Inverse Hall-Petch effect in quasi- and nanocrystalline materials

Inverse Hall-Petch effect/grain size softening in quasi- and nanocrystalline materials at ambient/low temperatures is attributed to mesoscopic (similar to a grain diameter or more) grain/interphase boundary sliding controlled flow. Equations for estimating the free energy of activation for the rate controlling process, the free volume fraction present in a basic sliding unit and the average number of grain boundaries that align to form a planar interface during superplastic deformation are given in some of our earlier papers. These predictions are verified here using experimental data pertaining to one quasi- and two nanocrystalline systems. The agreement between the predictions and the experimental observations is satisfactory. (C) 2014 Elsevier B.V. All rights reserved.