The growth of single crystal Terfenol-D crystals
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INTRODUCTION
THErare earth iron compound known as Terfenol-D was developed ~'2'3to achieve the high magnetostriction strains of the TbFe2 compound with a reduced magnetic crystal anisotropy energy achieved by introduction of Dy. The resulting ternary alloy of composition around Tb0.3Dy0.TFe2 is expected to melt peritectically because both DyFe2 and TbFe2 are reported4,s to melt peritectically. For this and other reasons it has proven difficult to prepare large defect free single crystals of Terfenol-D. Since optimum magnetostrictive strain, A, is obtained along the (111) directions, it is desirable to be able to prepare single crystal rods with a ( l i d direction parallel to the rod axis. It is also advantageous to achieve a minimum defect concentration so that domain wall motion is not hindered by pinning mechanisms between defects and domain walls. In this paper we report on a series of experiments aimed at preparing low defect single crystal Terfenol-D. The brittleness of Terfenol-D is a strong function of the Fe stoichiometry of the compound, RFex, where R = Tb0.3Dy0.7. It has been found2'6 that as X becomes less than 2 the compound becomes less brittle. Previous work has shown that the floating zone technique produces steady state mass transport conditions;6 consequently, when X becomes less than 2 the solidifying material consists of two microconstituents, the RFe2 compound plus a rare earth rich microconstituent. The form of the rare earth microconstituent is found7 to vary within the sample; at some locations it is a eutectic mixture of RFe2 plus R metal, while at other locations it is pure R metal, which forms by a divorced eutectic reaction. The rare earth phase is interconnected throughout the crystal volume6 and, apparently, it is the presence of this ductile metal phase that reduces the brittleness below that of the pure RFe2 compound. Commercial interest for utilization of Terfenol-D favors the use of X values less than 2 in order to reduce brittleness. Therefore this work has concentrated on compositions with X in the 1.8 to 1.99 range.
J.D. ~;ERHOEVEN is Senior Metallurgist, Ames Laboratory, and Professor, Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011. E.D. GIBSON, Associate Metallurgist, O.D. McMASTERS, Metallurgist, and H.H. BAKER, Senior Research Technician, are with Ames Laboratory, Iowa State University, Ames, IA 50011. Manuscript submitted May 5, 1986.
METALLURGICALTRANSACTIONS A
EXPERIMENTAL
Samples of composition Tb0.3Dy0.yFex, with X from 1.8 to 1.99 were prepared by a Bridgman technique and a float zone (FZ) technique. In both cases the alloys were prepared from metals of the following purity, Fe-99.98 pct, Dy99.97 pct, and Tb-99.97 pct. The metals were initially alloyed together by arc melting on a water cooled copper plate, utilizing several meltings to prepare homogeneous finger-shaped ingots. Rods of aligned columnar grained material were then prepared from these fingers by the two different directional freezing techniques. In the
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