We propose and develop a model as an attempt to describe the local mechanism of mechanical alloying. This model is based on the observation of global parameters, such as the volumes of the various phases present in the material, and their transformations during the process. The model is applied to milled Fe-Cu and Fe-N experimental results obtained in previous works. In the milling of Fe-Cu system, four stages, whose features are in accordance with experimental observation, are identified:(i) energy storage; (ii) start of the reaction and speed up of it; (iii) decrease of the reaction rate and (iv) stationary stage. No energy storage stage is observed and no saturation is achieved in the milling of Fe-N system. Moreover, we propose a modification on the global kinetic law introduced by Matteazzi et al. based on the different nature of Fe-Cu and Fe-N systems. This modification allows the extension of the validity of this law.