## 3 Results  
The aluminium phase, the Al\({}_{8}\)Cu\({}_{4}\)Er phase in the fine eutectic, and the Al\({}_{3}\)Er phase were identified by SEM and X-ray diffraction analyses (Figure 1). During solidification, 1.6%Cu, 0.9%Mn, 0.3%Zr, and 0.2-0.3%Er were dissolved in the (Al) solid solution according to the point electron diffraction X-ray (EDX) analysis in SEM. As shown by the map element distribution (Figure 1a), the Cu-, Er-, and Mn-rich phases are presented in the as-cast microstructure. Unidentified peaks (marked with \"?\" in Figure 1b) were found in the XRD patterns. These peaks were not found in the XRD patterns of the alloy with the same composition without Mn addition (grey line in Figure 1b). These peaks probably correspond to the quaternary (Al,Cu,Mn,Er) phase. The atomic rotation of Cu/Mn/Er in this phase is 4/2/1 in accordance with point EDX SEM results. The possible compound Al\({}_{25}\)Cu\({}_{4}\)Mn\({}_{2}\)Er may match the quaternary (Al,Cu,Mn,Er) phase. The same morphology has an Al\({}_{15}\)(Fe,Mn)\({}_{3}\)Si\({}_{2}\) eutectic phase [22; 24]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Element** & **Al** & **Cu** & **Er** & **Mn** & **Zr** \\ \hline nominal & bal. & 5.0 & 3.4 & 0.8 & 0.3 \\ experimental & bal. & 5.0 & 3.2 & 0.9 & 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the alloy, wt %. A homogenization temperature of 605 \({}^{\circ}\)C was chosen to be in accordance with the solidus temperature of 614 \({}^{\circ}\)C (results of DSC analysis (Figure 1c)) of the investigated alloy. Figure 2 illustrates the evaluation of the phase size, morphology, and concentration of copper in the aluminium solid solution. The Al\({}_{8}\)Cu\({}_{4}\)Er eutectic phase fragmentized and grew from 1.8 \(\upmu\)m after 1 h of annealing to 2.6 \(\upmu\)m after 3 h of annealing (Figure 2). The quaternary (Al,Cu,Mn,Er) phase did not change in size or morphology. The maximum copper content of 2.2% in the aluminium solid solution was achieved after 3 h of annealing due to the dissolving of the non-equilibrium part of the copper content intermetallic phases. The investigated alloy was quenched after 3 h of homogenization treatment and aged at 150, 180, and 210 \({}^{\circ}\)C. The hardness Vickers (HV) vs. time curves of the aged alloy are presented in Figure 3. The hardness slightly increases during aging at the indicated temperatures due to a low content of copper in the aluminium solid solution. The copper content in the aluminium solid solution in the commercial alloys is usually about 4-5% [22,24,25]. Figure 4 demonstrates the precipitate types in the aged alloy at 210 \({}^{\circ}\)C for 5 h after quenching. Three types of precipitates were identified in the thin structure by TEM investigation via EDX analyses. Two types, Al\({}_{20}\)Cu\({}_{2}\)Mn\({}_{3}\) and Al\({}_{3}\)(Zr,Er), were nucleated during homogenization treatment at 605 \({}^{\circ}\)C and 9\({}^{\circ}\)(Al\({}_{2}\)Cu) during aging treatment at 210 \({}^{\circ}\)C. The Al\({}_{20}\)Cu\({}_{2}\)Mn\({}_{3}\) phase has a length of 100-250 nm and a width of 70-120 nm. The average diameter of the Al\({}_{3}\)(Zr,Er) precipitate phase is 35 nm. The rotation of Zr/Er in the Al\({}_{3}\)(Zr,Er) phase is about 1/1 in accordance with point EDX analyses. The metastable disc shaped 9\({}^{\circ}\)(Al\({}_{2}\)Cu) precipitates with a thickness of 5 nm and a diameter of 100-200 nm were mostly nucleated on the Al\({}_{3}\)(Zr,Er) precipitate phases (Figure 4). High resolution TEM image in Figure 4 demonstrates the 9\({}^{\circ}\)(Al\({}_{2}\)Cu) and Al\({}_{3}\)(Zr,Er) phases. Figure 1: (**a**) As-cast microstructure (SEM), (**b**) XRD patterns of the investigated alloy (black line) in comparison with the Mn-free alloy (grey line), and (**c**) DSC curves. Figure 4: Microstructure of the annealed alloy at 605 \({}^{\circ}\)C for 3 h and the aged alloy at 210 \({}^{\circ}\)C for 5 h, and the EDX spectra from Al\({}_{3}\)(Zr,Er) and \(\theta^{\circ}\)(Al\({}_{2}\)Cu) particles (TEM).