An Analytical Electron Microscopy Characterization of Melt-Spun Iron/Rare-Earth/Boron Magnetic Materials
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ANALYTICAL EL,•CION MICROSCOPY AN ]RON/RARE-EARTH/(OION MAGNTIC MATERIALS
CHARACTERIZATION
OF
MELT-SPUN
R.C. Dickenson & K.R. Lawless Dept. of Materials Science, Univ. of Virginia, Charlottesville VA 22901 G.C. Eadjiipanayis Dept. of Physics, Kansas St. Univ., Manhattan KN 66506 Iron/rare-earth/boron permanent magnet materials have recently been delveloped to reduce the need for the strategic element cobalt, which was previously the primary canponent of high-energy magnets. These materials are generally produced by annealing rapidly solidified ribbons or by conventional powder metallurgy techniques. This paper will report results from an analytical electron microscopy characterization undertaken to establish the relationship between the magnetic properties and the microstructure of two iron/rare-earth/boron (Fe/RE/B) alloys. Ribbons of FeT. 5PrB 1 0 and Fe7Tb1 5 Bs were produced by melt-spinning. To obtain optimum magnetic properties, 5oth alloys were then annealed at 7000C, the FePrB ribbons for 6 minutes and the FeThB ribbons for 90 minutes. Foils for transmission electron microscopy were prepared by ion-milling the ribbons on a cold stage and examined using a Philips 400T TEM/SM equipped with an energy dispersive x-ray unit. Fe 7 5 PrlBo 1 0 In
the
as-spun
state
Fe75Pr15B
was
found to have a finely
crystalline structure (Fig. la), but the grain size varied substantially with distance from the wheel side of the ribbon. Selected area diffraction (SAD, Fig. lb) was used to identify the primary crystalline phase as a-Fe. Two diffuse rings were observed with d-spacings of 0.33nm and 0.30m. The 0.33nm d-spacing has been interpreted as the combination of the strongest reflection from several Pr-borates, and the 0.3Onm d-spacing as an unidentified Pr-rich oxide with a glassy character which is stabilized by sane boron content [1]. In addition, relatively large (25nm) crystallites of a FOC Pr-rich phase (a4l.O4nm) were observed, though not shown here. The overall composition of this alloy is shown in the E)S spectrum (Fig. Ic). The FePrB ribbons were annealed to promote growth of the hard magnetic phase Fe1 4 PrpB (tetragonal: as-O.88nm, cl .22nm). An overview of the resulting structure (FTg.2a) revealed that the Fel 4 Pr 2 B phase was the major constituent with an average grain size of 85nm. The corresponding SAD pattern (Fig. 2b) confirms the presence of the hard magnetic phase. The Fe 4Pr 2 B phase, identified by (HI) (Fig.3b) and EDS (Fig. 3c), crystallized as equiaxed grains as shown in a [110] oriented bright field image (Fig. 3a). Lattice imaging (Fig. 3a) revealed few planar defects in the Fe 1 4 Pr 2 B phase, but same small 'inclusions' (dark spots) were observed. These inclusions were postulated to be n-Fe precipitates. Small amounts of at least two additional phases were observed, generally as smaller grains (17kOe) than from Nd or Pr based alloys. In the as-spun state, the ribbons were observed to have a glassy structure in the regions closest to the quenching wheel becoming crystalline (Fig. 6a) w
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