CRYSTAL-GROWTH OF IRON-BASED COMPOUNDS BY THE COLD-CRUCIBLE CZOCHRALSKI TECHNIQUE

Bulk single crystals of NbFe2, HoFe2 and (To/Dy)Fe-2 have been grown f rom a melt with a cold-crucible Czochralski technique. The as-grown cr ystals were characterised by X-ray diffraction, optical and scanning e lectron microscopy. The microstructural defects of the pseudo-binary ( To/Dy)Fe, crystals have been studied by transmission electron microsco py. The as-grown NbFe2 crystals with C14 hexagonal structure contain m any thermal cracks, but no secondary phases. Both X-ray lattice parame ter measurement and EDX/SEM analysis show consistent stoichiometric co mposition. However, both HoFe2 and (Tb/Dy)Fe-2 have C15 cubic Laves ph ase structure and form by peritectic reactions with a narrow separatio n between the liquidus and solidus lines. By adopting a slow growth ra te, a planar growth front has been maintained and bulk single crystals have been achieved. The as-grown crystals of (Tb/Dy)Fe-2 contain many needle-shaped Widmanstatten precipitates formed by a solid state reac tion. Microstructural study by transmission electron microscopy reveal ed the presence of many {111} plane stacking faults and a few twin bou ndaries. A large number of interfacial dislocation networks have been observed on the Widmanstatten precipitates. The presence of thermal cr acks, microstructural defects and Widmanstatten precipitates in the La ves phase compounds will strongly affect the measurement of the magnet ic properties of the crystals.