Toward a quantitative understanding of the mechanical alloying process

The mechanical alloying behavior of transition metal-based mixtures and the amorphization of equilibrium intermetallic compounds by milling were characterized on a quantitative basis as a function of the processing parameters. Different kinetic mechanisms rule the two processes. The former giving rise to a sigmoidal-shaped trend, while the latter is characterized by a continuous, gradual decrease of the conversion rate. It was found that the extent of the microstructure refinement determines the overall transformation rates and that the mechanical energy dose required to transform a given amount of the parent phases is a characteristic invariant quantity of the system undergoing amorphization. The atom fluxes at the impact, the sublayer distance affected, and the dimension of the amorphous domains formed were also determined. The emerged framework supports the view that diffusive phenomena and chemical driving forces play a minor role in the mechanical alloying processes. Conversely, the thermodynamic qualities of the reacting systems and the enthalpy gain remain the main factors in mechanically induced combustion reactions, provided a critical dispersion of the reactants is reached under milling.