Based on the general fracture mechanics approach, the microstructure of metals plays a prominent role on ductile fracture process. The experimental results show that after ECAPed process the inhomogeneous microstructures including large elongated and fine equiaxed grains are established 39. The evolution of the microstructure of specimens subjected to 1-16 passes ECAPed processes indicates a transition from a microstructure dominated by lamellar boundaries oriented parallel to the shear direction to an equiaxed grain structure without considering previous shear planes 40. After the first pass, the microstructure mainly is encompassed of parallel lamellar boundaries with strongly elongated sub-grains, see Fig 1 (a). The lamellar boundaries are mainly low-angled, oriented parallel to the shear direction and longitudinal extended in the boundaries by several microns. Lamellar boundaries similar to dense dislocation walls are classified as geometrically necessary boundaries, since they are essential to accommodate the glide-induced lattice rotation in the adjoining volume 41.
After two passes, the fraction of the strongly elongated lamellar boundary structure with respect to the equiaxed subgrains is notably lower. Prior lamellar boundaries are often truncated by different shear planes, which lead to more homogeneous microstructure, see Fig 1 (b). This can be expressed as a microstructure consisting of domains with large angle boundaries, separating larger areas of subgrains with small misorientations. After several passes the proportion of equiaxed subgrains is predominant and the boundaries are often no longer straight, but more ridged than in the specimens subjected to fewer passes.