It was recalled that predominant among the processing techniques that were now available for synthesizing bulk nanocrystalline materials was ball milling, which produced nano-structures via the structural decomposition of large-grained structures by severe cyclic deformation. It was well-known that, during milling, the grain size decreased with milling time; reaching a minimum grain size, dmin, which was characteristic for each metal. A dislocation model was developed here that predicted the value of dmin as a function of material parameters such as hardness, melting point and stacking-fault energy. The model was based upon the concept that dmin was governed by a balance between the hardening rate introduced by dislocation generation and the recovery rate arising from dislocation annihilation and recombination. It was demonstrated that the model provided possible explanations for several recent observations regarding the characteristics of dmin.

A Dislocation Model for the Minimum Grain Size Obtainable by Milling. F.A.Mohamed: Acta Materialia, 2003, 51[14], 4107-19