Thermodynamic model for solid-state amorphization of pure elements by mechanical-milling

Mechanical-milling induced solid-state amorphization (SSA) has been observed in some covalent metalloids (Si, Ge, Se), but never in pure metals at room temperature. In this paper, a thermodynamic model was proposed for the SSA of the pure elements. In the model, a large Gibbs free energy of a grain boundary, compared with the free energy of a crystalline–amorphous interface, provides driving force for the amorphization. The critical grain size required for the SSA, therefore, can be determined from the condition when the Gibbs free energy of the nanocrystalline state is equal to that of amorphous state. The predicted critical grain sizes for SSA generally agree with the experimentally observed values for nonmetallic elements, but are much smaller than the minimum grain sizes of the metallic elements obtained by long-time mechanical-milling.