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

Abstract

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.

1. 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.

2. 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.

3. Results

3.1. 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 Figure 2 and Figure 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.

3.2. 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.

3.3. 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 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. Binary crystal structure data of the Al–Sn–Y system at 473 K.

Phase	Pearson’s Symbol	Crystal Structure	Space Group	Lattice Parameters (nm)			Refs.
				a	b	c
Sn3Y	oC16	Gd4Sn11	Amm2	0.4345	0.4391	2.1937	[12]
Sn5Y2	oP14	Ge5Er2	Pmmm	0.4322	0.4409	1.9089	[12]
Sn2Y	oC12	Si2Zr	Cmcm	0.4398	1.632	0.4304	[12]
Sn10Y11	tI84	Ge10Ho11	I4/mmm	1.154	–	1.692	[12]
Sn4Y5	oP36	Ge4Sm5	Pnma	0.805	1.529	0.805	[12]
Sn3Y5	hP16	Si3Mn5	P63/mcm	0.8902	–	0.6536	[12]
αYAl3	hP8	Ni3Sn	P63/mmc	0.6276	–	0.4582	[26]
YAl2	cF24	Cu2Mg	$Fd\overline{3}m$	0.78611	–	–	[26]
YAl	oC8	CrB	Cmcm	0.3884	1.1522	0.4385	[26]
Y3Al2	tP20	Al2Zr3	P42/mnm	0.8239	–	0.7648	[26]
Y2Al	oP12	Co2Si	Pnma	0.6642	0.5084	0.9469	[26]
Y5Al3	hP16	Mn5Si3	P63/mcm	0.8787	–	0.6435	[20]

.

3.4. Al–Sn–Y Ternary System

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.

3.5. 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 Figure 11, Figure 12, Figure 13, Figure 14 and Figure 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. Details of the phase regions and typical samples in the Al–Sn–Y system at 473 K.

Phase Regions	Alloy Composition (at.%)			Phase Composition
	Sn	Al	Y
1	20	9	71	Y + Sn3Y5 + Y2Al
2	5	30	65	Y2Al + Sn3Y5 + Y5Al3
3	4	35	61	Y5Al3 + Sn3Y5 + Y3Al2
4	6.3	37	56.7	Y3Al2 + Sn3Y5 + AlY
5	6.5	47.8	45.7	AlY + Sn3Y5 + Al2Y
6	33.5	12.5	54	Al2Y + Sn3Y5 + Sn4Y5
7	39	10	51	Sn4Y5 + Al2Y + Sn10Y11
8	45	15	40	Sn10Y11 + Al2Y + Sn2Y
9	6.5	64	29.5	Sn2Y + Al2Y + α–Al3Y
10	20	59	21	α–Al3Y + Sn2Y + Al
11	60.5	12.5	27	Al + Sn2Y + Sn5Y2
12	64.5	12.5	23	Sn5Y2 + Al + Sn3Y
13	69.8	14.9	15.3	Sn3Y + Al + Sn

.

4. 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, Sn₃Y, Sn₅Y₂, Sn₂Y, Sn₁₀Y₁₁, Sn₄Y₅, Sn₃Y₅, AlY₂, Al₃Y₅, Al₂Y₃, AlY, Al₂Y and α–Al₃Y. No ternary compound was found.