Complex eutectic growth and Bi precipitation in ternary Sn-Bi-Cu and Sn-Bi-Ag alloys

Sn-34wt%Bi, Sn-34wt%Bi-2wt%Ag and Sn-34wt%Bi-0.7wt%Cu alloys have been directionally solidified (DS) under a broad range of solidification cooling rates. Microstructures have been characterized with emphasis on both eutectic growth and precipitation of Bi within the b -Sn dendritic matrix. The eutectic growth, for all alloys examined, is shown to be associated with the coexistence of coarse and fine lamellar structures with different length-scale of lamellar spacing ( λ ). Experimental growth relations of λ vs. the cooling rate have been proposed. The length-scale of the lamellae in the fine eutectic ranges from 0.8 to 2.5 m m while in the coarse eutectic from 1.8 to 4.0 m m. Taking as reference the Sn-Bi alloy, both the spacing between Bi precipitates ( l p ) and the fine eutectic spacing ( l fine ) increase with Cu and Ag additions, whereas l coarse remains roughly unaltered. Both ternary Sn-Bi-Ag and Sn-Bi-Cu alloys are shown to have worse distributions of both lamellae in the fine eutectic and of precipitates within Sn-rich dendrites, which resulted in decrease in hardness.


Introduction
The rapid technological development of recent years has produced numerous devices on a large scale with many utilities, providing an increase in the quantity and diversity of electrical and electronics equipment.Among various toxic metals present in electronic waste, such as cadmium; mercury; barium; lead, the latter is one of the most worrisome because of extensive use in soldering processes as in Sn-Pb alloys [1].Therefore, the development of lead-free solder alloys is recognized as an essential and urgent task in the industry of electronics.Sn-Bi, with or without additions of alloying elements, are considered promising alternative solders because of its favorable properties such as low melting temperature (<183°C), good mechanical properties and low coefficient of thermal expansion.These are known as important factors for lowtemperature soldering and PTH (Plating in Thought Holes) applications of high-performance in mainframe computers [2,3].
Although a considerable number of investigations have been devoted to examine microstructure, wettability and mechanical properties of Sn-Bi alloys, with and without minute amount of alloying elements, the effects of Ni [4], Cu [2], Zn [5] and Ag [6] additions on the Sn-rich+Bi-rich eutectic growth remain undetermined.Most of the alloys investigated to date are characterized by microstructures with prevalence of eutectic mixture in relation to the other phases, which means that the consistency of these alloys as solder materials depends on the morphology of the eutectic mixture and on the distribution of phases within it.Shen et al. [2] reported that the microstructure of a Sn-40Bi-2Zn-0.1Cu alloy has been refined due to additions of Zn and Cu.A large fraction of the Bi-rich phase appeared with fine globular shape rather than with the lamellar morphology, and hardness was shown to be improved.The addition of 0.5wt%Ni to the Sn-58wt%Bi alloy decreased the elongation to fracture when compared to that of the Sn-58wt%Bi alloy, due to the growth of Ni 3 Sn 4 intermetallic particles [4].In addition to that, degraded strength has been exhibited by this Nibearing alloy because the formation of coarsened Bi-rich phases.This coarsening is a concern in the case of Bi-rich phases, which are generally brittle in nature, as it can result in poor Wang et al. [5] investigated the additions of Zn, Zn-Al and Zn-P on the reliability of the Sn-40Bi alloy, and reported that the addition of these three types of elements can refine the microstructures and improve the mechanical strength.
A recent investigation [7] affirms that the eutectic phase of Sn-Bi alloys may exhibit lower mechanical strength when compared with that of the primary β-Sn phase.The mechanical properties of the two phases were calculated by the Abaqus software, which indicated that the eutectic phase has lower strength.This denotes that both eutectic fraction and eutectic characteristics such as morphology and size could play a fundamental role on the alloy soundness.
Although studies on phase equilibria and solidification behavior of Sn-Bi-Ag alloys are available in the literature [8][9][10][11], metallographic analyses of alloys cooled from the melt under equilibrium conditions have been barely shown.Other than, the microstructural evolutions of Sn-Bi-Ag and Sn-Bi-Cu solder alloys for non-equilibrium cooling conditions have not been reported so far.
The binary Sn-Bi eutectic alloy has essentially an irregular lamellar structure, which is also characterized as a complex regular microstructure, in which two phases are arranged in alternating not-flat plates [12][13][14].In a recent study [14], the authors found that the Sn-Bi eutectic can be seen as having a complex regular microstructure where two types of regions are observed: zones of a regular repeating pattern and other zones of random orientation.Similar structures have been reported for alloys of the Pb-Bi and Bi-Cd systems [15].
The effect of solidification cooling rate (Ṫ) on the formation and evolution of Sn-rich+Bi-rich eutectic in ternary Sn-Bi-X alloys have not been elucidated so far.It is well known that the evolution of solidification cooling rate controls the final arrangement of the eutectic structure.On the other hand, two eutectic characteristic sizes, coarse and fine, have been observed for a binary hypoeutectic Sn-Bi alloy [14].Experimental growth eutectic laws have been proposed for the Sn-52wt%Bi alloy relating the eutectic spacings (λ coarse , λ fine ) to the growth rate (V L ) by adopting a power function having a -1/2 exponent.

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The present study is focused on the characterization of eutectic structures in directionally solidified (DS) Sn-Bi(-Cu;-Ag) alloys.Experimental interrelations of eutectic spacings (λ coarse , λ fine ) and Ṫ will be discussed.The influences of alloying additions and cooling rate on the eutectic length scale and on the size of Bi-rich precipitates will be examined.

Experimental procedure
The solidification setup used in the experiments imposes a directional extraction of heat only through a water-cooled bottom made of low carbon steel, promoting vertical upward directional solidification.The casting assembly has been detailed in previous research works [16,17].The solidification experiments were carried out with Sn-34wt%Bi-0.7wt%Cu,Sn-33wt%Bi-2.0wt%Agand Sn-34wt%Bi alloys.The temperature profiles were recorded at different positions from the cooled bottom of the DS Sn-Bi-X alloys castings via the signals of a set of type J thermocouples.The thermocouples readings acquired during solidification allowed the evolution of temperature to be assessed.For each thermocouple in a specific position an accurate determination of the slope of the cooling curve has been performed.The solidification cooling rate was determined with basis on this slope considering the time immediately after the passage of the liquidus front.
Selected transverse (perpendicular to the growth direction) samples of the DS binary Sn-34wt%Bi and of the DS ternary Sn-34wt%Bi-0.7wt%Cuand Sn-33wt%Bi-2.0wt%Agalloys castings were polished (solution of alumina 1µm) and etched with a solution of 2mL HCl, 10mL FeCl 3 and 100mL H 2 O applied during 5-20s to reveal the microstructures.The two scales of eutectic spacings (λ coarse and λ fine ) were measured on the transverse sections.The intercept method was employed to measure not only the λ coarse / λ fine but also the spacing between Bi-rich precipitates, λ p , which developed on the core of the Sn-rich dendrites [18].Each microstructural spacing has been characterized by at least 40 measurements for each selected position along the length of the castings.A Field Emission Gun (FEG) -Scanning Electron Microscope (SEM-EDS), Philips (XL30 FEG) and a Scanning Electron Microscope (SEM-EDS), FEI (Inspect S50L) were used in order to analyze the features found in the microstructures.Hardness tests

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were performed on transverse sections of the samples of each alloy by using a test load of 500 g and a dwell time of 15s.A Shimadzu HMV-G20 model hardness tester was used.The X-ray diffraction (XRD) patterns for phases formed in the ternary Sn-Bi(-Ag,-Cu) alloys have been acquired by a Siemens D5000 diffractometer with a 2-theta range from 20° to 90°, CuKα radiation and a wavelength, λ, of 0.15406 nm.As can be seen in Fig. 2 the binary Sn-rich+Bi-rich eutectic structures in the microstructure of the Sn-34wt%Bi alloy and in the ternary Sn-Bi(-Cu,-Ag) alloys are characterized by two different length-scales: coarser and finer.Fine eutectic structures are indicated by black arrows in Fig. 2. It is worth noting that all microstructures are dominated by the eutectic mixture, which justifies giving a focus on such microstructural feature.The presence of coarse and fine regions in the present results seems to be associated with thermal instabilities related to local heat flow conditions, which are typical of non-equilibrium solidification.This means that fine eutectic structures can be established in a certain site, followed by coarse structures in the surrounding areas, caused by the effect of thermal instabilities during non-equilibrium solidification.

Results and discussion
Lamellar eutectic with fine and coarse lamellae have been observed along the entire length of the castings for all compositions examined.In the case of the Sn-34wt%Bi alloy very low fractions of fine eutectic are associated with positions very close to the cooled casting surface.In general, the finer eutectic can be found isolated between the eutectic coarse mixture and the Sn-rich dendrites.In addition, Fig. 2 shows the presence of Bi-rich precipitates in the microstructures decorating the Sn-rich dendrites in their own core (white arrows).These particles have either spheroidal or ellipsoidal morphologies as can be seen in Fig. 3. Trifoils of Bi and fishbone-like eutectic appear in isolated points close to the coarse eutectic, as can be seen in Fig. 2b and 2f (Black outline squares).The evolution of the eutectic spacing (λ) may be described by the classical relationship for growth of lamellar eutectics proposed by Jackson and Hunt [20]: λ= a × (V L ) -1/2 , where V is the growth rate, ½ is the exponent and "a" is a constant.Considering that Ṫ is given by a 'constant' x V L 2 [21], another power function becomes applicable: λ= b x (Ṫ) -1/4 where Ṫ is the cooling rate, -1/4 is the exponent, and "b" is a constant.The average λ coarse and λ fine values with their standard deviations are shown in Fig. 4 for all Sn-Bi based alloys evaluated.
The -1/4 exponent associated with the cooling rate is able to characterize λ coarse and λ fine .
Santos et al. [22] reported the same exponent for Zn-Sn solder alloys.A single growth law is The coarse eutectic spacing decreases with increasing cooling rate ranging from 1.8 to 4.0µm.
In contrast, two experimental growth laws are necessary to encompass the whole λ fine experimental scatter in the microstructures of the Sn-34wt%Bi, Sn-34wt%Bi-0.7wt%Cuand Sn-33wt%Bi-2.0wt%Agalloys.The lamellar spacing in the fine eutectic is smaller for the Sn-34wt%Bi alloy when compared with those of the Cu-and Ag-bearing alloys, which means that larger λ fine are associated with Ag and Cu additions.

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A large number of fine Bi particles have been formed during cooling of the three Sn-Bi based alloys, which can be noted through the SEM images in Figs. 2 and 3.It can be inferred that Bi atoms were firstly dissolved in the Sn phase and secondly precipitated during the cooling stage after solidification.The precipitation of Bi is considered an important feature that requires attention in Sn-Bi based alloys.The microstructural spacing between these Bi precipitates (λ p ) has been determined for the specimens associated with cooling rates of 12°C/s, 1.0°C/s and 0.2°C/s with corresponding average values depicted in Table 1.Soldering operations are quite similar to the present experimental conditions, i.e., fast cooling conditions in very early solidification stages imposed by the contact with the cold and unaltered substrate.Thus, the examined range of cooling rates is very representative of industrial processes.Microstructures become finer for higher cooling rates.A higher density of Bi precipitates characterizes the Snrich dendritic areas in the Sn-34wt%Bi alloy due to the corresponding lower λ values.
Additionally, the cooling rate affects the distribution of these Bi particles since the microstructural spacing increased as the cooling rate decreased (Table 1).Further, the addition of Cu and Ag to the Sn-Bi alloy induced higher λ values.Since microstructures of the three alloys are dominated by eutectic mixtures, hardness values were shown to be related to one of the eutectic features, which is λ fine .The experimental points can be seen in Fig. 5.A very slight tendency of increasing hardness is noted with decreasing λ fine .The addition of Cu and Ag has a deleterious effect on hardness, decreasing its

Conclusions
From the experimental results acquired in this investigation, the following conclusions can be drawn: •     Table 1.Fine and coarse lamellar spacing and precipitate spacing found for three different cooling rates in the Sn-Bi(-Ag,-Cu) alloys.

Figs 6 Fig. 1 .
Figs. 1a and 1b show examples of representative SEM images of ternary eutectic

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to represent the λ coarse experimental scatter considering the entire range of cooling rates.

2 Fig. 5 .
Fig. 5. Evolution of Vickers hardness as a function of the inverse of the square root of the fine eutectic spacing for the Sn-Bi(-Ag,-Cu) solder alloys.

•
Figure Captions and Table Heading

Fig. 1 .
Fig. 1.SEM images of transverse sections detailing the ternary eutectic structures found for: (a)

Fig. 2 .
Fig. 2. Growth of lamellar eutectic and co-existence of Bi-rich precipitates in the

Fig. 3 .
Fig. 3. Typical examples of distribution and morphologies of the Bi precipitates within the

Table 1 .
Fine and coarse lamellar spacing and precipitates spacing found for three different cooling rates in the Sn-Bi(-Ag,-Cu) alloys.