Co-deformation of Al and Ti by accumulative roll bonding (ARB) with intermediate heat treatments is utilized to prepare multi-layered Ti/Al sheets. These sheets show a high specific strength due to the activation of various hardening mechanisms imposed during deformation, such as: hardening by grain refinement, work hardening and phase boundary hardening. The latter is even enhanced by the confinement of the layers during deformation. The evolution of the microstructure with a special focus on

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... rain refinement and structural integrity is traced, and the correlation to the mechanical properties is shown. involves non-common deformation techniques, such as the application of severe plastic deformation (SPD) [7] [8] [9] [10] [11] or co-deformation of at least two different materials [12] [13] [14] [15] [16] . Co-deformation of two metallic materials, such as Al and Ti, bears the potential to gain improved material properties regarding strength and formability. Both material properties are strongly linked to the microstructure. Their evolution including the formation of texture in these materials represents the key aspect in developing materials with superior properties. In recent years, it has been shown that cold working to large plastic strains can impose ultra-fine grained microstructures [17] [18] [19] or even microstructures with features being in the nanometer range [20] [21] [22] . The small grain sizes, fine dislocation networks and/or small precipitates cause a significant contribution to the mechanical strength according to work and precipitation hardening, respectively [8, [23] [24] [25] . When applied in combination, these hardening mechanisms may even have a synergetic strength effect. When co-deformation is applied repetitively, as is the case for accumulative roll bonding (ARB) [26-28] and for accumulative swaging and bundling (ASB) [16, [29] [30] [31] [32] [33] , the density of phase boundaries increases with applied bonding or bundling cycles, respectively. The increasing number of phase boundaries is beneficial for two reasons. First, the phase boundary itself represents a barrier for dislocation movement [34] . Secondly, these phase boundaries limit possible grain growth. After deformation to very large plastic strains, the multi-phased materials have a mean phase boundary distance in the the sub-micrometer-range. These materials also show considerable strength at elevated temperatures when grain growth occurs in at least one phase, as grain growth is limited to the distance of the phase boundaries. Summarizing, high densities of grain and phase boundaries significantly contribute to the strength of metallic composites. Although it has already been shown that Ti/Al composite sheets can be obtained by ARB [35] [36] [37] [38] [39] [40] , this process remains difficult. A crucial issue is to retain individual continuous layers within the composite during accumulative deformation. Once the Ti layers are strain hardened beyond a certain limit, their formability is negligible when compared to the Al sheets. Consequently, necking of the Ti layers is observed, and the stretched Ti pieces remain rather stable in size within the continuously-deforming Al matrix [36, [41] [42] [43] . In contrast to this, the individual Ti and Al layers remain stable when an intermediate heat treatment (IHT) is applied between the rolling cycles. After eight ARB + IHT cycles, a fine multi-layered composite sheet is obtained. With respect to grain refinement, IHT is counter-productive, as it may cause recovery, recrystallization and grain growth. Consequently, a highly strengthened light weight material cannot be achieved with this procedure. In addition, it is necessary to emphasize that the individual layers become wavy with increasing deformation strain, i.e., the number of ARB cycles, which has been considered as a consequence of the evolution of the texture within the layers [44, 45] . To avoid necking, a high work hardening rate and a low strength level are necessary. During the first ARB cycle, these conditions are met, but already for the second cycle, the work hardening ability of Ti is already saturated and, thus, may be responsible for the necking of the Ti layers. An annealing treatment at 723 K for 90 min under vacuum conditions between each ARB cycle lowers the strength, and the strain hardening ability is restored. Optimum strain hardening conditions would possibly be restored above the transus temperature of Ti. However, this is not possible to apply to the composite, as this temperature is above the melting temperature of Al. At elevated temperatures, successful roll bonding of Al can be achieved using a lower thickness reduction. It can be expected that this would result in reduced necking of the Ti layers. However, the reduction of strength with rising temperature is less pronounced in Ti than in Al. Alternatively, the arrangement of rolls and the geometry of the rolling gap can be altered. In contrast to a four high rolling mill, which is symmetric with respect to the sheet plane, trio rolling with different sizes of the upper and lower rolls is asymmetric. Even in the case when the excenter velocities of the upper and lower rollers are identical, the difference in diameter causes shearing in the deformation zone and, thus, eases bonding and reduces necking. This second possibility of controlling the homogeneity framework of the European Centre for Emerging Materials and Processes (ECEMP), Contract No. 13795/2379. Author Contributions: Jan Romberg and Jens Freudenberger designed the experiments, collected and interpreted the data, wrote and edited the paper and contributed to all activities. Juliane Scharnweber and Andy Eschke contributed to scanning electron microscopy, including sample preparation, imaging and diffraction; together with Carl-Georg Oertel, they analyzed the results and fostered their interpretation. Hansjörg Bauder, Georg Plattner and Hans Krug performed the accumulative roll bonding experiments; they contributed to the discussion and interpretation of the results and adapted the rolling mill to the present requirements. Frank Holländer performed the hot rolling experiments. Hansjörg Klauß performed the tensile tests.