## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c} \hline
**Composition** & \(\sigma_{y}\) **(MPa)** & \(\sigma_{b}\) **(MPa)** & \(\epsilon_{p}\) **(\%)** \\ \hline Al\({}_{83}\)Cu\({}_{17}\) & - & 758 & - \\ \hline Al\({}_{90}\)Cu\({}_{10}\) & 392 & 571 & 2.0 \\ \hline Al\({}_{94}\)Cu\({}_{6}\) & 306 & 472 & 7.4 \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of yield strength (\(\sigma_{y}\)), tensile strength (\(\sigma_{b}\)) and tensile plastic strain (\(\epsilon_{p}\)) of Al-based alloys. \begin{table}
\begin{tabular}{c c c} \hline
**Structure** & **H (GPa)** & **E (GPa)** \\ \hline Eutectic (\(\alpha\)-Al + \(\theta\)-Al\({}_{2}\)Cu) & \(2.5\pm 0.2\) & \(86\pm 2\) \\ \hline \(\alpha\)-Al in Al\({}_{90}\)Cu\({}_{10}\) alloy & \(1.8\pm 0.2\) & \(64\pm 3\) \\ \hline \(\alpha\)-Al in Al\({}_{94}\)Cu\({}_{6}\) alloy & \(1.3\pm 0.2\) & \(40\pm 1\) \\ \hline \end{tabular}
\end{table} 
Table 2: Hardness (H) and elastic modulus (E) of different structures in alloys, measured by nano-indentation. Figure 3: (**a**) Engineering tensile stress-strain curves of alloys, and (**b**) nano-indentation load-displacement curves of the ultrafine lamellar eutectic matrices and \(\alpha\)-Al phase in alloys. Figure 2: The SEM-EDS analysis of \(\alpha\)-Al phase in alloys Al\({}_{90}\)Cu\({}_{10}\) (**a**) and Al\({}_{94}\)Cu\({}_{6}\) (**b**). same as that of the Al\({}_{83}\)Cu\({}_{17}\) alloy, due to the similar compositions and lamellar spacing of ultrafine eutectic matrix in the three alloys. While the hardness and E of the \(\alpha\)-Al phase in the Al\({}_{90}\)Cu\({}_{10}\) and Al\({}_{94}\)Cu\({}_{6}\) alloys are much lower than that of the ultrafine eutectic matrix (Table 2). Therefore, the yield strength of the ultrafine lamellar eutectic composites Al\({}_{90}\)Cu\({}_{10}\) and Al\({}_{94}\)Cu\({}_{6}\) obviously decreases with the precipitation of the \(\alpha\)-Al phase. As expected, the precipitated \(\alpha\)-Al phase can enhance the ductility of alloys. The Al\({}_{90}\)Cu\({}_{10}\) alloy exhibits obvious work hardening and 2% tensile plastic strain, while the plastic strain of the Al\({}_{94}\)Cu\({}_{6}\) alloy is significantly improved to 7.4%. Obviously, the volume fraction of the primary \(\alpha\)-Al phase has significant effects on the plasticity of the composites. For high enough volume fractions, the precipitated \(\alpha\)-Al dendrites not only bear more deformation in themselves, but also connect with each other and form a continuous network distribution of \"hand-in-hand\", which are beneficial to suppress the local instability propagation of shear bands and micro-cracks. In previous studies, [PERSON] et al. [28] also found that there was a critical content of 40 vol% primary soft dendrites, beyond which the ductility of the La-based BMG composites escalates rapidly. Moreover, as show in Figure 3b and Table 2, the hardness and E of the \(\alpha\)-Al phase in the Al\({}_{94}\)Cu\({}_{6}\) alloy are much lower than that of the Al\({}_{90}\)Cu\({}_{10}\) alloy, indicating it has much higher ductility and stronger work hardening ability. Under the process of deformation, the primary \(\alpha\)-Al phase with higher work hardening ability can promote the redistribution of stress and avoid excessive stress concentration, thus delaying the plastic instability and obtaining larger plasticity. [PERSON] et al. [29] also found that with the same \(\alpha\)-Al volume fraction and size, the tensile plasticity of the Cu-Al alloys with bimodal structures was significantly enhanced by increasing the work hardening ability of the micron-scale primary phase. These results indicate that the microstructure (volume fraction, size and morphology) and micromechanical properties (hardness, E and work hardening ability) of the micron-scale primary phase have a significant effect on the plasticity of the ultrafine lamellar eutectic composite. More quantitative analysis and discussion on the relationships between the microstructure, micromechanical features and macroscopic properties should be further studied in the future.