A quantitative description of the amorphisation behaviour by mechanical alloying

In the belief that further progress requires an adequate characterisation of both the milling dynamics and the reactive behaviour of mechanically induced co-deformation processes, we attempted to gain a quantitative description of the transformation behaviour as a function of the energy intensity of the mechanical treatment. Structural evolution of pure transition metals, binary mixtures and intermetallic compounds was studied. Sigmoidal curves characterised the conversion of elemental mixtures leading to either amorphous or crystalline phases, while a progressively decreasing transformation rate was observed in the case of the disordering behaviour of intermetallics. The variation of the milling intensity only affected the reaction rates and the transformation processes were found to be isokinetic as a function of the mechanical work performed on the system. A macrokinetic model was developed in order to relate the experimental findings to the impact sequence registered in the course of die milling treatment. Based on a statistical approach, the model provides a set of differential equations solutions to which describe the evolution of the product phase in terms of conventional rate expressions. The model allows one to reproduce the different kinetic behaviours by means of a single, unifying mathematical formalism. The experimental trends for both the comminution and phase transformation processes are well reproduced and the conventional best-fitting procedures permits to quantify the conversion rate through suitable kinetic constants, related to the fraction of the powder charge actually cold-worked at each impact. The rate constants are found to depend linearly on the average impact energy. In the case of Ti-based alloys with metals belonging to the first transition period, their values co-relate with the Poisson's ratio of the metal partner of Ti.