**3. Results**

#### *3.1. Characterization of as-Cast Structure*

The extremely high cooling rate during ACZ alloy solidification caused the formation of a favorable microstructure (Figure 2a) characterized by small size of dendritic cells, fine eutectic particles of Ca-containing phases and full dissolution of Zr in Al solid solution. The measured average size of the dendritic cells is ~4 μm (Figure 2b) which, according to other studies [22,29,30], corresponds to a cooling rate of ~10<sup>4</sup> K/s. Calcium-bearing eutectic particles corresponding to the quaternary eutectic (Al) + Al4Ca + Al10CaFe2 + Al2CaSi2 [26] are detected in the form of thin veins located along the boundaries of the aluminum dendritic cells. It should be noted that the as-cast structure does not contain needle-shaped inclusions, for example, iron-containing ones.

Remelting of EMC rod followed by pouring into a graphite mold (cooling velocity about 20 K/s) leads to coarsening of structure and the formation of some needle-like particles (Figure 2c). On the other hand, the structure of the alloy after slow solidification (in a furnace) differs considerably from that of the initial EMC rod. Along with the expected general coarsening of the structure, the phase composition of the alloy changes. In particular, needle-shaped Al3Fe phase inclusions and segregation of primary Al3Zr phase crystals (Figure 2d) that are absent in the EMC rod structure can be identified (Figure 2a,b) after slow solidification.

**Figure 2.** As-cast microstructure of alloy ACZ after various cooling velocities, SEM: (**<sup>a</sup>**,**b**) ~10<sup>4</sup> (12 mm EMC rod, R in Table 2), (**c**) ~20 K/s (10 mm × 40 mm × 200 mm ingot), (**d**) ~0.1 K/s (cooling in furnace).

#### *3.2. Effect of Cold Deformation and Annealing on Structure, Hardness and Electrical Conductivity*

Due to the fine structure of the eutectic, the as-cast EMC rod possesses high ductility, even during cold forming. During both rolling and drawing, apart from the formation of a fibrous grain structure, fragmentation of Ca-containing eutectic particles can also be observed. Their size is not greater than 1 micron and they are uniformly distributed in the aluminum matrix (Figure 3a,b). Annealing at up to 450 ◦C inclusively does not lead to a significant change in the size of Ca-containing eutectic particles. However, the structure remains non-recrystallized. Coarsening of particles is observed at higher temperatures (Figure 3c,d).

As expected, cold deformation leads to a hardening of the ACZ alloy—the hardness increases up to 65HV for strip and up to 70 HV for wire (states R and W, respectively, see Table 3). A surprising result is a significant increase in the electrical conductivity—up to ~ 25 MS/m (both for strip and wire). In an earlier study for a Ca-free Al-0.6% Zr-0.4% Fe-0.4% Si alloy obtained by a similar process, this effect was not observed [22].

According to EMC rod hardness data obtained during annealing (Figure 4a), hardening reaches the highest level at 450 ◦C annealing stage temperature (the R450 state). Further increase in the annealing temperature leads to a significant decrease in HV, which is mainly due to the coarsening of the Al3Zr precipitates [5–11,16]. Deformation hardening was retained upon strip annealing to 450 ◦C. At this temperature, the hardness of the EMC rod is the same as for the cold rolled strip (Figure 4a). Further increase in the annealing temperature leads to a significant softening due to the formation of a recrystallized structure. At the maximum annealing temperature used, 600 ◦C, they had approximately the same

hardness (32–33 HV) due to the coarsening and transformation of the Al3Zr precipitates to the equilibrium D023 phase [6,15].

**Figure 3.** Microstructure of wire alloy ACZ in (**<sup>a</sup>**,**<sup>c</sup>**) 3 mm wire and (**b**,**d**) 2 mm strip: (**a**), as-drawn, (**b**) as-rolled, (**<sup>c</sup>**,**d**) annealed at 600 ◦C, SEM, (**a**) W, (**b**) S, (**c**) W600, (**d**) S600, designations see Table 2.

**Table 3.** Mechanical and electrical properties of ACZ wire alloy.


Decomposition of the aluminum solid solution with the formation of L12 (Al3Zr) nanoparticles during annealing promotes the increase in electrical conductivity (EC), as shown in Figure 4b. At the same time, the difference between the EC values for the EMC rod and for the 2 mm strip remains approximately the same at all annealing temperatures up to 500 ◦C inclusively (2.5–3.0 MS/m). At this temperature, the maximum EC for a 2 mm strip is reached, but it corresponds to the softening stage (Figure 4a). Considering the maximum hardness and high electrical conductivity in the cold rolled strip after annealing at 450 ◦C, we used the same heat treatment for the prepared wire.

**Figure 4.** (**a**) Hardness (HV) and (**b**) electrical conductivity (EC) curves for EMC rod (R) and cold rolled strip (S) subjected to stepwise annealing in accordance with the processing route shown in Table 2 (see R (temperature)/S (temperature) designation).

TEM microstructure of the ACZ alloy was examined for cold rolled strip in the S350 ◦C (Figure 5a,b) and S450 ◦C (Figure 5c,d) states to confirm the main structure changes described. According to obtained data for the both states, the fine individual particles of the eutectic Ca-containing intermetallics (dark in appearance) with a less than 1 μm size can be detected at sub-grain boundaries confirming their high pinning ability. However, detailed analysis revealed very few subtle particles in the S350 ◦C state (Figure 5b) which can be attributed to the initial stage of L12-Al3Zr phase formation. This assumption meets well with the electrical conductivity data presented in Figure 4b. In contrast, for the S450 ◦C state an exceptionally high number density and uniform distribution of the L12-Al3Zr phase nanoparticles are detected (Figure 5d,e). The result obtained suggests almost complete decomposition of the aluminum solid solution which converges well with data on the maximum electrical conductivity (Figure 4b) and hardness of the EMC rod (Figure 4a).

$$\bf{(a)}$$

(**b**)

**Figure 5.** *Cont*.

**Figure 5.** TEM structure of wire alloy ACZ (strip) and after annealing according to (**<sup>a</sup>**,**b**) S350 ◦C and (**<sup>c</sup>**–**<sup>e</sup>**) S450 ◦C regimes, (**<sup>a</sup>**–**d**) bright field, (**e**) dark field and diffraction patterns.

(**e**)

#### *3.3. Properties of Wire*

Mechanical and electrical properties of the wire ACZ alloy in as-drawn and annealed states are given in Table 3. In the initial state, the alloy has a good combination of strength (UTS ~ 280 MPa and YS ~ 250 MPa) and ductility (El ~ 4%), however, the electrical conductivity is small (~43% IACS). Annealing of the 3 mm wire according to the modes given in Table 2 allows one to increase the EC value significantly. In this case, the EC values are approximately the same as for the cold rolled strip (Figure 4b). This suggests that the decomposition of (Al) proceeds in a similar way. As can be seen from Table 3, the experimental alloy in the W450 state has the best combination of strength (UTS ~ 200 MPa and YS ~ 180 MPa), elongation (El ~ 12%) and electrical conductivity (54.7% IACS).

Fractography of wire samples after a tensile test revealed a fine-dimpled ductile structure of the fracture surface (Figure 6). Calcium-bearing particles found inside the dimples (Figure 6a,b, BSE mode) are much smaller than the average diameter of the dimples. It should also be noted that no oxides or nonmetallic inclusions were observed which meets well with a previous study [22] confirming melt refining tendency when using EMC technology.

(**b**) 

**Figure 6.** Fracture surfaces of experimental wire alloy (**a**) in initial (W) and (**b**) annealed (W450) states, SEM, **left**—backscattered electron image, **right**—secondary electron image, SEM.
