Effect of creep parameters on the steady-state flow stress of pure metals processed by high-pressure torsion

The correlation between the flow stress and grain size for severely deformed metals remains undefined because the conventional Hall-Petch relationship ignores the expected contributions from thermally-activated phenomena in nanomaterials and also it fails to explain the reported strain softening of metals having low melting temperatures. In this study, the contribution of thermally-activated creep mechanisms to the room temperature flow stress is evaluated for 31 pure metals processed by high-pressure torsion. The steady-state grain size and the hardness of these metals are first compared to theoretical predictions from high temperature creep mechanisms. It is shown that these mechanisms are not able to predict the flow stress, although the data from metals with low melting temperatures fall close to the theoretical prediction for high-temperature grain boundary sliding suggesting a possible explanation for the unusual softening of these metals. Nevertheless, a detailed analysis demonstrates that a modified model for grain boundary sliding at low temperature provides the capability of correctly predicting the flow stresses for metals having both high and low melting temperatures. The results confirm the significance of thermally-activated phenomena in determining the flow stress of nanomaterials processed by severe plastic deformation.