Limit of dislocation density and ultra-grain-refining on severe deformation in iron

It is well-known that severe deformation to metals causes a direct grain refinement of the matrix without special heat-treatments due to the mechanism of dynamic continuous recrystallization (DCR). However, the microstructural revolution during severe deformation is seemed to be different depending on the deformation mode, namely the direction of deformation. In general, multi-directional deformation is thought to be effective for the grain refinement caused through DCR. For instance, ultra-grain-refinement to 10nm has already achieved by mechanical milling treatment in a steel powder and hardness of the mechanically milled steel powder is increased to around HV12GPa by such a marked grain refinement. On the other hand, hardness of iron never exceeds HV4GPa by the mode of uni-directional deformation such a conventional cold rolling. In this paper, a limit of dislocation density is discussed for iron which is severely strained by the mode of uni-directional deformation, and also the importance of multi-directional deformation on DCR will be mentioned in association with a significant work hardening behavior in mechanically milled iron powder.