The strengthening mechanism due to copper (Cu) particles was discussed in terms of the interaction between dislocation and Cu particles in aged Fe-Cu alloys. Since Cu particles are softer than the iron matrix, its interaction with dislocation is different from that with the Orowan mechanism. The moving dislocations can cut the soft Cu particles and pass through them when the bowing angle reaches some critical value (q c ), and the precipitation strengthening due to Cu particles is expressed as a function of mean particle spacing (l) and the q c (p/2 gives Orowan stress). The q c increased with increasing the size of Cu particles and reached p/2 when the Cu particle size became 70 nm. This means that the precipitation strengthening due to Cu particles is dependent on not only l but also the Cu particle size, and 70 nm is the minimum Cu particle size for obtaining the Orowan stress.

The limit of dislocation density was investigated by means of mechanical milling (MM) treatment of an iron powder. Mechanical milling enabled an ultimate severe deformation of iron powder particles and dislocation density in the MM iron powder showed the clear saturation at around the value of 1016m-2. On the other hand, the relation between hardness and dislocation density was examined in cold-rolled iron sheets, and the linear Bailey-Hirsch relationship; HV[GPa]=0.7+3×10-8ρ1/2 was obtained in the dislocation density region up to 3×1015m-2. Extrapolation of the Bailey-Hirsch relationship indicated that the dislocation strengthening should be limited to about 3.7GPa in Vickers hardness which corresponds to about 1.1GPa in 0.2% proof stress.

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The microstructure development during plastic deformation was reviewed for iron and steel which were subjected to cold rolling or mechanical milling (MM) treatment, and the change in strengthening mechanism caused by the severe plastic deformation (SPD) was also discussed in terms of ultra grain refinement behavior. The microstructure of cold-rolled iron is characterized by a typical dislocation cell structure, where the strength can be explained by dislocation strengthening. It was confirmed that the increase in dislocation density by cold working is limited at 10 , which means the maximum hardness obtained by dislocation strengthening is HV3.7 GPa. However, the iron is abnormally work-hardened over the maximum dislocation strengthening by SPD of MM because of the ultra grain refinement caused by the SPD. In addition, impurity of carbon plays an important role in such grain refinement: the carbon addition leads to the formation of nano-crystallized structure in iron.

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