## 3 Results and Discussion  
The XRD patterns and optical microscopies of the alloys are presented in Figure 1. As shown in Figure 1a, the three alloys show very similar diffraction patterns, indicating that all the alloys are composed of \(\alpha\)-Al solid solution and \(\theta\)-Al\({}_{2}\)Cu phase. However, the microstructures of alloys are obviously different. The Al\({}_{83}\)Cu\({}_{17}\) alloy exhibits a typical ultrafine lamellar eutectic microstructure, in which the white \(\alpha\)-Al and black \(\theta\)-Al\({}_{2}\)Cu are arranged alternately. The lamellar spacing is about 0.6 \(\upmu\)m, as show in Figure 1b. In a previous study, [PERSON] et al. [27] reported that the lamellar spacing was 0.2-0.3 \(\upmu\)m for the Al\({}_{83}\)Cu\({}_{17}\) alloy which was only for 1 mm diameter rod-shaped samples prepared by Cu mold casting. Obviously, the cooling rate in present study is much lower than previously reported. However, the increase in lamellar spacing is not very significant, only from 0.2-0.3 \(\upmu\)m to 0.6 \(\upmu\)m, which is very beneficial to industrial production. The hypoeutectic Al\({}_{90}\)Cu\({}_{10}\) alloy shows a typical composite structure, in which white particles are uniformly embedded in the eutectic matrix. According to the EDS analysis (see Figure 2), the primary phase is enriched in Al and can be identified as the \(\alpha\)-Al phase with the average compositions of Al\({}_{92.9}\)Cu\({}_{7.1}\). The volume fraction and average grain size of the \(\alpha\)-Al phase in the Al\({}_{90}\)Cu\({}_{10}\) alloy is about 44% and 7 \(\upmu\)m, respectively. It is noteworthy that the eutectic matrix is still composed of the ultrafine lamellar \(\alpha\)-Al + \(\theta\)-Al\({}_{2}\)Cu eutectic microstructure, and the lamellar spacing is similar to the Al\({}_{83}\)Cu\({}_{17}\) alloy. For the Al\({}_{94}\)Cu\({}_{6}\) alloy, the volume fraction of \(\alpha\)-Al phase increases to 74%, and the grain morphology changes from particle to the dendrite. While the average compositions of the primary \(\alpha\)-Al phase is Al\({}_{97.8}\)Cu\({}_{2.2}\) in the Al\({}_{94}\)Cu\({}_{6}\) alloy, which obviously has a lower Cu content than that of the Al\({}_{90}\)Cu\({}_{10}\) alloy. Figure 3 presents the engineering stress-strain curves and nano-indentation load-displacement curves of the alloys. The corresponding mechanical properties are summarized in Tables 1 and 2. As shown, the Al\({}_{83}\)Cu\({}_{17}\) alloy exhibits the highest hardness and tensile strength, which are 2.5 GPa and 758 MPa, respectively, because of its completely ultrafine lamellar eutectic structure. However, the Al\({}_{83}\)Cu\({}_{17}\) alloy fails catastrophically without any plasticity. In the previous study [27], the Al\({}_{83}\)Cu\({}_{17}\) alloy with finer lamellar spacing also fails in a brittle manner under the room compressive tests. This brittle fracture is mainly due to the lack of a work hardening mechanism, resulting in highly localized deformation. Figure 1: (**a**) X-ray diffractometry (XRD) patterns of alloys and their optical microscopy (OM) micrographs of the (**b**) Al\({}_{83}\)Cu\({}_{17}\), (**c**) Al\({}_{90}\)Cu\({}_{10}\) and (**d**) Al\({}_{94}\)Cu\({}_{6}\). The up-insets in (**b–d**) show their magnified views of the corresponding microstructures. When some primary \(\alpha\)-Al particles are precipitated on the ultrafine lamellar eutectic matrix as in the case of Al\({}_{90}\)Cu\({}_{10}\) and Al\({}_{94}\)Cu\({}_{6}\), the alloys present lower tensile strengths (Table 1). In general, like most composites, we proposed that the deformation behaviors of these ultrafine lamellar eutectic composites should follow a rule-of-mixtures relationship. According to the nano-indentation analysis, the hardness and E of the ultrafine eutectic matrix in the Al\({}_{90}\)Cu\({}_{10}\) and Al\({}_{94}\)Cu\({}_{6}\) alloys are almost the same as in Al\({}_{90}\)Cu\({}_{10}\) and Al\({}_{94}\)Cu\({}_{6}\) alloys.