*3.1. The Grain Refinement during SPD and Annealing*

Extreme grain refinement was observed in the deformed hybrid samples, especially in the outer copper cladding where friction added to the severity of the shear strain. The microstructure, after two ECAP passes (Route BC), followed by annealing, is shown in Figure 2. Due to SPD, the grain size in the aluminium rod was reduced to 410 ± 150 nm and in the copper sheath to 180 ± 90 nm. The annealing did not cause any grain growth in the copper as the temperature–time schedules were chosen to be within the region of thermal stability suggested for high purity copper in [13]. The presence of Fe precipitates decorating the grain boundaries in the aluminium also prevented grain growth in the aluminium rod. Therefore, the annealing resulted in recovery of the dislocations within the ultrafine grains of both constituents of the hybrid material and formation of a clean (free of dislocation within the grain interior) ultrafine microstructure with sharp boundaries.

**Figure 2.** SEM images (back-scattered electrons) of the microstructure after two ECAP passes (Route BC) and annealing. (**a**) Aluminium core; (**b**) copper sheath.
