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Particle fracture and phase transformations induced by electrical discharge assisted mechanical milling

Calka A., Wexler D.

Inżynieria Materiałowa

2004

R. XXV, nr 3

465-468

A novel device for milling, incorporating high voltage, low current electrical discharges was constructed and its application for materials processing investigated [1, 2]. This type of milling has been found to result in rapid fracture rates, enhanced mechano-chemical reactions and novel reaction paths. We present recent studies of fracturing, agglomeration and phase formation using this method applied to a vibrational rod mill. The effect of spark discharge milling condition on particle size and surface morphology was investigated for a number of different materials including; alumina, NiZr, Ni plus Si, Mg-Zn alloy of eutectic composition. In these experiments, variations in milling vibrational amplitude resulted in variations in nominal average spark length for samples discharged milled under repeated impact. It was confirmed that during discharge milling, rapid fracturing occurs wervery short milling times and is accompanied by the formation of both fine particles and agglomerates. Large vibrational amplitudes tended to promote increased particle agglomeration in both ceramics and metals, while discharge milling with lower vibrational amplitudes promoted the formation of finer particles and smaller agglomerates. In the case of alumina, particle coarsening and spheroidisation was believed to result from repeated sintering of individual particles. For metals, alloys and metallic glasses, the tendency for coarsening and formation of spherical particles resulted from some combination of partial melting and deformation. Spark milling of Mg-Zn eutectic decomposition product was found to result in formation of the metastable eutectic phase, Mg7Zn3.

Electrical discharge assisted mechanical milling in materials processing: formation of nano-structures in carbon and boron.

Całka A., Wexler D.

Archives of Materials Science

2004

Vol. 25, nr 4

333-342

A novel device for ball milling, incorporating high voltage, low current electrical discharge was constructed and its application for material processing investigated. Recent results of investigations involving spark discharge milling of carbon and boron under various atmospheres and electrical conditions are presented and discussed. Samples including activated carbon, graphite, and crystalline boron powder were given electrical discharge milling treatment under Ar and N2 atmospheres. A range of nano-fragments was observed using transmission electron microscopy. Discharge milling of activated carbon in Ar, with small amounts of Fe powder, was found to promote the formation of multi-walled carbon nanotubes, while milling of the same activated carbon in nitrogen lead to the formation of carbon encapsulated iron nanoparticles. Experiment performed on graphite with Fe, discharge milled in Ar revealed the formation of both graphite nanoparticles and rolled-sheet graphite nanostructures. Milling of boron powder under a high voltage discharge was found to lead to the formation of fragmented particles of boron and unusual boron feather-like nanorods.