Formation of Amorphous and Nanostructural Powder Particles from Amorphous Metallic Glass Ribbons Using Ball Milling and Electrical Discharge Milling

In this paper both electric discharge assisted milling [1, 2] and conventional mechanosynthesis techniques were applied to investigate the effects of milling conditions on the fracture and agglomeration of amorphous CoSiB ribbons produced by planar flow casting. The effect of spark energy on particle shape and size produced by discharge milling was studied. Conventional milling in inert atmosphere for extended periods generally leads to the formation of porous powder particle aggregates, each particle comprised of small amorphous or, after extended milling times, nanocrystalline elements. The mechanism of agglomeration was believed to originate from repeated fracture, deformation and cold welding of individual ribbon elements. In contrast to conventional milling, spark discharge milling was found to induce the formation of predominantly sub-micron single particles of amorphous powder. The morphology of individual particles varied from sub-micron irregular shaped particles to remelted particles, depending on selection of vibrational amplitude during discharge. For high vibrational amplitudes and high energy input a wider range of particles as produced. These included sub-micron particles, remelted particles and welded agglomerates, and nano-sized particles produced as a fume and collected during discharge milling under flowing argon. These results combined with observations that most re-melted particles produced by discharge milling were also amorphous confirmed that extremely high heating and cooling rates are associated with discharge milling of metals. They also confirm the potential of electrical discharge milling as a new route for the synthesis of ultrafine and nanosized powder particles from amorphous ribbon, for possible processing into 3-D shapes.