**4. Conclusions**

EBSD analysis confirmed that the development of UFGs by ARB process is carried out via several mechanisms at different levels of strain. Grain subdivision as well as development of sub-grains is the major mechanism at the early stages of ARB. Strain induced transition of low angle to high angle grain boundaries and the formation of a thin lamellar structure occurs at the medium levels of strain. Finally, the progressive fragmentation of these thin lamellar structures into more equi-axed grains is the dominant mechanism at relatively high strains which leads to reduction of grain size to less than 500 nm. The presence of second phase particles in AA3003 aluminum sheets promotes the grain refinements and accelerates the occurrence of dynamic recovery. With an increasing number of cycles, the overall texture intensity increases and a very strong texture is developed which exhibits a limited number of sharp components. Large lattice rotation around these particles leads to the increase of local misorientation, evolution of a much weaker texture and development of a more homogeneous submicrometre grain structure in the AA3003 alloy. The Dillamore and the copper components are the main texture components of this material and they are sharpened with increasing the number of cycles. This texture evolution could be modeled with the Alamel crystal plasticity model and taking into account the specific geometry of the ARB process.

Generally, with increasing the number of cycles, the intensity of the α fiber decreased while the intensities of the β and τ fibers increased. It is shown that in the early stages of the ARB process the intensity of the β fiber, running from copper over S to the brass orientation, is almost homogeneous, but with increasing the number of cycles the intensity of the copper component gradually grows at the expense of the brass and S components.

It is demonstrated that due to incremental deformation in ASB process, high value of strain rate without considerable temperature rise is applied, leading to a high degree of Zener– Hollomon parameter, as a characteristic of this SPD process. The grain structure of commercially pure aluminum is refined by this process and an average grain thickness and length of 186 and 419 nm are developed after the four cycles of ASB, respectively. TEM micrographs and SAD patterns as well as the EBSD analysis indicate that this grain refinement occurred by formation of subgrains at early stages of severe deformation followed by increase of the misorientations by transition of low angle to high angle grain boundaries at the next stages.

The characterization of mechanical properties revealed that the strength of the aluminum sheets considerably increased by the first two ARB cycles which is attributed to work hardening caused by increase in dislocation density and formation of subgrains. It was also included that the strength of ARBed aluminum sheets obeys the Hall–Petch relationship and corresponds well with microstructural variations. Microhardness distributions across the thickness of the tubes illustrate that because of high equivalent strain at outer regions, the hardness is increased from the inner regions to the outer ones. Due to periodical entrance of the external layer within the thickness and the consequent hardness saturation, the hardness and its homogeneity is increased with increase of the ASB cycles. The yield and tensile strengths of the material are significantly increased up to the values of 194 and 235 MPa, respectively. The scheme of hardness development leads to decrease of the ratio of tensile strength to yield strength and consequently to decrease of the uniform elongation.
