Gradient Structure in High Pressure Torsion Compacted Iron Powder

Iron powder was compacted successfully into a solid disk by a severe plastic deformation process - the high pressure torsion (HPT) - at room temperature. The compaction was done in two steps: first axial compaction, then shear deformation by rotating the bottom part of the HPT die while maintaining the axial force constant. The homogeneity of shear strain across the thickness of the disk was examined by local strain measurement, showing a gradient distribution. The strain gradient leads to the formation of three regions throughout the thickness of the disk: Region U, that is undeformed in torsion (top part - in contact with the fixed punch), Region T, that is a transition or intermediate region where the shear strain is increasing, and Region S, which is severely deformed (bottom part - in contact with the rotating die) which is uniformly and heavily sheared. The observation of microstructure and Vickers hardness in the three regions showed a non-uniform distribution, corresponding to the strain gradient. Crystallographic texture measurements confirmed the presence of a typical shear texture with increasing strength in Regions T and S. Stress-strain curve was obtained from the local shear strain and microhardness measurements. The strain hardening curve of the HPT-compacted pure iron showed higher flow stress than the torsion deformed solid pure iron (IF steel).