Interpretation of Nanoscale Softening in Terms of Dislocation-Accommodated Boundary Sliding

Consideration of the mechanical data on nanocrystalline (nc) materials shows the presence of a critical grain size, d _c , such that for d larger than d _c , strength increases with decreasing grain size, and for d smaller than d _c , strength decreases with decreasing grain size, i.e., nanoscale softening occurs. By combining conventional Hall–Petch behavior for larger grains and a new deformation process based on dislocation-accommodated boundary sliding for smaller grains, it is shown that nanoscale softening can be predicted and that the critical grain sizes d _c for nc-Cu and nc-Ni are 25 and 13 nm, respectively. These findings are discussed with reference to the experimental data reported for both metals and the values for d _c estimated from other models and computer simulations.