Deformation mechanisms in ultrafine-grained metals with an emphasis on the Hall–Petch relationship and strain rate sensitivity

Ultrafine-grained materials display almost no strain hardening, an enhanced strain rate sensitivity and grain boundary offsets during plastic deformation. It is expected that dislocation climb is active in order to enable prompt recovery. The present analysis proposes a deformation mechanism that includes these effects and follows from the mechanism for high temperature grain boundary sliding. This mechanism predicts the relationship between strain rate, flow stress, grain size, temperature and basic material properties such as the Burgers vector modulus, the shear modulus and the grain boundary diffusion coefficient. The model may be used to estimate the final grain size achieved by severe plastic deformation and the strain rate sensitivity. An analysis shows that the predicted behavior agrees with the data from multiple experimental investigations and provides a good estimate of the Hall–Petch slope for different materials which includes breakdown and inverse Hall–Petch behavior under some conditions. The incorporation of a threshold stress provides an opportunity to predict the relationship between flow stress and grain size for a broad range of grain sizes, strain rates and temperatures. An excellent agreement is observed between the predictions of the model and experimental data for Al, Cu, Fe (α), Fe(γ), Mg, Ni, Ti and Zn.