Microstruoture-Property Relationships in Magenqubnch Magnets
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RIOROSTRUOTURE-PROPERTY RELATIONSHIPS IN•L@NgQUBNCH RAGNBTS RAJAK. RISHRA Physics Department, 0eneral Motors Research Laboratories, Warren, MI 48090-9055
ABSTRACT Transmission electron microscopy has been used to characterize the microstructure of Nd-Fe-B magnets produced by melt-spinning and subsequent hot-pressing/die-upsetting. For a material of starting composition Nd .1.. ••Fe .•1 n..B ^., the basic microstructure of melt-spun, hot-pressed and dieupse• magnets .U• consists of two phases. In the optimally processed melt-spun ribbons and hot-pressed samples, small and randomly oriented Nd2Pel•B grains are surrounded by a thin noncrystalline Nd-rlch phase. The die-upse• material consists of closely stacked flat NdoFAB grains surrounded by a second phase of approximate composition NdTFe•. No 1•1 1FeAB• phase is observed in these materials, but it can form if'th• chemicHl'co•p•sition and/or processing parameters are varied. In all these materials, Lorentz microscopy reveals that magnetic domain walls are pinned by the second phase. The differences in the hard magnetic properties of the three kinds of RAONEqUBNOH magnets closely correlate with the differences in the distribution of Nd2Fel4B crystallites and the pinning sites.
INTRODUCTION Ever since the discovery of the hard magnetic Nd-Fe-B materials[I-4] there has been growing scientific and technological interest in this field. Hundreds of papers and articles have been publlshed[5], and Nd-Fe-B permanent magnets have become commercially available. Today these magnets are principally produced by two different methods: one starting with melt-spun ribbonsIll and the other via powder metallurgy[2]. RAGNEqUENOH is the trade name designating the group of magnets produced by General Rotors. Three different grades of MAGNEQUENCH magnets can be made from the rapidly quenched Nd-Fe-B ribbons by (i) resin bonding, (ii) hot-pressing and (iii) die-upsetting[6]. In this paper we shall survey the available information on the microstructural features and their interactions with magnetic domains in the •GNEQUENCH magnets. The microstructure of the sintered magnets will be referred to for comparison• The paper ends with a discussion on the still unanswered yet important questions about the microstructural variables requiring further research. The main magnetic phase in all types of Nd-Fe-B magnets is the tetragonal NdoFe..B phase[7]. However, depending on the grain size, • • • 14 . . .. . . crystalllnlty, grain orientations, phase dlstrlbutzons, etc., widely varying hard magnetic properties are possible. The oriented J4ACNE•UBNCH magnets produced by die-upsetting[8] and the sintered magnets (which are oriented in a magnetic field before sintering) exhibit similar energy product values of over 45 RGOe and intrinsic coercivity values of over 12 kOe[9]. The bonded and hot-pressed RAGNEQUENCH magnets are isotropic and have energy products and coercivities around 15 RGOe and 20 kOe respectively[6]. The microstructural features responsible for such wldely varying properties are summar
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