TEM and XAS Characterization of Hard Magnetic Phase in Nd-Fe Alloys
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TEM and XAS Characterization of Hard Magnetic Phase in Nd-Fe Alloys G. Kumar1, J. Eckert2, W. Löser1, P. Schilling3, E. Ma4, C. Mickel1, L. Schultz1 1 IFW Dresden, Institute for Metallic Materials, D-01171 Dresden, Germany. 2 TU Darmstadt, Department of Materials and Earth Sciences, Physical Metallurgy Division, D64287 Darmstadt, Germany. 3 Department of Mechanical Engineering, University of New Orleans, New Orleans, USA. 4 Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, USA. ABSTRACT Nd80Fe20 alloys were prepared by copper mold casting and directional solidification (DS) to obtain a coarse microstructure, which can be identified unambigously. The mold-cast samples exhibit a fine globular eutectic structure and show high room temperature coercivity (~ 5 kOe). Transmission electron microscopic (TEM) investigations reveal the presence of fcc (a = 5.32 Å) Nd-rich and nanocrystalline non-cubic Fe-rich phases. The directionally solidified (DS) samples exhibit large grains (composition close to NdFe2). The NdFe2 grains (formed in the DS samples) show clear magnetic domains with uniaxial anisotropy indicating that the NdFe2-type phase observed in the present samples is not a cubic phase. X-ray absorption spectroscopy (XAS) was used to analyze the local configuration around Fe atoms in the mold-cast samples. The XAS patterns do not match with the bcc and fcc standards, which supports the presence of NdFe2 phase with uniaxial symmetry in the DS samples. INTRODUCTION Nd-Fe together with Sm-Co is the most important binary system for magnetic materials. Despite this, the formation mechanism of many metastable phases and their structure-property relationship remain unknown for eutectic Nd-Fe alloys [1-3]. The main difficulty in characterizing these alloys is the occurrence of a very fine microstructure and several metastable phases stabilized by impurities due to the high reactivity of Nd and the positive heat of mixing between Nd and Fe [3,4]. Hard magnetic properties in as-cast Nd-rich Nd-Fe and Nd-Fe-Al alloys are often correlated to the phases with unidentified structures, hitherto known as A1 and µ, respectively [3-7]. According to the binary Nd-Fe phase diagram, two stable intermetallic phases i.e., Nd2Fe17 and Nd5Fe17 are known, which are magnetically soft [2]. However, the formation of a third NdFe2 Laves-type phase under atmospheric conditions is not known with certainty due to the lack of structural data [8]. The main aim of the present work is the structural and magnetic characterization of a hard magnetic phase in hypereutectic Nd80Fe20 alloys prepared by mold casting and directional solidification. EXPERIMENTAL Prealloyed Nd80Fe20 ingots were obtained by melting nominal amounts of high purity elemental constituents in an arc-melting furnace under argon atmosphere. Cylindrical rods of 3 mm diameter were prepared by remelting the ingots in a quartz crucible and ejecting the melt
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into a copper mold. For the directional solidification experiments, the ingots were
