Analytical Electron Microscope Study of High- and Low-Coercivity SmCo 2:17 Magnets
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ANALYTICAL ELECTRON MICROSCOPE STUDY OF HIGH- AND LOW-COERCIVITY SmCo 2:17 MAGNETS
JOSEF FIDLER , J. BERNARDI AND P. SKALICKY Institute of Applied and Technical Physics, Karlsplatz 13, A-1040 Vienna, Austria.
University
of Technology,
ABSTRACT Sintered, precipitation hardened SmCo 2:17 magnets contain a multiphase microstructure. Our electron microscopic investigations reveal that the size of the rhombic, cellular precipitation structure and the formation of cell interior and cell boundary phases is determined by the nominal composition of the alloy and the postsintering heat treatment conditions and primarily control the intrinsic coercivity of the magnet. Selected area electron diffraction together with high resolution electron microscopy showed a high density of basal stacking faults (microtwinning) of the cell interior phase of low coercivity (iHc < 700 kA/m) magnets with a (c/a)*- ratio of the basic structural unit of > 0.843. High coercivity magnets (HK>) 1000 kA/m), containing a high density of the platelet phase perpendicular to the c-axis, exhibit cell diameters up to 200 nm with a (c/a)*-ratio of the basic structural unit of the cell interior phase of ( 0.843.
INTRODUCTION Commercially available permanent magnet materials may be divided into hardferrites, AlNICo-magnets and rare earth (RE)-permanent magnets (PM). There exist two groups of REPM, the RE-cobalt magnets and the recently developed RE-iron magnets. The RE-cobalt magnets can be divided into five types depending on whether the magnet has a single-phase or a two-phase microstructure. The ideal microstructure of the single phase magnets consists of aligned single-domain grains with a SmCo 5 - or Sm2 CO1 7 -crystal structure. Two types of precipitation hardened magnets can be distinguished: the one type contains 2:17-precipitates in a 1:5-matrix, the other type forms l:5-precipitates in a 2:17-matrix. Besides these magnets there are the bonded magnets, in which the single domain particles are embedded in a non magnetic phase. Rare earth-cobalt magnets are produced by a powder netallurgical process with complicated post-sintering heat treatment procedures in the case of precipitation hardened SmCo 2:17-magnets. In fact, the norinal composition, the sintering and annealing parameters determine the intrinsic coercivity of SmCo 2:17 magnets. To understand the microstructure and the reasons for high coercivity of sintered SmCo 2:17 magnets transmission electron microscopy has widely been used [1-5]. The purpose of this work is to compare the microstructure of low coercivity (iHc 1000 kA/m) were choosen for transmission electron microscopy. Commercial sintered SmCo 2:17 magnets can also be divided into high iHc and low iHC magnets (fig.1). In order to study the influence of the effect of the nominal Samarium content on the microstructure and coercivity a set of samples was prepared and given a systematically varying heat treatment by THYSSEN Edelstahlwerke, FRG with a Sm-content of 23.9 wt% (±H&=250 kA/m), 24.8 wt% (iHc ) 1700 kA/m) and 25.7 wt% (iH=
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