## 1 Introduction  
Traditional methods to increase the strength of Al alloys mainly include solid solution strengthening and precipitation hardening. In recent years, ultrafine-grained (UFG) and nanograined (NG) materials have proven to be effective means to strengthen aluminum alloys. Severe plastic deformation (SPD) techniques such as equal channel angular pressing (ECAP), accumulative roll bonding (ARB) and high pressure torsion (HPT) can refine aluminum alloys to UFG/NG materials [1; 2; 3; 4; 5], which can significantly improve the strength of the alloy. However, due to the low dislocation storage capacity of UFG/NG materials, the tensile ductility is often insufficient [2; 3]. In addition, the above methods have their own disadvantages; for example, it is difficult to handle large samples using ECAP and HPT and the plates are easy to separate during ARB. Therefore, to improve the processing performance and commercial applications of UFG/NG aluminum alloys, it is essential to improve their work-hardening ability while ensuring their high strength, which can help delay local deformation (necking) under tensile deformation and thus improve uniform elongation.  
At present, many methods have been used to improve the work-hardening ability of UFG/NG aluminum alloys: (i) the introduction of a bimodal structure of coarse grains and nanosized grains,in which the nanosized grains mainly enhance the strength and the coarse grains provide ductility [6; 7]; (ii) the preparation of a large number of nanotwins to accommodate additional dislocations [8; 9; 10] and (iii) the introduction of nanoparticles/precipitates to effectively prevent dislocation movement [11; 12]. Different methods have been used to build bimodal structures to improve the uniform elongation of UFG/NG materials [6; 7] but these methods are inefficient and technically difficult. In addition, tensile ductility sometimes increases at the expense of strength [10]. In contrast, the introduction of nanoparticles/precipitates into UFG/NG aluminum alloys is a potential method to achieve both high strength and good tensile ductility. For example, cryorolling combined with low-temperature aging can simultaneously improve the strength and tensile ductility of nanogenized 7075 aluminum alloys [12]. Introducing a large number of nanoprecipitates into the matrix by low-temperature reaging is conducive to the accumulation of dislocations during the tensile process, which can significantly improve the uniform elongation of nanogenized alloys. However, this rolling method needs to be carried out in a cryogenic liquid nitrogen environment and a certain time of cryogenic treatment is needed between each pass, which involves high risk and high energy consumption. Therefore, it is of great significance to improve the mechanical properties of UFG/NG materials based on existing processing methods.  
The above discussion shows that UFG/NG alloys with nanoparticles/precipitates not only exhibit enhanced strength but also possess good ductility. The main purpose of this paper is to achieve high strength and good ductility in a UFG/NG Al-Cu-Mn alloy through the design of a composite nanostructure (mainly involving UFG and NG, dislocations of specific density and high density of nanoparticle/precipitates). The composite nanostructures were prepared by a thermomechanical treatment process. The process included solution treatment, multipass cold rolling and aging. The microstructure evolution of the alloy during the process was quantitatively analyzed by confocal laser scanning microscopy (CLSM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The formation of composite nanostructures in this alloy is discussed. In addition, the relationship between the microstructure parameters and mechanical properties of the alloy is also discussed in detail.