278 M.Richert et al.Materials Science and Engineering A260 (1999)275-283 1mm ED Fig.4.Macrostructure in the longitudinal section at different accumulated strains:(a)3.6:(b)4.:5;(c)22.5:(d)30.6.The extrusion direction is marked ED [12]. 3.3.Microstructure (TEM) crobands (MBs)[12].The DDW/MBs bound cell blocks containing ordinary dislocation cells.Within The microstructure at low strain (s=0.9)shown in many of the cells dislocation tangles can be observed. Fig.5 is a characteristic deformation structure with The banded structure observed by TEM shows no extended Dense Dislocation Walls (DDWs)and Mi- relationship to the banded structure observed optically278 M. Richert et al. / Materials Science and Engineering A260 (1999) 275–283 Fig. 4. Macrostructure in the longitudinal section at different accumulated strains: (a) 3.6; (b) 4.;5; (c) 22.5; (d) 30.6. The extrusion direction is marked ED [12]. 3.3. Microstructure (TEM) The microstructure at low strain (o=0.9) shown in Fig. 5 is a characteristic deformation structure with extended Dense Dislocation Walls (DDWs) and Mi￾crobands (MBs) [12]. The DDW/MBs bound cell blocks containing ordinary dislocation cells. Within many of the cells dislocation tangles can be observed. The banded structure observed by TEM shows no relationship to the banded structure observed optically