**4. Discussion**

When comparing the basic characteristics of the new ACZ alloy and the previously studied Al-0.6% Zr-0.4% Fe-0.4% Si alloy [22] obtained under the same conditions, it can be seen that at close values of strength and electrical conductivity, the heat resistance of the Ca-containing alloy is significantly higher (450 vs. 400 ◦C) despite a smaller content of Zr (0.5 vs. 0.6 wt.%). Indeed, the hardness of the Ca-free alloy decreases down to 40 HV after annealing at 450 ◦C, which is significantly lower than that for the new alloy with calcium (Table 3). Obviously, the reason for the increase in the heat resistance is that the ACZ alloy contains calcium. Therefore, study of the effect of this element deserves special consideration. Considering that zirconium forms only the Al3Zr phase (stable and metastable modifications), to understand the distribution of calcium, iron and silicon between phases, it is necessary to consider the Al–Ca–Fe–Si system. Earlier [26], the structure of this phase diagram in the range of Al-Ca alloys (concentration of Ca much higher than Fe and Si) was reviewed but other fields (with small Ca content) were not studied. Using the calculation in the Thermo-Calc software [31], as well as the results of previously published works [22,24–26,30,32] and additional experiments, we proposed structures of the Al–Ca–Fe–Si system in the aluminum corner including distribution of phases in the solid state (Figure 7a) and polythermal projection (Figure 7b). According to the proposed version, this system contains five four-phase regions: I—(Al) + Al4Ca + Al10CaFe2 + Al2CaSi2, II—(Al) + Al10CaFe2 + Al2CaSi2 + Al3Fe, III—(Al) + Al2CaSi2 + Al3Fe + Al8Fe2Si, IV—(Al) + Al2CaSi2 + Al8Fe2Si + Al5FeSi, V—(Al) + Al2CaSi2 + Al5FeSi + (Si). From the distribution shown in Figure 7a it follows that the Al2CaSi2 phase is present in all regions of this quaternary system. Considering the low solubility of Ca in (Al), this means that even at a small amount of calcium in Fe and Si containing alloys, the formation of this particular ternary compound is inevitable. Excess calcium should lead to the formation of the Al4Ca and Al10CaFe2 phases. The influence of calcium on the equilibrium phase composition of the Al-0.5% Fe-0.25% Si alloy (i.e., with the same concentrations as in the ACZ alloy) is reflected in Table 4. The calculation results show that the phase composition of the base ternary alloy has a very high sensitivity to the calcium content. In this case, the ACZ alloy must certainly be in region I. However, for nonequilibrium solidification, the phase composition can differ greatly from the equilibrium one. This is largely due to the occurrence of incomplete peritectic reactions. As follows from the polythermal projection (Figure 7b), there are three invariant peritectic and three eutectic reactions in the aluminum corner of the Al–Ca–Fe–Si system. The compositions of the liquid phase and the temperatures of these reactions are given in Table 5.

**Figure 7.** Phase diagram of Al–Ca–Fe–Si system in aluminum corner: (**a**) distribution of phases in in the solid state and (**b**) polythermal projection. Point **1** corresponds to the chemical composition of the ACZ alloy.


**Table 4.** Effect of Ca concentration on phase composition of alloy Al-0.5% Fe-0.25% Si (calculated at 200 ◦C).

**Table 5.** Calculated parameters of invariant reactions in the Al–Ca–Fe–Si system.


1 In quasi-ternary section.

As follows from the polythermal projection (Figure 7b), in a quaternary alloy containing 0.8% Ca, 0.5% Fe and 0.25% Si (point 1 in Figure 7b), after primary crystallization of (Al), eutectic reactions L→(Al)+Al3Fe and L→(Al) + Al3Fe + Al2CaSi2 (line E3–P1), and then peritectic L + Al3Fe→(Al) + Al10CaFe2 + Al2CaSi2 (point P1) one should proceed. The incompleteness of the latter explains the presence of needle-like particles in the microstructure of the slowly solidified ACZ alloy (Figure 2d). With an increase in the cooling rate, the phase boundaries shift towards higher iron content (dashed line in Figure 7b), therefore the Al3Fe phase is not formed and it is absent in the as-cast microstructure of the EMC rod (Figure 2a,b). The solidification of this quaternary alloy (and hence the ACZ alloy) should end via eutectic reactions with the formation of three Ca-containing phases (point P1). To further confirm the proposed structure of the Al–Ca–Fe–Si system, several quaternary alloys were annealed at 600 ◦C. As can be seen from Figure 8a, in the alloy containing 0.8% Ca, 0.5% Fe and 0.25% Si, only globular particles are detected, which, according to the distribution maps for Ca (Figure 8b), Fe (Figure 8c) and Si (Figure 8d) elements, can be identified as Al4Ca, Al10CaFe2 and Al2CaSi2.

To confirm the effect of cold deformation on the electrical conductivity of the ACZ alloy mentioned above, additional studies on the effect of calcium on EC of both as-cast ingots and cold rolled sheets containing 0.5% Fe and 0.25% Si at varying calcium concentrations (Table 1) were carried out (Figure 9). As can be seen from Figure 9, the difference between the ingot and cold rolled sheet is small at small Ca content but at 0.75–1% Ca it reaches ~2 MS/m, i.e., similar to the ACZ alloy (Figure 4b). Taking into account the low solubility of Ca in (Al) and the invariability of the phase composition during deformation, this effect can probably be caused by the influence of the dislocation structure, vacancies and other defects of the crystal structure. This requires special study. A slight increase in EC should be noted with the addition of 0.1% Ca to the ternary alloy. This can be explained by a decrease in the concentration of Si in (Al) due to the formation of Al2CaSi2 compound.

**Figure 8.** Microstructure of alloy Al-1% Ca-0.5% Fe-0.25% Si (ingot 10 mm × 40 mm × 200 mm) after annealing at 600 ◦C: (**a**) SEM, (**b**–**d**) electron microprobe analysis (EMPA) mapping (**b**—Ca, **c**—Si, **d**—Fe).

**Figure 9.** Electrical conductivity of Al–Ca–Fe–Si alloys containing 0.5% Fe and 0.25% Si vs. calcium content curves: R—EMC rod, S—cold rolled strip.
