Solid State Phase Equilibria of an Al–Sn–Y Ternary System

A complete understanding of the solid-state phase equilibria of the ternary Al–Sn–Y system is essential for the development of both Al-based structural materials and Sn-based lead-free solders. In this work, the phase relationships in the Al–Sn–Y ternary system at 473 K were investigated mainly by means of X-ray powder diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) with energy disperse spectroscopy (EDS) analysis. The existence of 12 binary compounds, namely Sn3Y, Sn5Y2, Sn2Y, Sn10Y11, Sn4Y5, Sn3Y5, AlY2, Al3Y5, Al2Y3, AlY, Al2Y and α–Al3Y, was confirmed. Controversial phases (Sn5Y2 and Al3Y5) were found in this work. This isothermal section consisted of 15 single-phase regions, 27 two-phase regions and 13 three-phase regions. No ternary compounds were found and none of the other phases in this system revealed a remarkable solid solution at 473 K.


Introduction
Al-based alloys, which consist of Al-Pb and Al-Sn, are widely used for sliding bearing applications due to their good load carrying capacity, fatigue resistance, wear resistance and sliding properties [1][2][3]. However, because of toxic Pb, environmental legislation has driven manufacturers to eliminate Pb from bearing alloys. Thus, the focus has been concentrated on Al-Sn alloys. Al-Sn based alloys are simple eutectic binary alloy systems with solid solutions of a wide range of compositions and are well known as soft tribological alloys [3]. However, the main challenges of Al-Sn based alloys are that the strength of alloys is generally low and can easily form a near-continuous large Sn zone that weakens the interface bonding [4]. Abundant attempts, such as alloying addition, to improve preparation methods have been made to overcome those drawbacks. Al-Sn-Si [5], Al-Sn-Bi [6] and Al-Sn-Mg [4] alloys have been researched with the aim of enhancing the strength of Al-based bearing alloys. It is well known that the addition of small amounts of rare earth elements can improve the microstructures and properties of aluminum alloys [7][8][9]. Meanwhile, Sn-Al eutectic alloy has the potential to be a new system of lead-free solder because it is similar to existing systems, such as the Sn-Zn system and the Sn-Cu system. Rare earth (RE) is an important kind of alloying additive for metallic materials which can significantly improve the properties of alloys by affecting microstructure and refining grain. The ternary Al-Sn-Y system [10] has been reported before but it is only part of the section (65 at.% Y or less) at room temperature, which is not enough for the application of alloys at high temperatures. Therefore, a complete knowledge of the phase diagram of the ternary Al-Sn-Y system is essential for a better understanding of this system. The work presented in this article aims to determine the Al-Sn-Y phase equilibrium at 473 K. It is expected that this study will give further insights into the Al-Sn-Y ternary system for practical applications.

Materials and Methods
Aluminum (99.9 wt.%), tin (99.9 wt.%) and yttrium (99.99 wt.%) were prepared as raw materials. The alloy compositions of all the samples are plotted in Figure 1. Some components were repeatedly designed for the ideal results. The samples (each 1.5 g) were prepared in an electric arc furnace under an argon atmosphere and a water-cooled copper crucible. In order to obtain a homogeneous composition, each sample was melted three times. For most alloys, the weight loss was generally less than 1 wt.% after being melted. All the samples were sealed in an evacuated quartz tube for homogenization treatment. The alloys which contained more than 50 at.% Sn were homogenized at 673 K for 20 days. Then, the alloys were cooled down to 473 K and maintained for 30 days. Others were kept at 873 K for 10 days and then cooled slowly to 473 K and maintained for 30 days. Finally, all the samples were quenched with ice-water.
Materials 2018, 11, x FOR PEER REVIEW 2 of 14 microstructure and refining grain. The ternary Al-Sn-Y system [10] has been reported before but it is only part of the section (65 at.% Y or less) at room temperature, which is not enough for the application of alloys at high temperatures. Therefore, a complete knowledge of the phase diagram of the ternary Al-Sn-Y system is essential for a better understanding of this system. The work presented in this article aims to determine the Al-Sn-Y phase equilibrium at 473 K. It is expected that this study will give further insights into the Al-Sn-Y ternary system for practical applications.

Materials and Methods
Aluminum (99.9 wt.%), tin (99.9 wt.%) and yttrium (99.99 wt.%) were prepared as raw materials. The alloy compositions of all the samples are plotted in Figure 1. Some components were repeatedly designed for the ideal results. The samples (each 1.5 g) were prepared in an electric arc furnace under an argon atmosphere and a water-cooled copper crucible. In order to obtain a homogeneous composition, each sample was melted three times. For most alloys, the weight loss was generally less than 1 wt.% after being melted. All the samples were sealed in an evacuated quartz tube for homogenization treatment. The alloys which contained more than 50 at.% Sn were homogenized at 673 K for 20 days. Then, the alloys were cooled down to 473 K and maintained for 30 days. Others were kept at 873 K for 10 days and then cooled slowly to 473 K and maintained for 30 days. Finally, all the samples were quenched with ice-water. All of the homogenized samples were ground into powder and then measured with the help of a Rigaku D/Max 2500V diffractometer (Rigaku, Tokyo, Japan) with CuKα radiation and a graphite monochromator operated at 40 kV, 200 mA. The microstructures and phase analyses were determined by scanning electron microscopy (SEM, Hitachi, Tokyo, Japan) with energy disperse spectroscopy (EDS, Hitachi, Tokyo, Japan) analysis. The temperature of the phase transition was determined using a differential scanning calorimeter (Netzsch, Bavaria, Germany), which was performed in an aluminum crucible with a flowing argon atmosphere as a reference substance between room temperature and 1373 K. The heating and cooling rate used was 10 K·min -1 . All of the homogenized samples were ground into powder and then measured with the help of a Rigaku D/Max 2500V diffractometer (Rigaku, Tokyo, Japan) with CuKα radiation and a graphite monochromator operated at 40 kV, 200 mA. The microstructures and phase analyses were determined by scanning electron microscopy (SEM, Hitachi, Tokyo, Japan) with energy disperse spectroscopy (EDS, Hitachi, Tokyo, Japan) analysis. The temperature of the phase transition was determined using a differential scanning calorimeter (Netzsch, Bavaria, Germany), which was performed in an aluminum crucible with a flowing argon atmosphere as a reference substance between room temperature and 1373 K. The heating and cooling rate used was 10 K·min −1 .

Sn-Y Binary System
For the Sn-Y binary system, the existence of the five phases, i.e., Sn 3 Y, Sn 2 Y, Sn 10 Y 11 , Sn 4 Y 5 and Sn 3 Y 5 , are accepted without question. However, the existence of the phase Sn 5 Y 2 is controversial. In the Sn-Y phase diagram revised by Okamoto [11], the phase Sn 5 Y 2 was discovered at the range of temperature from 273 K to 798 K and the structure of the Sn 5 Y 2 phase was reported in detail, which is in good agreement with the findings of Tang et al. [12]. In that work, the Sn-Y system was investigated by thermodynamic modeling. The Sn 5 Y 2 phase was considered to have the same structure as the Ge 5 Er 2 phase, and the lattice parameters were 0.4322 nm (a), 0.4409 nm (b) and 1.9089 nm (c). But the existence of the Sn 5 Y 2 phase was questioned by Mudryk et al. [13]. When they investigated the R-Fe-Sn ternary systems (R-Y, Gd) at 670 K, Chen et al. [10] and Zhan et al. [14] also reported the same results that phase Sn 5 Y 2 was not found. However, Romaka et al. [15] later confirmed the existence of the Sn 5 Y 2 phase in the Sn-Ni-Y ternary system at 670 K.
In this work, the samples (60.5 at.% Sn, 12.5 at.% Al, 27 at.% Y, 64.5 at.% Sn, 12.5 at.% Al and 23 at.% Y) were prepared to verify the existence of the Sn 5 Y 2 phase. Figure 2 shows the X-ray powder diffraction (XRD) pattern of the sample (60.5 at.% Sn, 12.5 at.% Al and 27 at.% Y), which illustrates the existence of Sn 2 Y, Sn 5 Y 2 and Al. Figure 3 shows the pattern prepared with the atomic proportion of 64.5 at.% Sn, 12.5 at.% Al and 23 at.% Y. It indicates the existence of the three phases-Sn 3 Y, Sn 5 Y 2 and Al. In Figures 2 and 3, the Sn phase was found. A possible reason for this is that when the Sn content is higher, it is easy to separate out tin whiskers during annealing over a longer time [16,17]. However, the XRD patterns of the samples clearly showed the existence of the Sn 5 Y 2 phase, which was confirmed in the Al-Sn-Y ternary system at this investigated temperature.

Sn-Y Binary System
For the Sn-Y binary system, the existence of the five phases, i.e., Sn3Y, Sn2Y, Sn10Y11, Sn4Y5 and Sn3Y5, are accepted without question. However, the existence of the phase Sn5Y2 is controversial. In the Sn-Y phase diagram revised by Okamoto [11], the phase Sn5Y2 was discovered at the range of temperature from 273 K to 798 K and the structure of the Sn5Y2 phase was reported in detail, which is in good agreement with the findings of Tang et al. [12]. In that work, the Sn-Y system was investigated by thermodynamic modeling. The Sn5Y2 phase was considered to have the same structure as the Ge5Er2 phase, and the lattice parameters were 0.4322 nm (a), 0.4409 nm (b) and 1.9089 nm (c). But the existence of the Sn5Y2 phase was questioned by Mudryk et al. [13]. When they investigated the R-Fe-Sn ternary systems (R-Y, Gd) at 670 K, Chen et al. [10] and Zhan et al. [14] also reported the same results that phase Sn5Y2 was not found. However, Romaka et al. [15] later confirmed the existence of the Sn5Y2 phase in the Sn-Ni-Y ternary system at 670 K.
In this work, the samples (60.5 at.% Sn, 12.5 at.% Al, 27 at.% Y, 64.5 at.% Sn, 12.5 at.% Al and 23 at.% Y) were prepared to verify the existence of the Sn5Y2 phase. Figure 2 shows the X-ray powder diffraction (XRD) pattern of the sample (60.5 at.% Sn, 12.5 at.% Al and 27 at.% Y), which illustrates the existence of Sn2Y, Sn5Y2 and Al. Figure 3 shows the pattern prepared with the atomic proportion of 64.5 at.% Sn, 12.5 at.% Al and 23 at.% Y. It indicates the existence of the three phases-Sn3Y, Sn5Y2 and Al. In Figures 2 and 3, the Sn phase was found. A possible reason for this is that when the Sn content is higher, it is easy to separate out tin whiskers during annealing over a longer time [16,17]. However, the XRD patterns of the samples clearly showed the existence of the Sn5Y2 phase, which was confirmed in the Al-Sn-Y ternary system at this investigated temperature.

Al-Y Binary System
For the Al-Y binary system, the existence of the five phases, i.e., Y2Al, Y3Al2, YAl, YAl2 and YAl3, are accepted without question. Bailey [18] reported early on that two structurally related polymorphic forms of Al3Y have been corroborated-a low temperature form with the hexagonal Ni3Sn-type structure (α-YAl3) and a high temperature form with a rhombohedra BaPb3-type structure (β-YAl3). At the temperature of this work, the YAl3 phase is α-YAl3. The Al-Y binary system was also investigated thermodynamically by Lukas [19]. The Y5Al3 phase was found by Lukas and the structure of Y5Al3 phase was identified by Richter et al. [20]. Liu et al. [21] also experimentally investigated the Al-Y phase diagram and failed to confirm the existence of the Y5Al3 phase. After that, in the studies of many ternary systems, such as Al-Fe-Y [22], Al-Zr-Y [23], Al-Sb-Y [24] and Al-Sn-Y [10], the Y5Al3 phase was not found. However, Liu et al. [25] thermodynamically assessed the Al-Zn-Y system and found that the binary compound Y5Al3 forms through the reaction L + Y3Al2 ⇔ Y5Al3 + YZnAl at 997 K. However, the temperature range of Y5Al3 has not been clearly identified.
In order to obtain a believable result, the samples (4 at.% Sn, 35 at.% Al, 61 at.% Y, 5 at.% Sn, 30 at.% Al and 65 at.% Y) were prepared. Figure 4 shows that the XRD pattern of the sample (4 at.% Sn, 35 at.% Al and 61 at.% Y) illustrates the existence of Y3Al2, Y5Al3 and Sn3Y5. The XRD pattern of the sample (5 at.% Sn, 30 at.% Al, 65 at.% Y) illustrates the existence of Y2Al, Y5Al3 and Sn3Y5, as shown in Figure 5, which indicates the existence of the Y5Al3 phase.

Al-Y Binary System
For the Al-Y binary system, the existence of the five phases, i.e., Y 2 Al, Y 3 Al 2 , YAl, YAl 2 and YAl 3 , are accepted without question. Bailey [18] reported early on that two structurally related polymorphic forms of Al 3 Y have been corroborated-a low temperature form with the hexagonal Ni 3 Sn-type structure (α-YAl 3 ) and a high temperature form with a rhombohedra BaPb 3 -type structure (β-YAl 3 ). At the temperature of this work, the YAl 3 phase is α-YAl 3 . The Al-Y binary system was also investigated thermodynamically by Lukas [19]. The Y 5 Al 3 phase was found by Lukas and the structure of Y 5 Al 3 phase was identified by Richter et al. [20]. Liu et al. [21] also experimentally investigated the Al-Y phase diagram and failed to confirm the existence of the Y 5 Al 3 phase. After that, in the studies of many ternary systems, such as Al-Fe-Y [22], Al-Zr-Y [23], Al-Sb-Y [24] and Al-Sn-Y [10], the Y 5 Al 3 phase was not found. However, Liu et al. [25] thermodynamically assessed the Al-Zn-Y system and found that the binary compound Y 5 Al 3 forms through the reaction L + Y 3 Al 2 ⇔ Y 5 Al 3 + YZnAl at 997 K. However, the temperature range of Y 5 Al 3 has not been clearly identified.
In order to obtain a believable result, the samples (4 at.% Sn, 35 at.% Al, 61 at.% Y, 5 at.% Sn, 30 at.% Al and 65 at.% Y) were prepared. Figure 4 shows that the XRD pattern of the sample (4 at.% Sn, 35 at.% Al and 61 at.% Y) illustrates the existence of Y 3 Al 2 , Y 5 Al 3 and Sn 3 Y 5 . The XRD pattern of the sample (5 at.% Sn, 30 at.% Al, 65 at.% Y) illustrates the existence of Y 2 Al, Y 5 Al 3 and Sn 3 Y 5 , as shown in Figure 5, which indicates the existence of the Y 5 Al 3 phase.

Sn-Al Binary System
There was no compound found in the Sn-Al system. Figure 6 shows that the XRD pattern of the sample (69.8 at.% Sn, 14.9 at.% Al, 15.3 at.% Y) illustrates the existence of Sn, Al and Sn3Y. The crystal

Sn-Al Binary System
There was no compound found in the Sn-Al system. Figure 6 shows that the XRD pattern of the sample (69.8 at.% Sn, 14.9 at.% Al, 15.3 at.% Y) illustrates the existence of Sn, Al and Sn3Y. The crystal

Sn-Al Binary System
There was no compound found in the Sn-Al system. Figure 6 shows that the XRD pattern of the sample (69.8 at.% Sn, 14.9 at.% Al, 15.3 at.% Y) illustrates the existence of Sn, Al and Sn 3 Y. The crystal structure data of the intermetallic compounds in the Sn-Y, Al-Y and Sn-Al binary systems at 473 K are given in Table 1. structure data of the intermetallic compounds in the Sn-Y, Al-Y and Sn-Al binary systems at 473 K are given in Table 1.

Al-Sn-Y Ternary System
For the Al-Sn-Y ternary system, the Al 3 Sn 9 Y 8 ternary compound was detected by Chen et al. [10] at room temperature. In order to verify the existence of the Al 3 Sn 9 Y 8 phase at 473 K, the samples (48.5 at.% Sn, 10 at.% Al, 41.5 at.% Y; 41 at.% Sn, 15 at.% Al, 44 at.% Y; 45 at.% Sn, 15 at.% Al, 40 at.% Y) were prepared. The XRD patterns of the samples clearly indicated the existence Sn 2 Y, Sn 10 Y 11 and YAl 2 , as shown in Figure 7. Thus, the Al 3 Sn 9 Y 8 ternary compound was not detected in this work.
For the Al-Sn-Y ternary system, the Al3Sn9Y8 ternary compound was detected by Chen et al. [10] at room temperature. In order to verify the existence of the Al3Sn9Y8 phase at 473 K, the samples (48.5 at.% Sn, 10 at.% Al, 41.5 at.% Y; 41 at.% Sn, 15 at.% Al, 44 at.% Y; 45 at.% Sn, 15 at.% Al, 40 at.% Y) were prepared. The XRD patterns of the samples clearly indicated the existence Sn2Y, Sn10Y11 and YAl2, as shown in Figure 7. Thus, the Al3Sn9Y8 ternary compound was not detected in this work.

Isothermal Section
According to the XRD, SEM/EDS and differential scanning calorimeter (DSC) analysis, the isothermal section of the Al-Sn-Y ternary system at 473 K is shown in Figure 8. This isothermal section consists of 15 single phase regions, 27 binary phase regions and 13 ternary phase regions. No ternary compounds were found and none of the other phases in this system revealed a remarkable homogeneity range at 473 K. Figure 9 shows the XRD pattern of the sample (20 at.% Sn, 9 at.% Al, 71 at.% Y) indicating the existence of Sn3Y5, Y2Al and Y. The XRD pattern of the equilibrated sample with a stoichiometric composition of 6.5 at.% Sn, 47.8 at.% Al, 45.7 at.% Y indicated the existence of Sn3Y5, Al2Y and YAl, as shown in Figure 10. In addition, Figure 11 shows the XRD pattern of the sample (39 at.% Sn, 10 at.% Al, 51 at.% Y) indicating the existence of Sn4Y5, Al2Y and Sn10Y11. Figure  12 shows the XRD pattern of the sample (6.5 at.% Sn, 64 at.% Al, 29.5 at.% Y) indicating the existence of α-Al3Y, Al2Y and Sn2Y. The XRD results confirm that nine binary compounds, namely Sn3Y, Sn2Y, Sn10Y11, Sn4Y5, Sn3Y5, Y2Al, Y3Al2, YAl, YAl2 and α-YAl3, exist in this system at 473 K. The SEM photographs (as shown in Figures 11-15) also clearly display the existence of some phases (identified

Isothermal Section
According to the XRD, SEM/EDS and differential scanning calorimeter (DSC) analysis, the isothermal section of the Al-Sn-Y ternary system at 473 K is shown in Figure 8. This isothermal section consists of 15 single phase regions, 27 binary phase regions and 13 ternary phase regions. No ternary compounds were found and none of the other phases in this system revealed a remarkable homogeneity range at 473 K. Figure 9 shows the XRD pattern of the sample (20 at.% Sn, 9 at.% Al, 71 at.% Y) indicating the existence of Sn 3 Y 5 , Y 2 Al and Y. The XRD pattern of the equilibrated sample with a stoichiometric composition of 6.5 at.% Sn, 47.8 at.% Al, 45.7 at.% Y indicated the existence of Sn 3 Y 5 , Al 2 Y and YAl, as shown in Figure 10. In addition, Figure 11 shows the XRD pattern of the sample (39 at.% Sn, 10 at.% Al, 51 at.% Y) indicating the existence of Sn 4 Y 5 , Al 2 Y and Sn 10 Y 11 . Figure 12 shows the XRD pattern of the sample (6.5 at.% Sn, 64 at.% Al, 29.5 at.% Y) indicating the existence of α-Al 3 Y, Al 2 Y and Sn 2 Y. The XRD results confirm that nine binary compounds, namely Sn 3 Y, Sn 2 Y, Sn 10 Y 11 , Sn 4 Y 5 , Sn 3 Y 5 , Y 2 Al, Y 3 Al 2 , YAl, YAl 2 and α-YAl 3 , exist in this system at 473 K. The SEM photographs (as shown in Figures 11-15) also clearly display the existence of some phases (identified by EDS). Constitutions of the ternary phase regions and compositions of the typical alloys are given in Table 2.

Conclusions
The isothermal section of the Sn-Al-Y ternary system at 473 K was experimentally constructed in this work. This isothermal section consists of 15 single-phase regions, 27 two-phase regions and 13 three-phase regions. The existence of 12 binary compounds was confirmed; namely, Sn 3 Y, Sn 5 Y 2 , Sn 2 Y, Sn 10 Y 11 , Sn 4 Y 5 , Sn 3 Y 5 , AlY 2 , Al 3 Y 5 , Al 2 Y 3 , AlY, Al 2 Y and α-Al 3 Y. No ternary compound was found.