Mechanical alloying (MA) is a non-equilibrium processing route for the synthesis of metastable phases. The technique has been widely used for solid state amorphization. It has been observed that the mechanical alloying process can lead to amorphization in systems exhibiting both positive and negative heat of mixing. As the process involves energizing the system, we discuss how the system uses this input energy for the synthesis of metastable phases. The technique is independent of the starting condition of the material being milled, like elemental powders or intermetallic compound. Hence, such an equilibrium which is forced onto the system is termed as "dynamic equilibrium". We also discuss the different forms in which the system can store part of the input energy. Simple calculations have been performed to estimate the individual contributions to the non-chemical energy. The calculations show that the grain boundary energy (-5000 J/mol) is the dominant one. Thus, the system can store maximum energy in the form of grain boundaries with long time milling. This leads to the generation of grain boundaries and results in the refinement of grain size. A model for the grain size refinement has been proposed. The refinement in grain size can be explained as a combination of deformation, fracturing, folding and rotation.