Faculty of Science, Department of Physics, Bijenięka 32, 10002 Zagreb, Croatia
High energy mechanical ball milling, which
includes mechanical alloying (MA) and mechanical grinding (MG), has been
extensively used in the last 20 years as a non-equilibrium processing method
analogous to the previous interest in very rapid solidification cooling
(rates from 106 to 1010 K/s) for synthesizing, at room temperature, all
kinds of materials.
Both mechanical alloying of dissimilar powders and mechanical grinding of single composition powder have resulted in a wide variety of entirely new metastable alloys including amorphous alloys, nanocrystalline materials, extended solid solutions, alloys from elements with a widely different melting point, all sorts of compounds and composites and otherwise difficult to process novel materials, and even those alloys which are immiscible by conventional processing methods.
The crystallite size of MA and MG materials decreases rapidly with milling time to reach a saturation value, generally in the range from 1 to 30 nm, attributing the name nano-sized materials to MA and MG end products.
During the milling process the interaction between milling balls and powder particles can be characterized by processes like cold-welding, plastic deformation and further fragmentation of the particles. A high defect structure of the lattice, the immense magnification of the boundary surface and high diffusion rate leads to low activation energies for the transformation of the structure.
The current state of the mechanism of the metastable phase formation during milling in systems which exhibit a negative heat of mixing of the components will be given from the thermodynamic and kinetic viewpoints. On the other hand, although phase formation in systems with positive heats of mixing is far from being understood, a possible explanation will also be given.
Various application areas where the MA technology could be used will be illustrated as well.