High-pressure torsion of iron with various purity levels and validation of Hall-Petch strengthening mechanism

Impurity atoms have a significant effect on the strength of metals processed by severe plastic deformation (SPD), but their strengthening mechanism is still under argument. To gain an insight into the strengthening mechanism, iron samples with different purity levels such as 99.96% (IF steel), 99.94% (Armco steel), 99.88% and 97.78% and with different initial states (bulk, powder and ball-milled) were processed by high-pressure torsion (HPT). The steady-state hardness and tensile strength for the materials with the micrometer and submicrometer grain sizes reasonably followed the Hall-Petch relationships reported earlier for pure iron and mild steels. However, the nanograined materials followed an inverse Hall-Petch relationship. It was shown that the occurrence of softening by the inverse Hall-Petch effect can be significantly avoided by stabilizing the grain boundaries using carbon atoms. These findings indicate that the extra hardening by impurity atoms is mainly due to the grain-boundary strengthening mechanism.