Abstract
AbstractAmorphous metallic powders can be formed by mechanical alloying in a high-energy ball mill. Starting from the elemental, crystalline powders, ball milling first produces powder particles with a characteristically layered microstructure. Further milling leads to an ultrafine composite in which amorphization by solid state reaction takes place. The glass-forming range has been determined in detail for Fe-Zr and Ni-Zr. In Fe-Zr it differs completely from rapidly quenched amorphous samples. A systematic study of alloys of 3d transition metals with Zr and Ti shows that the glass-forming ability depends critically on a large negative free enthalpy of mixing. The results lead to the conclusion that amorphization by mechanical alloying is based on a solid state reaction and occurs under a metastable thermodynamic equilibrium neglecting the existence of intermetallic phases. Measurements of the superconducting transition temperature and Möβbauer studies show the structural similarity of mechanically alloyed and rapidly quenched amorphous samples. Finally mechanical alloying of FeZrB and NdFeB is described. Whereas FeZrB becomes amorphous after an additional annealing, a microcrystalline powder with very high coercivity is formed for NdFeB.
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