Investigation of the Microstructural Evolution During Large Strain Cold Working of Metals by Means of Synchrotron Radiation—A Comparative Overview

Abstract

Dislocation densities, arrangements and long range internal stresses in cold rolled polycrystalline Cu, Ni, and Al, as well as in cold torsioned Fe, were determined by X-ray diffraction profile analysis using synchrotron radiation. At different deformation degrees, scanning measurements across single grains with a focal spot of less than 50 μm were carried out, in order to inform on the features of the deformation induced substructure. At small deformations including stage III, the dislocation densities and internal stresses are uniform within single grains while at higher deformations in stages IV and V, the dislocation densities and long range internal stresses exhibit opposite fluctuations. In stage IV these fluctuations correspond to the formation of polarized tilt walls (PTW’s) from polarized dipole walls (PDW’s). In contrast to the PDW’s, the PTW’s cause a much higher misorientation in between adjacent lattice areas. This transformation, which is the main element of the progressing fragmentation process during large strain deformation, occurs in all metals studied regardless of dislocation mobility and/or lattice type. Approaching higher strains in stage V, however, these parameters gain some importance especially when the deformation occurs in an iterative way. If the mobility is high, marked static recovery takes place between the single deformation passes, which results in decreases of both dislocation density and local internal stress, and no formation of PTW’s is observed.

Gradient plasticity constitutive model reflecting the ultrafine micro-structure scale: the case of severely deformed copper

“…Such behaviour was observed using in situ deformation in synchrotron radiation experiments. The cell walls were shown to change their structure from polarised dipole walls to polarised tilt walls, which allows storing of a higher dislocation density (Zehetbauer et al, 2002). It is also possible that part of those dislocations residing in the cell interiors is accommodated in the walls, which explains the decrease and increase of the respective dislocation densities under ECAP straining up to 12 passes.…”

Grain Size, Misorientation, and Texture Evolution of Copper Processed by Equal Channel Angular Extrusion and the Validity of the Hall–Petch Relationship

“…[22] It is suggested that these processes occur simultaneously along with the increase in misorientation angle among subgrains, which is again part of the recovery mechanism involving dislocations in the cell wall, which rearrange themselves in order to screen the long-range stress field. [11,46] While the observation of 1-to 4-lm large grain aggregates (Figure 4) after N = 16 does not significantly change the mean grain size value due to the small number of such large grains, a non-negligible volume fraction may influence the final mechanical properties. Indeed, the lower YS measured in N = 8, 12, and 16 specimens may be also explained by the presence of such recovered deformation substructures.…”

“…In contrast to these observations, XRD peak broadening analysis has led to a description of the microstructure [9,10,11] based on the coherent scattering of domains. Because no coherency results from X-rays scattered from different cells, the microstructural features with the shortest average spacing between misorientations are measured.…”

“…High-resolution XRD via synchrotron radiation allows for a further distinction between cells, which with increasing deformation reflects a transformation of cell walls with a Burgers vector parallel to the cell walls (polarized dipole wall) to cell walls with perpendicular Burgers vectors (polarized tilt wall) and, thus, qualitatively yields an estimate of the degree of misorientation of cells. [11] A more straightforward definition of the microstructural features is derived by EBSD measurements. There, grains and subgrains can be differentiated using their misorientation angle, which is >15 deg (or HAGBs) and <15 deg (or low-angle grain boundaries (LAGBs)), respectively.…”

Analysis of the Deformation Behaviour of Cu Processed by High Pressure Torsion

“…The growth of the dislocation density in the cell walls should lead to some qualitative changes in the structure of the lattice . M. Zehetbauer and others [18] made a supposition about a gradual transformation of dipole walls into tilted ones. The process is accompanied by an increase of misorientations between neighbouring cells (Fig.…”