Directional solidification and heat treatment of terfenol-D magnetostrictive materials
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I.
INTRODUCTION
STUDIES by Clark and coworkers I~'2j have established that the cubic Laves-phase compounds RFe2, where R = a rare earth element, exhibit very large magnetostrictive strains. The most useful of these materials from the standpoint of energy efficiency is the compound called Terfenol-D, TbxDyl_xFey, where x ~ 0.3 and y ~ 1.95. This material can exhibit magnetostrictive strains approaching 2000 ppm with relatively low hysteresis losses and has many potential applications, particularly for sonar transducers. Optimum magnetostrictive properties are found for single crystals, and since the easy axis of magnetization for Terfenol-D is (111), it is desirable to produce single-crystal rod having such an axial orientation, but this goal has proven to be very difficult. In the present study, techniques have been developed to produce grain-aligned rods of Terfenol-D by directional solidification techniques. Previous work I3j has established that the dendrites of Teffenol-D grow with a sheet morphology having growth directions of (112) with {111} planes parallel to the dendrite sheet plane. It was also found that directionally solidified material tended to develop elongated grains having a (112) direction parallel to the growth axis. The present work was undertaken to develop directional solidification techniques which would produce relatively large rods (diameters > ~2.5 cm) with lengths > ~-25 cm having grains elongated along the entire length of the rods. This paper presents the magnetostrictive properties of the elongated polycrystalline material and compares them to those of single-crystal rods prepared by float zone melting (ZM) techniques, as well as to bi- and tricrystals prepared by the ZM technique. J.D. VERHOEVEN, Professor, Department of Materials Science and Engineering, and Senior Metallurgist, Ames Laboratory, E.D. GIBSON, Associate Metallurgist, Ames Laboratory, and J.E. OSTENSON, Associate Physicist, Ames Laboratory, are with Iowa State University, Ames, IA 50011. O.D. McMASTERS, General Manager, is with Edge Technologies Inc., ETREMA Division, Ames, IA 50010. Manuscript submitted October 13, 1989. METALLURGICAL TRANSACTIONS A
In the course of these studies, it was found that the magnetostrictive properties could be significantly improved by a simple heat treatment. Experiments are presented here which attempt to establish why the heat treatment is effective and also determine its relative effectiveness in single vs polycrystalline materials.
II. D I R E C T I O N A L S O L I D I F I C A T I O N DEVELOPMENT The directional solidification was carried out by a method which involves a bottom pouring technique. A hole was drilled into the bottom of a container crucible, and a closed-end alumina thermocouple protection tube was inserted into the hole from above to act as a stopper rod, as shown in Figure 1. The Terfenol-D alloy was first prepared by arc melting the elements together into fingers of the desired composition in an inert atmosphere on a water-cooled copper hearth. Several of these fin
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