*3.2. Al-Ca-Mn-Ni Alloys*

In a prior study [12], Al-Ca-Ni alloys were reported with a ternary Al9CaNi compound primary, crystals of which have a compact shape. It was assumed that nickel could improve the complex physical and mechanical properties of Al-Ca-Mn ternary alloys. Accordingly, Al-8Ca-2Mn-(1-4)Ni alloys were studied in detail (Table 1). The calculated liquidus projection of the Al-Ca-Mn-Ni system at 8% Ca is shown in Figure 6. Some physical and mechanical properties of the as-cast alloys are presented in Table 3.

**Figure 6.** Calculated liquidus projection of the Al-Ca-Mn-Ni system at 8% Ca.

**Table 3.** Calculated phase transformation temperatures, hardness and electrical conductivity of the as-cast Al-Ca-Mn-Ni alloys.


The Al-8Ca-2Mn-1Ni alloy solidified in the furnace ambience exhibits two types of primary crystals including grey-coloured Al4Ca platelets and compact bright Al10CaMn2 crystals (Figure 7a). The latter contains a minor amount of nickel which likely substitutes manganese. In addition, ternary [(Al)+Al4Ca+Al8Ca(Ni,Mn)2] and binary [(Al)+Al4Ca] eutectics are observed in the structure. Due to the increase in the mean atomic number the Ni-rich phase is brighter in the BSE image as compared to other phases. EMPA revealed that some Mn dissolves in the Al8CaNi2 phase. In the furnace cooled Al-6Ca-3Mn-1Ni alloy, the primary Al8Ca(Ni, Mn)2 crystals have a compact shape and linear size of less than 10 μm (Figure 7b). Apart from the primary crystals, the presence of two binary eutectics [(Al)+Al4Ca] and [(Al)+Al8Ca(Ni,Mn)2] is detected.

**Figure 7.** SEM images showing the primary crystals appeared in the as-cast Al-Ca-Mn-Ni alloys: (**a**) Al-8Ca-2Mn-1Ni (slow cooled); (**b**) Al-6Ca-3Mn-2Ni (slow cooled); (**c**) Al-8Ca-2Mn-1Ni (10 K/s); (**d**) Al-6Ca-3Mn-2Ni (10 K/s); (**e**) Al-8Ca-2Mn-2Ni (slow cooled); (**f**) Al-8Ca-2Mn-4Ni (10 K/s).

The structure of the Al-8Ca-2Mn-1Ni alloy formed at a cooling rate of 10 K/s (Figure 7c) represents a perfect natural composite and therein equiaxed small (no more than 10–15 μm) Al10Ca(Mn,Ni)2 ternary compound particles are evenly distributed on the superfine eutectic background. The latter contains about 8% Ca and 0.8% Mn. Similarly, the Al-6Ca-3Mn-2Ni alloy structure consists of the eutectic and two types of primary crystals established to be Al10Ca(Mn, Ni)2 of compact angular shape and Al6(Mn,Ca,Ni) of coarse acicular shape (Figure 7d and Table 4). It is apparent that if comparing two alloys, the Al-8Ca-2Mn-1Ni alloy has the most favourable structure as it contains fine primary crystals of less than 10–12 μm uniformly distributed in the ultrafine eutectic matrix. In the alloy Al-8Ca-2Mn-2Ni, the Al10Ca(Mn, Ni)2 primary crystals appeared along with bright Al8Ca(Ni, Mn)2 equiaxed crystals, having visible facets (Figure 7e). Their amount increases with Ni content. In Al-8Ca-2Mn-3Ni and Al-8Ca-2Mn-4Ni alloys manufactured at a cooling rate of 10 K/s (Figure 7f), the structure consists of dispersed eutectic and uniformly distributed compact crystals of two types including grey Al10Ca(Mn, Ni)2 and light Al8Ca(Ni, Mn) (Table 4).

**Table 4.** Composition of primary crystals according to EMPA analysis.


For assessing processability at deformation the experimental Al-6Ca-3Mn, Al-8Ca-2Mn, Al-8Ca-2Mn-(1-4)Ni alloys were chosen as their microstructure best fits the NMMC. Before hot rolling, the ingots of all selected alloys, as well as the FM180 ingot, were annealed at a temperature of 500 ◦C for 3 h to improve ductility. While the FM180 alloy has been established as a casting alloy, some research reported its wrought processing. Novel Al-Ca hypereutectic alloys are expected to be uni-materials, a good choice either for casting or rolling. After annealing a substantial decrease in hardness and an increase in EC value are both due to sufficient change in eutectic particles shape.

During annealing, the fine eutectic-origin intermetallics acquire a rounded shape, the hardness of the alloys reasonably decreases and the electrical conductivity increases (Table 5). It is prominent that the FM180 alloy showed less response to spheroidisation due to the coarser eutectic structure.


**Table 5.** Hardness and electrical conductivity of the alloys in the annealed condition.

Furthermore, the Al-6Ca-3Mn, Al-8Ca-2Mn and FM180 ingots of 15 mm in thickness were rolled at 480 ◦C on a duo-mill 260. The rolling was carried out at a 10% reduction for each pass. In 11 passes, the Al-6Ca-3Mn and Al-8Ca-2Mn alloys reached 2.06 and 2.22 mm in thickness without any breaks and other defects (Figure 8a). The FM180 was rolled down to 3.3 mm after 10 passes and broke down during the 11th pass (Figure 8b). While the FM180 alloy is a foundry alloy, hot stamping is also applied for manufacturing pistons [20,21]. Nevertheless, the ductility of this material appeared to be insufficient for obtaining quality hot-rolled sheet products. The Al-8Ca-2Mn-(1-4)Ni alloys were rolled at 500 ◦C. As for the Al-8Ca-2Mn-(2-4)Ni samples, they showed transverse cracks after the first two passes. Conversely, the Al-8Ca-2Mn-1Ni alloy was rolled down to 3 mm in thickness without any visible cracks including those on the edge surfaces (Figure 8c). The primary intermetallics in all Ca-bearing alloys do not change their size and shape after deformation but become redistributed and aligned in the rolling direction. When it comes to the FM180 alloy, multiple cracks appeared in the silicon particles (Figure 9). The Al-6Ca-3Mn and Al-8Ca-2Mn hot-rolled sheets were annealed at 500 ◦C for 1 h and next subjected to cold rolling down to a thickness of 0.5 mm.

**Figure 8.** Hot-rolled sheets: (**a**) Al-6Ca-3Mn; (**b**) Al-8Ca-2Mn; (**c**) Al-8Ca-2Mn-1Ni; (**d**) FM180.

**Figure 9.** SEM images showing the microstructures of the hot-rolled sheets (arrows indicate the rolling direction): (**a**)—Al-6Ca-3Mn (longitudinal direction); (**b**)—Al-8Ca-2Mn (longitudinal direction); (**c**)—Al-8Ca-2Mn-1Ni (transversal direction); (**d**)—FM180 (longitudinal direction).

Then, the tensile test specimens were cut from Al-6Ca-3Mn and Al-8Ca-2Mn hot-rolled and cold-rolled sheets and a Al-8Ca-2Mn-1Ni hot-rolled sheet. Additionally, unbroken parts of the FM180 hot-rolled sheet were taken for testing. Mechanical and some physical properties of the sheets are given in Table 6. In terms of mechanical properties in the hot-rolled state, the quaternary Al-8Ca-2Mn-1Ni alloy is superior as compared to others.


**Table 6.** Physical and mechanical properties of the hot-rolled and cold-rolled sheets.

#### *3.3. Coefficient of Thermal Expansion of the Ternary and Quaternary Alloys*

The alloys with the most favourable microstructure were selected for thermal expansion studying at 25–500 ◦C. According to [22], the CTE value of a binary hypereutectic Al—18%Ca alloy in the temperature range of 20–100 ◦C is 13.8 × 10−<sup>6</sup> 1/◦C. It is known from the literature that in the temperature range of 30–200 ◦C, the CTE value of the permanent mould cast hypereutectic Al alloys containing 17–19%Si is in the range 20·10−6–22 × 10−<sup>6</sup> 1/◦C [23]. As is shown in Table 7, Ni alloying slightly changes CTE value in the temperature range 20–100 ◦C. Meanwhile, the Al-8Ca-2Mn and Al-8Ca-2Mn-1Ni alloys after 400 ◦C exhibit a decrease in CTE down to (19–20) × 10−<sup>6</sup> 1/◦C.


**Table 7.** The CTE values of the alloys exhibited the most favourable microstructure.
