Study of Boundary Free Tb-Dy-Fe Magnetostrictive Materials
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183 Mat. Res. Soc. Symp. Proc. Vol. 360 01995 Materials Research Society
levitation cold crucible in CZ technique, has been developed to manufacture bulk boundary free Tb-Dy-Fe single crystals[ 41. The relations between the microstructure and growth conditions will be discussed in this paper. EXPERIMENTAL Samples of composition TbxDy1-xFel.95-2.0, with x from 0.27 to 0.32, were prepared by the magnetic levitation cold crucible in CZ technique. Investigations were focused on the relations between their crystallized structure and growth conditions. Favorable growth parameters were also explored. Large bulk boundary free Tb-Dy-Fe single crystals (see Figure 1) have been prepared and its microstructure has been investigated by X-ray diffraction and backscattering techniques.
Figure 1. CZ Tb-Dy-Fe single crystal. RESULTS AND DISCUSSION
RFBe Laves Phase Tb-Dy-Fe pseudobinary phase diagram[5 ] (see Figure 2) shows that RFe 2 Laves phase (LP) could only be achieved over an extremely narrow region. In addition the TbxDylxFe2 pseudobinary compound was reported to melt peritectically. During the crystallization process, the solidification front will grow easily from the melt alloy by dendrite or cellular crystals to form a twinned sheets structure. The second phase such as R (rare earth rich phase) will appear between the sheets. Hence to optimize the kinetic conditions of crystaflized front and grow with flat front instead of dendrite or cellular front is a promising method to eliminate the defects formed in the microstructure of TbxDyl-xFe2 alloys. It has been found that the twinned sheets structure could appear in the alloys when their crystallized rate were too fast(see Figure 3), even though their compositions were just inside the Laves phase region. This phenomenon could be explained as the moving rate of solid and liquid interface was too fast such that a dendrite growth front formed during the crystallized procedure. The situation is quiet similar to what happens in the procedure of
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structure of CZ Tb-Dy-Fe pure phase is not the same as the formal RFe 2 Laves phase structure. A superlattice phenomenon, which may cause a lower symmetry, was found for the first time by X-ray crystallographic analysis in the CZ Tb-DyFe single crystal. But the detailed information has not yet been obtained through the experiments until now. Further researches will be carried on the detail crystallographic structure of the superlattice, and the relationship between structure and performances of the single crystal in the future work. Magnetostrictive Performances The pure Laves phase have been obtained by eliminating boundaries in CZ Th-Dy-Fe single crystal. Hence its domain walls will move more easily than in polycrystal or oriented crystals That makes the CZ single crystal have better magnetostrictive behavior than the polycrystal or oriented crystal with the same composition, especially for its lower field magnetostriction. For instance direction TbO.3 DY0 .7 Fel.9 5 boundary free single
crystal exhibits the performence shown
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