sintering heat treatment, employed to optimize the coercivity, is also compared for the two systems. EXPERIMENTAL Powder samples were prepared from a nominal Nd1 Fe7 B0 , and an alloy in which 3.5 wt% (approximately 10 atomic %) of Nd Ias fe~laced by Dy in the alloying step. Both of these powder samples were prepared under identical conditions and contained typically 0.5 to 0.6 wt% oxygen prior to pressing and sintering. 4.0 wt% Dy 0 (3.5 wt% Dy equivalent) was hand-mixed with the former, and both t~ps of magnets were sintered at 1070'C for an hour and annealed in two steps [4]. The first step of the annealing process consisted of 900'C for 2 hours and the second step varied between 550 and 680 0C for an hour. Samples were rapidly quenched in a cooler part of the furnace in between the two steps. After the magnetic measurements, the sintered samples were analyzed for 0 by a vacuum fusion technique. Boron, Nd, Dy and Fe were analyzed chemically by inductively coupled plasma (ICP) spectroscopy. A wavelength dispersive spectrometer was employed with the electron microprobe to scan for Dy.
Mat. Res. Soc. Symp. Proc. Vol. 96. c1987 Materials Research Society
194
electropolished Samples for using TEM analyses a solution were mechanic
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