From simple binary to complex multicomponent eutectic alloys

The eutectic solidification of almost all binary and majority of key ternary alloy systems have been studied and modelled extensively. The development of eutectic microstructure in ternary, multicomponent and high entropy alloys have generated potential engineering alloys with superior mechanical/magnetic properties that outperform their traditional binary eutectic counterparts due to refined microstructure and/or the presence of dual hard/soft phase mixture. Currently, our understanding of the eutectic solidification is mainly restricted to alloy systems having upto 3 constituents (eg. ternary eutectic). There exists a knowledge gap in our understanding of the solidification behaviour of high order multicomponent eutectic alloys. This review article gives a brief background of the development of eutectic alloys from binary to senary multicomponent systems, together with an overview of recent development of complex microstructures of aluminium based multicomponent alloys with five or more constituents at/near eutectic compositions. Although the number of crystalline phases coexisted in the Al-based eutectic alloys increases with increasing number of


Introduction
Eutectic alloys refer to a unique class of material system comprised of more than one constituent and they exhibit the lowest temperature of melting/freezing as compared to the melting point of any of the constituents. The word eutectic is derived from the Greek word "eutektos" which implies easily melted and it was first used by Guthrie 1 in 1884. Eutectic systems are found everywhere in nature, and have been reported in a wide range of materials including organic 2,3 , ceramic 4

Previous and current research of eutectic alloys
Eutectic solidification of binary alloys 910111213141516 involves the nucleation and growth of a two-phase mixture. The morphology of such regular two-phase eutectic microstructure (e.g. lamellar, rod-like, fibrous etc.) depends on materials characteristics such as faceted/non-faceted nature, entropy of fusion and proportion of each solid phase, as well as the solidification conditions 17,18 . Such two-phase eutectic microstructure is not limited to binary alloy systems but are also found in ternary (e.g. 4 Al-Si-Cu 19 , Ag-Cu-Ge 20 , Ti-Sn-Fe 2122 , Co-Fe-Zr 12 , Fe-Si-Ti 23  alloys. There are hundreds or more eutectic/near eutectic alloys 51 found in metallic systems comprising of upto 6 constituents. Table 1 gives a selected list of these eutectic alloys from binary to senary systems. It is interesting to see that the total number of coexisted phases present in the eutectic microstructure is below four even for the senary alloys.

Search for eutectic composition.
Not all binary alloys can form eutectic alloys. This is because the valence electrons of the constituents are not always compatible with the formation of joint crystal lattice. However, these binary eutectic alloys are frequently formed at near simple composition ratio 52 of their constituents, such as 8/1, 5/1, 3/1, 2/1 and 3/2 53 . This was explained previously by the short-range atomic ordering of the eutectic liquid structure into specific arrangements of icosahedral clusters 53,54 . Recently, a new structural tool based on the cluster-plus-glue-atom model 55 was used to derive the composition formulas of binary eutectics. The eutectic compositions and temperatures of many binary and ternary alloys can be extracted from experimentally determined equilibrium phase diagrams listed in handbooks 56,57 .
If all the eutectic phases formed are known beforehand, the eutectic composition and eutectic temperature, TE, can be predicted using the following Schroeder-van Laar equation 58,59,60 : where Xi is the molar fraction of the i th phase, TE is the eutectic temperature of the alloy and Ti is the melting temperature of the i th phase. This approach has been applied to 16 binary (e.g. Al-Ga, Al-Ge-Al-Zn, Au-Ge, Pb-Sb etc.) and 6 ternary (e.g. Al-Ge-Sn, Cd-Pb-Sn etc.) eutectic alloys 61 . In general, the theoretic prediction agrees well with the general trends of eutectic temperature and compositions studied over a wide range of alloy systems. However, there are discrepancies between the predicted and the experimental data, which may be due to the assumption of ideal mixing in the Schroeder-van Laar equation. For quaternary alloys, the eutectic points can be determined by twodimensional sections set construction 62 (tie-lines method) on the model of T-x-y-z and the intersection of the four surfaces of primary crystallization of the components based on data from binary and ternary alloy systems 59 . However, they rely on prior knowledge of all co-existed eutectic phases.
An alternative way to search the eutectic point, is to use the CALPHAD (CALculation of PHAse Diagrams) software (e.g. Thermocalc, Pandat, JMatPro, FactSage) 63 , which is based on several geometric methods or mathematical formalisms to generate the phase diagrams. These programs use the properties of the individual components, single points of experimental data of mixtures and high order thermodynamic database 64 by extrapolation from the lower order constituent binary and ternary systems. For quinary alloys, the phase diagrams can be computed by fixing four elements first and then adjusting the 5th element 65,66,67,68 . There is a still huge challenge to design/locate the eutectic compositions in high-order constituent alloy systems without prior knowledge of eutectic phases.
In our studies, the initial prediction of the eutectic composition of Al-Cu-Si-Mg quaternary alloy was determined using Thermocal software to achieve the lowest melting point for the solidification of a four-phase microstructure by adding Mg, together with the adjustment of Cu and Si contents of known ternary Al-Cu-Si ternary eutectic composition, as shown in Figure 1. The predicted eutectic composition of Al-Cu-Si-Mg was found to be Al-12at%Cu-7at%Si-3at%Mg. This is similar to the reported Al-14at%Cu-7at%Si-3at%Mg 50 eutectic composition. and Al76Cu14Si7Mg3(quaternary) eutectic alloys subjected to a 20K/min cooling cycle.

From binary Al-Cu to quaternary Al-Cu-Si-Mg systems
They consisted of a single exothermic DSC peak, corresponding to eutectic solidification.
The eutectic solidification peak occurs at 558°C in Al84Cu16 and decreases to 518°C in Al76Cu14Si7Mg3. The heats of fusion (i.e. area under the peak) of ternary and quaternary eutectic alloys are found to be 448J/g and 398J/g, respectively. This corresponds to the decreasing eutectic temperature with increasing number of elements in the alloy system. The direct proportional relationship between melting temperature and heat of fusion has been reported in Al-Cu binary alloy 69 . Figure 2b shows XRD spectra of suction cast Al84Cu16, Al81Cu12Si7 and Al76Cu14Si7Mg3 alloys. The number of eutectic phases increased from two (e.g. α-Al and θ-Al2Cu) in binary to four (e.g. α-Al, θ-Al2Cu, Si, Q(Al4Cu2Mg8Si7)) in the quaternary alloys. α-Al and θ-Al2Cu are two predominant eutectic phases present in all three alloys. 9> and θ-Al2Cu ˂1 1 0>, respectively. However, such four-phase microstructure changed to a mixture of equiaxed α-Al, θ-Al2Cu and Si phases, together with platelets of Q phase, as shown in Figure 4c. The sizes of Si, θ-Al2Cu and Q phases reduced to 40-90nm, 50-100nm and 100nm width by 100-250nm length, respectively.
The orientation relationship (OR) of eutectic θ-Al2Cu, α-Al and Q phases in the cellular region exhibited the following orientation relationship as determined by T-EBSD technique ( Figure 5): The α-Al/θ-Al2Cu eutectic phases in quaternary alloy retained a similar OR to that reported in binary Al-Cu system 9 . Hence in the (ɑ-Al + θ-Al2Cu+ Q-phase + Si) quaternary eutectic, Si faceted phase grows independently, while α-Al, θ-Al2Cu and Q-phase grow cooperatively, maintaining the OR. Table 2 shows a list of multicomponent alloys prepared by suction casting and characterised in this study. Figure 6 shows DSC traces of these multicomponent alloys subjected to a 20K/min heating cycle. The 5-element multicomponent alloy consisted of a single endothermic peak, corresponding to the eutectic reaction. However, the DSC traces of 11-element and 13-element alloys consisted of multiple endothermic peaks overlapping each other. In all cases, solidification of primary phase from the melt has not been observed by DSC studies. However, the heat of fusion (i.e. area under the peak) of 5-element multicomponent alloy is 337J/g, which is higher than 257J/g in 11element and 272J/g in 13-element alloys. This is contributed to the lower onset melting temperature of 11-and 13-element multicomponent alloys as compared to 5-element multicomponent alloy. from EDX spectrum (Figure 8c). This is similar to T-phase (Al6MgCu) intermetallic compound but with the incorporation of a minor amount Ni, giving Al6(MgCuNi).

Multicomponent eutectic alloys beyond quaternary Al-Cu-Si-Mg system
However, the grey phase is an unknown compound but has a composition of Al-8 at%Cu-5at%Mg-2at%Ca-0.6%Ni-0.6at%Mn compound, as shown in EDX spectrum (Figure 8d). identified. The formation of these phases involves three stages as illustrated by the three overlapping DSC melting peaks in Figure 6. The solidification behaviour of such alloy is very complex and further studies are being carried out to understand their microstructural evolution.

Hardness of binary eutectic to multicomponent alloys
The combination of refined microstructure and presence of Q-phase has led to the moderate increase in average hardness from 267HV in Al84Cu16 to 292HV in Al77Cu13Si6Mg3Ni1, as shown in Figure 12. However, 11-and 13-elements multicomponent alloys exhibited average hardness values of 367HV and 380HV, respectively. Such significant increase in hardness is contributed by a complex microstructure of multiple phases including solid solutions and intermetallic compounds.

Conclusions
This review article has shown our strong understanding and knowledge of eutectic alloys in the low order constituent binary and ternary systems. When the alloy systems become more complex and are based on a large number of constituents, our knowledge of eutectic solidification is limited and it is restricted mainly to two-phase eutectic microstructure. This is partly due to the difficulty of finding eutectic compositions in high order constituent alloy systems without any prior knowledge of co-existed eutectic phases. Our recent attempt to search for eutectic compositions in alloy systems beyond five constituents have led to the following conclusions: • The melting temperature and latent heat of fusion decreased with increasing number of components in the alloy system.