Effect of Dy Additions on Microstructure and Magnetic Properties of Fe-Nd-B Magnets
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EFFECT OF Dy ADDITIONS ON MICROSTRUCTURE AND MAGNETIC PROPERTIES OF Fe-Nd-B MAGNETS R.Ramesh#, G.Thomas# and B.M.Ma' # Department of Materials Science and Mineral Engineering and Materials and Molecular Research Division, Lawrence Berkeley Laboratory, University of California, Berkeley, Ca 94720 * Crucible Research Center, Pittsburgh Pa 15230. ABSTRACT: This paper addresses the effect of Dy addition upon the magnetic properties, microstructure and microcomposition of Fe-Nd-B magnets. It is shown that increasing additions of Dy causes the remanence, Br to decrease linearly. The intrinsic coercivity, iHc, increases sharply for small additions of Dy, but the increase is not proportional for higher Dy contents. The iHc increases almost linearly with the effective anisotropy field of the RE2 Fe 1 4 B phase until the Dy content is about 10% of the total rare earth content. Above this concentration, there is a strong deviation from linearity. Various types of possible concentration profiles of the substituted rare earth are suggested. It is also argued that preferential segregation of Dy to the interfaces could be beneficial in increasing the nucleation field. Morphologically there is no apparent effect of Dy on the microstructure. However, inthe 5 atomic % Dy sample, Dy rich oxides were observed. It is shown through Energy Dispersive Xray Spectroscopy (EDXS) line profiling that Dy partitions preferentially into the RE2 Fe 14 B phase in all the cases. No segregation of Dy to the interphase interfaces has been detected. INTRODUCTION: Current industrial and academic interest in Fe-Rare Earth-B permanent magnets is well evidenced by the number of conference symposia and workshops being held on this class of magnets. Since the discovery of these magnets[I -3], considerable advances have been made in the understanding of their structure, microstructure and properties [4,5], although some questions still remain regarding the actual site for initiation of magnetization reversal. One of the efforts now is in improving the temperature dependent properties through alloying additions. These efforts are being made to offset the high temperature Mat. Res. Soc. Symp. Proc. Vol. 96. '1987 Materials Research Society
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coefficient of iHc and Br, due to the low Curie temperature,Tc, of the RE2 Fe 1 4 B phase. Thus, Dy,Tb,AI additions have led to significant improvements inthe intrinsic coercivity, while Co addition has helped in raising the Curie temperature of the RE2 Fe 14 B phase[6-12]. It has also been shown that addition of small amounts of Zr can still improve the anisotropy field[1 3]. However, heavy rare earths such as Dy or Tb decrease the magnetization due to the ferrimagnetic coupling between the rare earth atom and the Fe atoms. Thus, the effect of these additions in the case of single crystal samples, on the magnetic properties are well understood. Recently, Hirosawa et.al[1 4] have examined the relationship between iHc and HA of heavy rare earth (Dy and Tb) substituted Fe-RE-B magnets and also the influence of temperatur
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