Correlation between the behavior of nanocrystalline HCP metals and the dislocation model for the minimum grain size obtainable by milling

Recently, a dislocation model that quantitatively relates the minimum grain size obtainable by milling, dmin, to several physical parameters, such as the activation energy for self-diffusion and the stacking fault energy, in a nanocrystalline (nc) material was developed. The development of the model was based on the suggestion that the minimum average grain size, dmin, is the result of a balance between the formation of dislocation structure and its recovery by thermal processes. The predictions of the above model were found to be in good agreement with experimental data and trends reported for nc-FCC and nc-BCC metals. In this paper, the validity of the model to the description of the behavior of nc-HCP metals for which data on the minimum grain size, dmin, are available is examined. Also, the general applicability of the model to other severe plastic deformation (SPD) processes is discussed.