Inverse Hall–Petch effect in quasi- and nanocrystalline materials

• The origin of grain size softening/inverse Hall–Petch (HPE) effect is traced.
• Data concerning one quasicrystalline and two nanocrystalline materials are analyzed.
• The equations needed for the analysis are taken from our earlier publications.
• The agreement between the predictions and experimental results is good.

Inverse Hall–Petch effect/grain size softening in quasi- and nanocrystalline materials at ambient/low temperatures is attributed to mesoscopic (~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.