Description of the New Eutectic Al-Ca-Cu System in the Aluminum Corner

Abstract

The structure of the new ternary eutectic Al-Ca-Cu system considered as a replacement for the ternary eutectic system Al-Ce-Cu widely used for additive manufacturing has been studied using experimental techniques. The liquidus projection of the Al-Ca-Cu system in the aluminum corner has been suggested based on experiential studies of the microstructure and phase composition of model alloys. The suggested structure of the diagram has two quasi-binary sections: (Al)-Al₂₇Ca₃Cu₇ and (Al)-Al₈CaCu₄ and three invariant eutectic transformations: L→(Al) + (Al,Cu)₄Ca + Al₂₇Ca₃Cu₇ (at 5.6 wt.% Ca, 4.5 wt.% Cu, 595 °C), L→(Al) + Al₂₇Ca₃Cu₇ + Al₈CaCu₄ (at 2.2 wt.% Ca, 13.5 wt.% Cu, 594 °C) and L→(Al) + Al₈CaCu₄ + Al₂Cu (at 0.5 wt.% Ca, 34 wt.% Cu, 544 °C). The limit solubility of copper in aluminum solid solution (Al) at 530 °C reaches ~5.1 wt.% in the ternary phase field (Al) + Al₈CaCu₄ + Al₂Cu and drops to ~2.4 wt.% in the (Al) + Al₈CaCu₄ + Al₂₇Ca₃Cu₇ ternary phase field. For the example of the model ternary hypoeutectic alloys with a predominant content of the eutectic (Al,Cu)₄Ca phase, it has been shown that the system is promising for designing new eutectic-type alloys with a natural composite structure.

1. Introduction

Al-Cu-based alloys are widely used in load-bearing applications at room and relatively high temperatures (up to 225 °C) due to their good strength-to-weight ratio and toughness after artificial aging [1,2,3]. However, the wide solidification range typical of these alloys leads to various casting defects such as hot-tearing, shrinkage, and microporosity [4]. In contrast, eutectic Al-Re based alloys (Re = rare earths) have excellent resistance to hot-tearing [5], and thus high casting manufacturability. Earlier studies revealed [6,7,8] that the addition of Re in Al-Cu based alloys allows one to increase the resistance to hot-tearing, thus improving the castability due to the formation of a eutectic structure. Moreover, Al-Cu-Re based alloys have a good combination of mechanical properties at room and elevated temperatures [8,9] and exhibit high processability for additive manufacturing (AM) techniques [10], such as selective laser melting. Indeed, the high cooling rates used for AM provide for the formation of nano-sized eutectic or quasi-eutectic structures (in hypereutectic alloys [11]) which in turn provide for the effective strengthening by the Orowan mechanism [12,13], for which dislocations bypass the intermetallic particles by looping around them. For example, new Al-9Cu-6Ce and Al-9Cu-6Ce-1Zr (wt.%) alloys obtained by laser powder bed fusion AM have an ultra-fine eutectic structure consisting of an aluminum matrix and a eutectic intermetallic phase identified as Al₈Cu₃Ce [14,15]. The structure formed provides for a high strength at room and elevated temperatures of up to 400 °C.

However, despite the described advantages of the Al-Cu-Ce alloys, their fundamental drawback is the necessity of adding high concentrations of the relatively expensive rare earth Ce. We believe that a calcium (Ca) additive can serve as an adequate replacement for rare earth metals. Indeed, calcium is a widespread, environmentally-friendly alkaline earth metal. Ca forms a eutectic-type system with aluminum and exhibits a very low solubility in Al (<0.01 at.%) [16], even after the sever plastic deformation [17] of the Al-Ca based alloy. According to numerous studies, as-cast [18,19,20] and wrought [17,21,22] Al-Ca-based alloys have a fine eutectic structure and a good combination of mechanical properties and manufacturability. The ternary Al-Ca-Cu system is also of interest, but it has been poorly studied [23,24]. It is well-known that information on the structure of phase diagrams is required for the informed design of new alloys. According to an earlier study [25], four intermetallic compounds, i.e., (Al,Cu)₄Ca, Al₂₇Ca₃Cu₇, Al₈CaCu₄ and Al₂Cu, can be in equilibrium with (Al) in the Al-Ca-Cu ternary system. The chemical composition and crystallographic structure of these compounds have recently been reported [25]. The presence of this many compounds in equilibrium with aluminum determines the very complex structure of this ternary diagram, and it is hardly possible to completely describe it in a single study. For comparison, the Al-Cu-Ce ternary system, where two phases (Al₁₁Re₃ and Al₈ReCu₄ [8,26]) are considered to be in equilibrium with aluminum, has been studied for the last decade, but nevertheless there is still controversy regarding its structure.

Thus, the aim of this work is to provide a first insight into the new system and to reveal the main features inherent in the structure of its phase diagram, and also to deliver sufficient information on the phase equilibria and transformations that determine the structure of the Al-Ca-Cu alloys upon solidification and high temperature annealing.

2. Materials and Methods

Several model alloys of the Al-Ca-Cu system (

Table 1. Actual chemical composition of the experimental alloys.

№	Designation	Actual Concentrations, wt.%
		Al	Ca	Cu
1	Al1.0Cu8.0Ca	balance	8.8	1.6
2	Al2.2Cu6.7Ca	balance	6.6	2.5
3	Al3.4Cu5.6Ca	balance	5.6	3.6
4	Al5.6Cu3.5Ca	balance	3.9	6.6
5	Al9.0Cu3.5Ca	balance	3.5	9.0
6	Al10Cu0.5Ca	balance	0.35	9.0
7	Al10Cu1.5Ca	balance	1.2	9.0
8	Al14Cu2.5Ca	balance	2.5	14.0
9	Al14Cu6.0Ca	balance	6.5	13.0
10	Al18Cu6.0Ca	balance	6.0	15.0
11	Al30Cu1.0Ca	balance	1.0	27.0
12	Al36Cu1.0Ca	balance	1.5	35.0
13	Al9.0Cu0.5Ca	balance	0.4	8.2
14	Al9.0Cu1.5Ca	balance	0.8	8.5
15	Al14Cu1.0Ca	balance	1.1	13.2
16	Al14Cu7.0Ca	balance	8.3	14.2
17	Al3Ca0.5Cu	balance	2.7	0.5
18	Al3Ca1Cu	balance	3.0	1.0

and Figure 1) were chosen and prepared for studies. The alloys were prepared from 99.99% aluminum in a resistance furnace (GRAFICARBO) with a graphite crucible. Aluminum was placed in the crucible, and after its melting, copper in the form of pure metal (99.9 wt.% Cu) and calcium in the form of the binary Al-15%Ca master alloy were introduced into the melt. After the melting of the main components, the melt was held for 5–10 min to obtaining a homogeneous composition, and the metal was then cast into a 10 × 20 × 180 mm graphite mold. The cooling rate in the mold was about 10 K/s. For achieving a close-to-equilibrium state at least at the beginning of the solidification, some alloys were remelted and slowly solidified in the furnace chamber at a cooling rate of about 0.02 K/s.

The Al10Cu0.5Ca and Al10Cu1.5Ca alloys were subjected to homogenization annealing at 530 °C for 10 h. Al3Ca0.5Cu and Al3Ca1Cu ingots without preliminary annealing were subjected to hot rolling (at 300 °C) to 2 mm sheet samples with an 80% reduction. The ingots were heat treated in muffle electric furnaces with a temperature control accuracy of ~3 K.

The microstructure was examined by means of scanning electron microscopy (SEM, TESCAN VEGA 3) and electron microprobe analysis (EMPA, OXFORD AZtec). The samples were polished for the studies. The metallographic samples were ground with SiC abrasive paper and polished with 1 µm diamond suspension. A solution of 1% hydrogen fluoride (HF) water was used for etching.

X-ray diffraction (XRD) data were collected using CoKα radiation and treated with a software package [27]. The specimens for the X-ray diffraction study were polished specimens cut from part of the ingots.

The solidification pattern of some alloys was studied by means of experimental cooling curves recorded using an AKTAKOM–2006 registering unit (Lutron Electronic Enterprise Co., Ltd., Taipei City, Taiwan) and chromel–alumel thermocouples.

The Vickers hardness (Hv) was measured using a DUROLINE MH-6 setup (METKON Instruments) with a load of 1 kg and a dwell time of 10 s. Room-temperature tensile tests of as-processed wire specimens were conducted on a Zwick Z250 universal testing machine (the loading rate was 10 mm/min). The flat proportional samples without grippers and with a size of 110 × 10 × 2 mm were used for the tensile tests.

In order to facilitate the preliminary analysis of the ternary system, a thermodynamic calculation of the liquidus projection of the Al-Ca-Cu system (Figure 1) was carried out using the Themo-Calc software and the TTAL5 database [28]. It can be seen that the calculation suggests the existence of two intermetallic compounds: Al₄Ca and Al₂Cu, in equilibrium with aluminum and one invariant eutectic phase transformation L→(Al) + Al₄Ca + Al₂Cu in this corner.

3. Results and Discussion

3.1. Liquidus Projection

Experimental plotting of the liquidus projection in the system requires that the primary crystallization areas of the phases be identified. According to earlier data [25] and the experimental results of this study, three intermetallic Ca-containing phases, i.e., (Al,Cu)₄Ca, Al₂₇Ca₃Cu₇ and Al₈CaCu_4, can be in equilibrium with the aluminum solid solution (Al). In order to reveal the primary crystallization areas for each of the phases, the structure of some alloys pertaining to the respective areas was studied in detail. The primary crystallization areas were identified by the type of primary crystals observed in the structure.

Hypereutectic alloys were chosen for identifying the primary crystallization area of the phases. In the Al1.0Cu8.0Ca alloy, the primary crystals observed have a typical acicular structure, and according to spectral analysis for rapidly and slowly cooled alloys (

Table 2. Chemical composition of the phases detected in the alloys obtained at various cooling rates (Vs) upon solidification.

Alloy	Vs, K/s	Chemical Composition, at.%			Phase Identification
		Al	Ca	Cu
Al1.0Cu8.0Ca	10	balance	19.6	2.0	(Al,Cu)4Ca
	0.02	balance	19.3	2.1	(Al,Cu)4Ca
Al2.2Cu6.7Ca	10	balance	19.3	3.3	(Al,Cu)4Ca
	0.02	balance	19.0	3.8	(Al,Cu)4Ca
Al14Cu6.0Ca	10	balance	19.0	8.0	(Al,Cu)4Ca
		balance	8.0	19.0	Al27Ca3Cu7
	0.02	balance	19.0	8.0	(Al,Cu)4Ca
		balance	8.0	19.0	Al27Ca3Cu7
Al18Cu6.0Ca	10	balance	19.2	8.3	(Al,Cu)4Ca
		balance	7.9	18.9	Al27Ca3Cu7
	0.02	balance	8.0	20.0	Al27Ca3Cu7
Al14Cu2.5Ca	10	balance	8.0	20.0	Al27Ca3Cu7
	0.02	balance	8.0	20.0	Al27Ca3Cu7
Al30Cu1.0Ca	10	balance	6.6	30.1	Al8CaCu4
Al36Cu1.0Ca	10	balance	6.0	30.5	Al8CaCu4
	0.02	balance	7.8	33	Al8CaCu4
Al3.4Cu5.6Ca	0.02	balance	19.3	6.5	(Al,Cu)4Ca
Al5.6Cu3.5Ca	0.02	balance	-	2.0	(Al)
		balance	19.1	7.6	(Al,Cu)4Ca
		balance	7.8	17.6	Al27Ca3Cu7
Al9.0Cu3.5Ca	0.02	balance	-	1.8	(Al)
		balance	19.7	8.1	(Al,Cu)4Ca
		balance	7.4	18.2	Al27Ca3Cu7

), these crystals are the (Al,Cu)₄Ca phase. According to [25], the (Al,Cu)₄Ca phase is a solid solution based on the Al₄Ca phase with dissolved Cu atoms substituting Al. The primary crystals form against the fine eutectic background in the rapidly cooled alloy (Figure 2a). Slow cooling leads to the coarsening of the eutectic which, however, still remains relatively fine (Figure 2b). The Al2.2Cu6.7Ca alloy containing less Ca and more Cu also has a hypereutectic structure with less elongated and generally finer primary crystals (Figure 2c) which can also be clearly identified as the (Al,Cu)₄Ca [25] phase (Table 2). The eutectic looks uniform but much coarser. In the Al14Cu6.0Ca alloy with an excess of copper, a small fraction of another type of coarse crystals bright in appearance and having a compact spherical shape are observed along with the primary (Al,Cu)₄Ca phase crystals (Figure 2e). According to the EMPA data (Table 2), the solubility of copper in the (Al,Cu)₄Ca phase is the highest compared to that observed for the above-mentioned alloys. EMPA also revealed the chemical composition of the second type of crystals observed. Their phase is described by the formula Al₂₇Ca₃Cu₇ [25]. After the slow solidification of the Al14Cu6.0Ca alloy (Figure 2f), only (Al,Cu)₄Ca phase primary crystals are observed (marked by arrow). The latter fact suggests that the Al14Cu6.0Ca alloy pertains to the (Al,Cu)₄Ca phase primary field but is close to the (Al,Cu)₄Ca/Al₂₇Ca₃Cu₇ boundary.

The Al14Cu2.5Ca and Al18Cu6.0Ca alloys were analyzed in order to clarify the boundaries of the primary crystallization area of the Al₂₇Ca₃Cu₇ compound. For the Al18Cu6.0Ca alloy, two types of coarse primary crystals are also observed (Figure 3a). The bright faceted crystals in Figure 3a are identified as the Al₂₇Ca₃Cu₇ phase, whose chemical composition perfectly matches that of the crystals in the Al14Cu6.0Ca alloy considered above. The second type of crystals, which are grey and elongated, are the (Al,Cu)₄Ca phase. The XRD data for this alloy (Figure 4) confirmed its phase composition consisting of both of the extra phases. However, after slow solidification (Figure 3b), only Al₂₇Ca₃Cu₇ phase primary crystals are observed. The latter fact confirms that the alloy pertains to the primary crystallization area of the Al₂₇Ca₃Cu₇ phase, but is close to the (Al,Cu)₄Ca/Al₂₇Ca₃Cu₇ boundary. After accelerated cooling, the Al14Cu2.5Ca alloy has a near-eutectic structure (Figure 3c) with lone Al₂₇Ca₃Cu₇ phase primary crystals. The presence of those crystals in the structure was also confirmed by XRD analysis (Figure 4). Along the Al₂₇Ca₃Cu₇ phase crystals, the patterns typical of the Al₈CaCu₄ phase [25] were also observed. After slow solidification, the alloy has a hypoeutectic structure (Figure 3d) with a high eutectic fraction. These facts demonstrate that the alloy is located at the boundary between two (Al)/Al₂₇Ca₃Cu₇ primary crystallization areas and concurrently near a ternary eutectic point.

The Al30Cu1.0Ca and Al36Cu1.0Ca alloys were analyzed in order to clarify the primary crystallization area boundaries of another ternary Al₈CaCu₄ compound that is in equilibrium with the aluminum in this system. The microstructure of both alloys contains bright primary crystals (Figure 5a,b) which, according to the spectral analysis (Table 2), has an excess of copper as compared to the Ca-containing phases considered above. The chemical composition of these crystals obtained after slow solidification is somewhat different from that obtained after accelerated solidification. The latter fact can be coupled with the relatively fine structure of the crystals obtained after solidification, which prevents accurate measurements. Thus, according to the data for the slowly solidified alloy, the observed primary crystals correspond perfectly well to the Al₈CaCu₄ phase [25]. One can also note that the slow solidification of the Al36Cu1.0Ca alloy (Figure 5c) also revealed the presence of lone Al₈CaCu₄ phase primary crystals against the background of the eutectic. The latter fact confirms that the alloy is located near the eutectic point.

Finally, the structure of the hypoeutectic alloys with primary aluminum solid solution (Al) crystals was studied. One can see that the Al3.4Cu5.6Ca, Al5.6Cu3.5Ca and Al9.0Cu3.5Ca alloys have a fine hypoeutectic structure (Figure 6). The alloys were also obtained at slow solidification, which coarsens the structural components. The latter fact makes it possible to carry out spectral analysis to determine the chemical composition of crystals. The data (Table 2) suggest that the eutectic in the alloys consists of the (Al,Cu)₄Ca and Al₂₇Ca₃Cu₇ phases. An XRD analysis of the Al5.6Cu3.5Ca alloy (Figure 4) confirmed the presence of these extra phases. Due to the relatively fine structure of the crystals, the measured composition of the Al₂₇Ca₃Cu₇ crystals is somewhat different from the one accepted in [25], but it can still be identified.

The aforementioned experimental data allow for hypothesizing regarding the boundaries of the primary crystallization areas of the phases considered. It is worthy of note that the analysis did not reveal any traces of peritectic transformations. The structure of the eutectic type diagram is therefore suggested to have two quasi-binary sections: (Al)-Al₂₇Ca₃Cu₇ and (Al)-Al₈CaCu₄ (Figure 7). Indeed, the arrangement of the alloys on the diagram in accordance with the type of primary crystals observed in the structure suggests the existence of three invariant eutectic and two quasi-binary eutectic phase transformations (

Table 3. Coordinates of invariant phase transformations.

Phase Transformation	Point on the Diagram	Chemical Composition, wt.%			T, °C
		Al	Ca	Cu
L→(Al)+ (Al,Cu)4Ca+Al27Ca3Cu7	E1	balance	5.6	4.5	595
L→(Al)+ Al27Ca3Cu7+Al8CaCu4	E2	balance	2.2	13.5	594
L→(Al)+ Al8CaCu4+Al2Cu	E3	balance	0.5	34	544
L→(Al)+ Al27Ca3Cu7	eX	balance	2.8	11.3	596
L→(Al)+ Al8CaCu4	eY	balance	1.8	14.7	596

). In comparison with another eutectic Al-Ce-Cu system containing one quasi-binary section (Al)-Al₈CeCu₄ and two invariant eutectic transformations (at ~12% Cu, 7% Ce, E1; ~32% Cu, 0.5% Ce, E2) in the aluminum corner, the new Al-Ca-Cu system is preferable for the design of aluminum matrix composites with an excess of eutectic. Indeed, the E1 point in the new Al-Ca-Cu system is located at the copper content typical of industrial alloys, whereas a much higher copper concentration is needed for the Al-Ce-Cu system (~12% Cu for the E1 eutectic point). Moreover, taking into account the very similar content of the third component (7% Ce and 6% Ca) at the E1 point in the systems, the use of cheaper and lighter calcium instead of cerium is preferable. One should note that the estimated position of E3 in Al-Ca-Cu is very close to that of the respective E2 points in the Al-Ce-Cu system. It can therefore be concluded that the position of E3 is calculated with an acceptable accuracy. In comparison with another copper containing ternary system in which the third component has a very limited solubility in aluminum, i.e., Al-Cu-Fe [29], the position of this point is also quite close (~32.5% Cu, 0.3% Fe) to the respective point in the new system.

To assess the temperature of the invariant phase transformations, some alloys from the corresponding phase fields were subjected to thermal analysis in order to plot their cooling and heating curves (Figure 8). The temperature of invariant transformation is visualized as a platform at a constant temperature. The Al9Cu3.5Ca and Al14Cu7Ca alloys were chosen since they pertain to the (Al) + (Al,Cu)₄Ca + Al₂₇Ca₃Cu₇ ternary phase field, and their solidification must end at the temperature of the E1 eutectic transformation. Thermal analysis showed (Figure 8a) that the L→(Al) + (Al,Cu)₄Ca + Al₂₇Ca₃Cu₇ eutectic transformation temperature is about ~595 °C, which is ~22 °C lower than that of the L→(Al) + (Al,Cu)₄Ca eutectic transformation in the binary system [16]. The next two Al9Cu1.5Ca and Al14Cu2.5Ca alloys pertain to the other ternary phase field, (Al) + Al₂₇Ca₃Cu₇ + Al₈CaCu_4, and their solidification must end at the temperature of the E2 eutectic transformation. An analysis revealed that the latter temperature is also close to 594–595 °C. One should note that since the positions of the eutectic points e_X and e_Y in the quasi-binary sections are quite close to the invariant eutectic point E2, the difference in the temperatures should also be negligible. Thus, the temperatures of the e_X and e_Y transformations can be assumed to be a few degrees higher than that of E2. Additional precision studies are required for a more accurate determination of these temperatures. One should note that the thermal curves of the Al14Cu2.5Ca alloy contain one more very narrow platform at ~544 °C (marked by arrow in Figure 8b). This transformation is a prime consequence of the elevated solubility of copper in aluminum (to be discussed below) leading to the (L→(Al) + Al₈CaCu₄ + Al₂Cu) ternary eutectic transformation, which is non-equilibrium for this phase field. On the contrary, equilibrium solidification of the last two Al9Cu0.5Ca and Al14Cu1Ca alloys must end via this ternary transformation, the temperature of which is about 544 °C (Figure 8c). This temperature is close to that of the L→(Al) + Al₂Cu eutectic transformation in the binary system (548 °C) and the ternary eutectics at the respective points in the Al-Ce-Cu (545 °C) and Al-Ce-Fe (542 °C) systems.

3.2. Isothermal Section at 530 °C

For the Al-Ce-Cu system, the solubility of copper in (Al) reaches 5.7 wt.% in the (Al) + Al₈CeCu₄ + Al₂Cu ternary phase field and 5.7 wt.% in the (Al) + Al₈CeCu₄ quasi-binary section; therefore the alloys from these phase fields can be prone to precipitation hardening. Annealing temperatures in the range of 530–540 °C are commonly used for solid solution treatment before the quenching of the Al-Cu based alloys. Some alloys from the new system were studied after long-term annealing at 530 °C, which is expected to cause the dissolution of most of the nonequilibrium eutectic Al₂Cu phase. An analysis revealed that the copper solubility in (Al) in the (Al) + Al₂₇Ca₃Cu₇ + (Al,Cu)₄Ca ternary phase filed is very low and can be accepted as a few tenths of a percent. However, in the rest of the ternary phase field, the copper solubility in (Al) is high enough. Two alloys Al10Cu1.5Ca (Figure 9a) and Al10Cu0.5Ca (Figure 9b) were chosen for the study. The phase composition of the alloys was theoretically assessed based on a simple stoichiometric balance corresponding to the alloy composition. For the hypoeutectic Al10Cu1.5Ca alloy from the (Al) + Al₂₇Ca₃Cu₇ + Al₈CaCu₄ ternary phase field (

Table 4. The measured chemical composition of the aluminum matrix (Al) after homogenization and calculated fractions of extra phases.

Alloy	Chemical Composition, wt.%			Fractions of Extra Phases, wt.% (vol.%).
	Al	Cu	Ca	Al8CaCu4	Al2Cu	Al27Ca3Cu7
Al10Cu0.5Ca	balance	5.1 ± 0.2	-	6.2 (3.9)	3.3 (2.2)	-
Al10Cu1.5Ca	balance	2.4 ± 0.2	-	11.0 (7.0)	-	6.9 (5.7)

), an EMPA analysis of the aluminum matrix showed a copper solubility of up to ~2.4 wt.%. For the Al10Cu0.5Ca alloy pertaining to another ternary phase field (Al) + Al₈CaCu₄ + Al₂Cu (Table 4), the copper solubility is approximately at the solubility limit, i.e., ~5.1 wt.%, which is close to that for the binary Al-Cu and ternary Al-Ce-Cu systems at the same temperature. Thus, the alloys pertaining to the (Al) + Al₈CaCu₄ quasi-binary section and the (Al) + Al₈CaCu₄ + Al₂Cu ternary phase field can also be prone to precipitation hardening. However, for a noticeable precipitation hardening response, the content of copper in the alloy should be about ten times the calcium content. The latter fact makes the precipitation hardening of the Al-Ca-Cu ternary alloy less interesting due to the high content of expensive and heavy copper. However, the transition to more complex systems containing Al-Ca-Cu can help to solve this issue by rearranging the distribution of copper between the aluminum and eutectic phases. The obtained isothermal section of the system at 530 °C is presented in Figure 10.

3.3. Alloy Examples

The prospects of this system for designing new eutectic type alloys with a natural composite structure are demonstrated for the example of the Al3Ca0.5Cu and Al3Ca1Cu alloys. Indeed, as one can see from Figure 11, both alloys are hypoeutectic (Figure 11a,c) and have an ultra-fine eutectic component. Both alloys are near the boundary between the (Al) + (Al,Cu)₄Ca/(Al) + (Al,Cu)₄Ca + Al₂₇Ca₃Cu₇ phase fields with a predominant eutectic (Al,Cu)₄Ca phase. The alloys without preliminary annealing were subjected to hot rolling at a moderate temperature of 300 °C to 2 mm sheets (a total deformation degree of 80%). A microscopic analysis (Figure 11b,d) did not reveal any significant refinement of the structure upon deformation processing. Indeed, the as-cast eutectic colonies are still well resolved, but they are elongated in the rolling direction. The slight differences from the as-cast structure are due to the relatively low degree of deformation.

The hardness of both alloys after rolling is about the same, ~60 Hv. The obtained hot rolled sheets were also subjected to uniaxial tensile tests. The data on the mechanical properties are presented in Figure 12. The obtained level of mechanical properties is moderate yet acceptable for base model alloys. Indeed, in comparing these alloys with the closest counterparts containing a similar [30] or a higher [31] amount of calcium but more expensive rare earth metals (UTS 240–260 MPa, YS 85–205 MPa and δ 5.5–9.0%) [30,31] and nickel (UTS ~190–303 MPa, YS ~150–220 MPa and δ ~0.5–5%) [31] instead of copper, one can see similar mechanical properties, while the latter alloys contain additional hardening additives. One should also note that due to both the eutectic-type structure of the system and the presence of a high amount of numerous insoluble aluminides, the new system and the alloys may be promising for additive manufacturing techniques such as selective laser melting.

One should also note that a slight increase in the copper content from 0.5 to 1.0 wt.% leads to a minor increase in the strength and a much greater increase in the ductility. The origins of the latter fact are not clear, and more detailed studies are required to determine the causes underlying this phenomenon. It is possible that this can be associated with an expectable increase in the fraction of the Al₂₇Ca₃Cu₇ phase, leading to a modification of the eutectic or an increase in the solubility of copper in the (Al,Cu)₄Ca phase, leading to a change in the properties of the (Al,Cu)₄Ca compound.

4. Summary

(1)

The liquidus projection of the Al-Ca-Cu system in the aluminum corner was suggested based on experiential studies of the microstructure and phase composition of model alloys. The suggested structure of the diagram has two quasi-binary sections: (Al)-Al₂₇Ca₃Cu₇ and (Al)-Al₈CaCu_4, and three invariant ternary eutectic transformations: L→(Al) + (Al,Cu)₄Ca + Al₂₇Ca₃Cu₇ (at 5.6 wt.% Ca, 4.5 wt.% Cu, 595 °C), L→(Al) + Al₂₇Ca₃Cu₇ +Al₈CaCu₄ (at 2.2 wt.% Ca, 13.5 wt.% Cu, 594 °C) and L→(Al) + Al₈CaCu₄ + Al₂Cu (at 0.5 wt.% Ca, 34 wt.% Cu, 544 °C). The eutectic point in the quasi-binary sections is accepted to be as follows: L→(Al) + Al₂₇Ca₃Cu₇ (at 2.8 wt.% Ca, 11.3 wt.% Cu) and L→(Al) + Al₈CaCu₄ (at 1.8 wt.% Ca, 14.7 wt.% Cu).

(2)

A study of the copper solubility limit in (Al) revealed that the copper solubility in the (Al) + Al₂₇Ca₃Cu₇ + (Al,Cu)₄Ca ternary phase filed is small and can be accepted as a few tenths of a percent. In the (Al)+Al₂₇Ca₃Cu₇+ Al₈CaCu₄ ternary phase filed, the copper solubility reaches ~2.4 wt.%, while in the other ternary phase filed, (Al) + Al₈CaCu₄ + Al₂Cu is close to the solubility limit, i.e., ~5.1 wt.%, which is close to that for the binary Al-Cu and the ternary Al-Ce-Cu systems. Thus, the alloys pertaining to the (Al) + Al₈CaCu₄ quasi-binary section and the (Al) + Al₈CaCu₄ + Al₂Cu ternary phase field can be prone to precipitation hardening.

(3)

The prospects of this system for designing new eutectic type alloys with a natural composite structure were demonstrated for the example of the Al3Ca0.5Cu and Al3Ca1Cu alloys. The alloys have an ultra-fine eutectic structure based on the (Al,Cu)₄Ca eutectic phase. The alloys showed high manufacturability for moderate temperature hot rolling (300 °C). Uniaxial tensile tests carried out for the obtained 2 mm sheet alloys revealed the following mechanical properties: UTS up to 220 MPa, YS up to 180 MPa, and relative elongation up to 5.5%, which are acceptable for the model alloys.