Consideration of the mechanical data on nanocrystalline materials showed the presence of a critical grain size, dc, such that, for d > dc, the strength increased with decreasing grain size, and for d smaller than dc, strength decreased with decreasing grain size, i.e., nanoscale softening occurred. By combining conventional Hall–Petch behavior for larger grains and a new deformation process based upon dislocation-accommodated boundary sliding for smaller grains, it was shown that nanoscale softening could be predicted and that the critical grain sizes dc for nanocrystalline Cu and nanocrystalline Ni were 25 and 13nm, respectively. These results were analyzed with regard to reported experimental data on the metals and to the values for dc estimated using other models and computer simulations.

Interpretation of Nanoscale Softening in Terms of Dislocation-Accommodated Boundary Sliding. F.A.Mohamed: Metallurgical and Materials Transactions A, 2007, 38[2], 340-7