Macrosegregation in Bridgman growth of Terfenol-D and effects of annealing

Giant magnetostrictive material Terfenol-D (TbxDy1-xFe2-y, 0.27 less than o r equal to x less than or equal to 0.30, 0.05 less than or equal to y less than or equal to 0.1) was directionally solidified by vertical Bridgman met hod within a sealed quartz crucible. Macrosegregation of RE elements along the growth axis occurred in an abnormal way. RE elements concentration decr eased significantly with increasing axial distance, which is different from the typical macrosegregation with a distribution coefficient, ii, less tha n unity. This was due to the compositional inhomogeneity of the initial mel t. During the melting of the prealloyed ingot in the quartz crucible, eutec tic RE-rich phase melted first and flowed out into the gap between the cruc ible and the ingot. This RE-rich phase which flowed to the bottom did not m ix well with the melt by convection because the temperature distribution in the melt is quite stable and the density of the RE-rich phase is higher th an that of the REFe2 phase. Macrosegregation was reduced very much in the r od grown after the homogenization of the initial melt. Even though the magn etostrictive strain was very different from part to part in the as-grown ro d due to the macrosegregation, each specimen came to have a similar magneto striction after the heat treatment above the eutectic temperature (890 degr ees C). During the high-temperature annealing, excess RE-rich phase was obs erved to be melted and flowed out from the specimen leaving pores between R EFe2 matrices. Therefore, composition of the crystal approached to the stoi chiometic value regardless of the initial composition. That makes magnetost riction of the samples similar to one another. Melt extraction during the a nnealing is thought to be the result of poor wetting between the two phases . (C) 2000 Elsevier Science B.V. All rights reserved.