Effect of the Elemental Content of Bi and Pb on the Properties of Sn-Bi-Pb Low Melting Point Alloys

Abstract

Using the tin alloy module in the material phase diagram and thermodynamic simulation software JMatPro, the phase compositions of different components of Sn-Bi-Pb low melting point alloys with the same Sn content were simulated based on the phase diagrams of Sn-Bi, Sn-Pb, and Pb-Bi binary alloys, and the influence of alloying element content on the melting characteristics of the alloys was investigated, which was used for the optimization of the components and the obtaining of low melting point alloys with excellent melting characteristics. The microstructure and melting characteristics of the optimized alloys were characterized by Scanning electron microscope (SEM), Thermogravimetry Analysis-Differential Scanning Calorimetry (TG-DSC), X-ray diffractometer (XRD), etc., and the influence of phase content on the mechanical properties was investigated. The results show that the Sn-Bi-Pb alloy possesses Sn-(Bi, Pb) phase, Bi-(Pb) phase and Pb₇Bi₃ intermetallic compound phase, and the simultaneous increase of the Bi element and decrease of the Pb element content to the Sn-Bi-Pb alloy obviously improves its tensile strength, but the elongation rate shows a decreasing trend, among which Sn50Bi30Pb has the optimal comprehensive performance, with a tensile strength of 38.60 MPa, an elongation of 61.13%, and a melting range of 94.0°C∼100.6°C.

1. Introduction

In practical applications, the binary and multivariate alloys composed of low melting point elements Bi, Pb, Sn, Cd, In, Ga, Zn, Sb, etc. are called low melting point alloys, whose melting points generally need to be lower or equal to the melting point of traditional Sn-Pb alloys (183.0°C), and the current low-temperature metals and their alloys are generally selected from the monomers with low melting points or from the binary alloys such as Pb-Bi, Na-K, Sn-Bi, Sn-Zn, Sn-In and other binary alloys [1, 2]. Low melting point alloys are now widely used in machinery, aerospace, automotive, electrical instrumentation, light industry, and atomic energy industry [3], such as automatic sprinklers, and fire and smoke alarm devices. Low melting point alloys have also been used in fixed and removable restorations in dental restorations due to their unique properties and high dimensional accuracy [4]. In addition, the nuclear industry has been using liquid metals as high-temperature heat transfer heat storage fluids since the 1940s [5], but due to the erosive nature of liquid metals on piping or vessel materials and their hazards [6], low melting point alloys have now begun to be selected as heat-carrying mediums in the liquid, gaseous, or ionized states [7–10].

The melting point range of low melting point binary alloy systems is generally located in the range of 80–250°C, and the multicomponent alloy systems are able to reach even lower phase transition temperatures [11, 12]. One of the more common Sn-Bi systems of low melting point alloys of eutectic composition of Sn-58Bi, a eutectic temperature of 138.6°C [13]. Sn-Bi alloys are currently one of the most promising low melting point alloy systems, China’s Bi resources are abundant, Bi metal cost is low [14], and because of the excellent mechanical properties and creep resistance has been a lot of research, due to the Bi element itself of the brittle and aggregation in the molten state of the However, due to the brittleness of the Bi element and the aggregation effect in the molten state, the reliability and toughness of Sn-Bi low melting point alloys in service are lower than other low melting point alloys [15], and it is also impossible for them to be processed into the corresponding products, which greatly impedes the application of a wide range of applications.

Therefore, in order to meet the demand for low melting points and improve the brittleness and aggregation effect of Bi element, this paper will choose Sn-Bi-Pb low melting point alloy system for research. The Sn-Bi-Pb alloys with different contents of Pb element were simulated by using the Sn-based alloy module of the material phase diagram and the thermal property simulation software JMatPro. The relative content of Bi and Pb elements is regulated, and the effects of Bi and Pb elements on the phase composition, melting characteristics, and thermal properties of Sn-Bi-Pb alloy are studied, and the optimal components are selected by horizontal comparison. The microstructure, phase composition, melting characteristics, and mechanical properties of the optimized alloy were studied, in order to provide a theoretical basis and basic data for the further improvement of the mechanical properties and melting characteristics of Sn-Bi-Pb alloy.

2. Experimental Procedures

2.1 Specimen preparation

In this experiment, Sn40Bi40Pb was used as the alloy matrix, and different contents of Sn and Bi were added to obtain Sn-Bi-Pb low melting point alloy. The Sn blocks (99.99% purity) and Bi blocks (99.99% purity) were proportionally weighed using an analytical balance and heated up to 400°C in a box-type resistance furnace for the melting of the alloy. During smelting, due to the contact between the molten alloy and the air, a dense oxide layer is formed on the surface, which isolates the air and prevents the alloy from further oxidation and volatilization, so that the Bi, Sn and Pb elements in the alloy do not form obvious oxides, maintaining the uniformity of the alloy. In addition, in order to make the structure more uniform, the stirring was carried out every 10 min. After the melting and holding for a period of time, it was poured into the graphite mold, and then air-cooled to room temperature to obtain the ingot.

2.2 Means of testing

The melting characteristics of the alloys were analyzed by a Simultaneous Thermal Analyzer (TG-DSC, Setaram Setsys Evo) at a heating rate of 5°C/min in N₂ flow protection of 30 mL/min. The alloy samples were protected by nitrogen gas, and heated at 25∼400°C with a heating rate of 5°C/min. The physical phase characterization of the alloy specimens was carried out by a Bruker D8 Advance X-ray diffractometer, with a Cu target as the radiation source, a swept range of 10∼100°, and a scanning rate of 5°/min.

Tensile specimens were prepared using a DK7720 EDM wire line cutting machine, and the dimensions of the tensile specimens are shown in Fig. 1. The tensile strength, plasticity, and elongation at break of the alloy were determined using an INSTRON 3382 electronic universal materials testing machine, with a tensile speed of 0.0075 mm/s. The tensile strength was taken as the average value of three specimens. The microstructure and tensile specimen fracture of the low melting point alloy were observed by optical microscope and FEI Quanta 650 FEG field emission scanning electron microscope (SEM), and energy spectrum analysis (EDS) was performed.

Fig. 1

Schematic diagram of specimen for tensile testing.

3. Results and Discussion

3.1 Alloy composition design

Combined with the phase diagram of Sn-Bi alloy, in order to ensure that the melting characteristics of the designed alloy meet the demand for a low melting point, Sn and Bi are the main elements, and Pb is the doping element. The metal Pb can significantly lower the melting point of the alloy and refine the coarse Bi phase in the Sn-Bi alloy to prevent the deterioration of toughness and plasticity caused by the agglomeration of the Bi phase.

The magnitude of the mixing enthalpy can be used as a basis for judging the difficulty of the reaction and also reflects the magnitude of the interatomic bonding [16]. As given in Table 1, the mixing enthalpies between the three elements composing the alloy, in which the mixing enthalpies of Sn with Bi and Pb are all positive, all show weak interactions, and the mixing enthalpies of Sn with Bi and Pb are inclined to form Sn-(Bi, Pb) solid solution, while the mixing enthalpy of Bi with Pb is 0 so that Bi and Pb are capable of forming both Bi-(Pb) solid solution and Pb-Bi intermetallic compounds.

Table 1 Chemical mixing enthalpies of the atomic pairs among the alloying elements (unit: kJ·mol⁻¹) [17].

Due to the presence of a large number of solid solutions within the alloy matrix, the main strengthening mechanism is solid solution strengthening, and the effect of solid solution strengthening depends on the relative interaction between solute atoms and dislocations and the number of solute atoms per unit slip area. Therefore, the two factors affecting the effect of alloying elements on solid solution strengthening are the difference in mass of solute and solvent atoms A = W_{solvent atoms}/W_{solute atoms}, which is approximated by the ratio of the number of solvent atoms to the number of solute atoms and the difference in the radius of solute and solvent atoms B = (r_{solute atoms} − r_{solvent atoms})/r_{solvent atoms}, which is related to the degree of lattice distortion caused by the solute atoms. In general, the larger these two values, the better the solid solution strengthening effect. The alloy solid solution strengthening effect parameter H_s can be expressed by the following formula.

  

\begin{equation} H_{\text{s}} = A \times B \end{equation} (1)

The solid solution strengthening parameter H_s of Pb for Sn is calculated to be 0.137, while that of Bi for Sn is 0.042, which shows that the former is one order of magnitude higher than the latter. It can be generally seen that for Sn solid solution, Pb has a higher solid solution strengthening effect compared with Bi.

Since the mixing enthalpy of Pb and Bi is smaller than that of Pb and Sn, the interaction of Pb and Bi is larger than that of Pb and Sn, and Pb has a higher solid solution strengthening effect, the element Pb should be used to replace part of the metal Bi in the alloy.

In summary, this study chooses Sn-Bi alloy as the base, and it is proposed to add Pb element into Sn-Bi alloy to constitute Sn-Bi-Pb ternary composition and its basic components are designed as w(Sn) = 20%, w(Bi) = 80%, and Pb element is added to ensure that the mass fraction of Sn element is 20%, so as to change the ratio of Bi and Pb. The mass fraction of each specific component is shown in Table 2 below.

Table 2 The composition of different Sn-Bi-Pb based alloys.

Figure 2 shows the steady-state phase diagrams of Sn-Bi-Pb alloys with different components, from which it is found that three phases exist in the Sn-Bi-Pb system alloys at the room temperature state (25°C), which are Sn-rich phase, Bi-rich phase, and ε-PbBi phase. Excess Pb and Bi are added to the Sn matrix, a part of the dissolution into solid solution, the formation of Sn-rich phase; and more than the limit of solubility of the part can not be dissolved, for the excess phase, part of the Pb dissolved in which the formation of solid solution, i.e., Bi-rich phase; and part of the Pb and Bi to form intermetallic compounds. Excess phase alloys generally have a reinforcing effect, its strengthening effect, and the excess phase of its properties, excess phase of strength, and hardness, the higher the strengthening effect, but hard and brittle excess phase content exceeds a certain limit, the alloy becomes brittle, but reduced mechanical properties. It can be seen that the Sn-rich phase is Sn-(Bi, Pb) solid solution phase, the Bi phase is the Bi-(Pb) solid solution phase; the ε-PbBi phase is intermetallic compounds phase, according to the phase diagram of Pb-Bi binary alloy, it can be seen that it is generated by the encapsulation reaction of Bi and Pb at 184.0°C to produce the stable phase Pb₇Bi₃.

Fig. 2

Phase content changes of Sn-Bi-Pb alloy at different temperatures (a) Sn58Bi22Pb, (b) Sn55Bi25Pb, (c) Sn50Bi30Pb, (d) Sn45Bi35Pb, and (e) Sn40Bi40Pb.

As can be seen from Fig. 2, the content of the Bi-(Pb) phase at room temperature is gradually decreasing with the increase of the content of the Pb element, and most of the Pb atoms are precipitated by forming Pb₇Bi₃ intermetallic compounds with Bi, while the rest of them are dissolved in Bi and Sn. The gradual increase of the Pb₇Bi₃ phase suggests that the Pb elemental is more inclined to form Pb₇Bi₃ intermetallic compounds than to form Bi-(Pb) solid solution with Bi element. As the temperature decreases from the solid-phase line to room temperature, the content of the Bi-(Pb) phase gradually increases and the Pb₇Bi₃ phase gradually decreases, indicating that the solubility of Pb in Bi is highly dependent on the temperature.

Figure 3 shows the melting characteristics of Sn-Bi-Pb alloys with different components. The solid-phase line of these five Sn-Bi-Pb alloys is 94.7°C, and the liquid-phase line temperature will show a decreasing and then increasing trend with the decrease of the content of Bi element and the increase of the content of the Pb element. Among them, the melting interval is the smallest when w(Bi) = 50% and w(Pb) = 30%, and the melting range is 1.4°C, so it can be concluded that the melting characteristics are optimal when Sn50Bi30Pb alloy.

Fig. 3

Solid state temperature, liquid state temperature, and melting range of Sn-Bi-Pb alloy by JmatPro.

Combined with the steady-state phase diagrams of Sn-Bi-Pb alloys with different components in Fig. 2 for analysis, the Pb₇Bi₃ phase has been completely transformed to liquid phase at the solid phase line temperature when w(Bi) > 50% and w(Pb) < 30%, and the Sn-(Bi, Pb) phase has been completely transformed to liquid phase from solid phase at roughly 101°C. Thus, it is the Bi-(Pb) phase that mainly affects the melting characteristics of the alloy. The reason is that the addition of the Pb element makes Pb and Bi form relatively few intermetallic compounds Pb₇Bi₃, which consumes part of Bi, leading to the shift of the Sn-Bi sub-eutectic system to the eutectic system, which leads to the decrease of liquid phase line temperature.

When w(Bi) = 50% and w(Pb) = 30%, Sn-Bi-Pb is in the equilibrium system, and the Sn-(Bi, Pb) phase, the Bi-(Pb) phase and the Pb₇Bi₃ phase in the alloy are almost completely transformed to liquid state at the same temperature, and the eutectic phase transition temperature is almost kept at 94.7∼96.1°C, so the liquid phase line temperature is the lowest.

In contrast, when w(Bi) < 50% and w(Pb) > 30%, it is the Pb₇Bi₃ phase that mainly affects the melting characteristics of the alloy. The principle of alloy melting is the bonds between metal atoms that are broken by energy. The more interfaces there are, the more metallic bonds between the atoms, so more energy is required when the alloy melts, leading to an increase in the enthalpy and a consequent increase in the melting point [18]. Pb and Bi form a large number of intermetallic compounds, Pb₇Bi₃, which changes the microform and phase composition of the alloy, and the increase in the number of alloy micro-interfaces increases the enthalpy of the phase transition, and the temperature of the eutectic phase transition is increased, which leads to the liquidus line temperature increasing.

By comparing the melting characteristics of the above alloys, it is concluded that the Sn-Bi-Pb alloy with excellent melting characteristics can be obtained when w(Pb) = 30%, i.e., the optimized component of the Sn-Bi-Pb alloy is Sn50Bi30Pb. In order to further investigate the combined effects of Bi and Pb elements on the alloys, therefore, the microstructures and mechanical properties of Sn40Bi40Pb, Sn50Bi30Pb, and Sn58Bi22Pb for the study of microstructure and mechanical properties.

3.2 Melting characteristics

Figure 4 shows the DSC curves of Sn-Bi-Pb low melting point alloys, and two heat absorption peaks appeared in all samples during the heating process. The temperature of the first peak is located between 81.3 and 84.6°C, which is small and smooth, and it can be inferred that it corresponds to the sub-eutectic reaction L → (Pb₇Bi₃/Sn)_eutectic at this time, the second peak is more sharp, and according to the turning point and the peak temperature, it can be inferred that it corresponds to the complete eutectic reaction L → (Pb₇Bi₃/Sn/Bi)_eutectic at this time. eutectic. It can be seen from the data in Fig. 5, that the solid-phase lines of the alloys are 94.0°C, 94.0°C, and 92.5°C, respectively. With the increase of Bi element content and the decrease of Pb element content, the liquid phase line of the alloys gradually increases, and the melting range difference shows a trend of decreasing and then increasing, in which the melting interval of Sn50Bi30Pb alloy is the smallest, and the melting range difference is only 6.6°C, which is in line with the melting range trend calculated by the phase diagram of JMatPro software.

Fig. 4

DSC curve of Sn-Bi-Pb low melting point alloy.

Fig. 5

Solid state temperature, liquid state temperature, and melting range of Sn-Bi-Pb alloy.

By analyzing the melting point results of the alloys, it is found that Pb and Bi in Sn50Bi30Pb alloy will form the intermetallic compound Pb₇Bi₃, which consumes part of the high melting point Bi elements, resulting in the Sn-Bi sub-eutectic system shifting to the eutectic system and approaching the eutectic system, so the melting point is close to the eutectic point, which in turn narrows the melting temperature interval. While Sn40Bi40Pb and Sn58Bi22Pb are sub-eutectic systems and per-eutectic systems, respectively, the melting intervals are larger than that of Sn50Bi30Pb alloy.

3.3 Microstructure and phase analysis

The results of X-ray diffraction (XRD) analysis of the alloy are shown in Fig. 6, which shows that the changes in the Pb and Bi contents did not generate new intermetallic compounds, and all of them are composed of Sn-(Bi, Pb) solid solution phase, Bi-(Pb) solid solution phase and Pb₇Bi₃ phase. It is consistent with the Bi-rich phase, Sn-rich phase, and ε-PbBi phase in the above analysis by phase diagram and the calculation results by JMatPro software.

Fig. 6

XRD pattern of Sn-Bi-Pb low melting point alloy.

Optical microscope images of the organization and morphology of Sn-Bi-Pb alloys are shown in Fig. 7, which shows that there are large differences in the organization and morphology of three different components of Sn-Bi-Pb alloys. Sn40Bi40Pb, Sn50Bi30Pb, and Sn58Bi22Pb alloys are organized as primary dendrites + (Bi+Pb) eutectic, but there are differences in primary dendrites of the three alloys, which are composed of larger fishbone-like Sn-(Bi, Pb) solid solution phases and smaller-sized island-like Bi+Pb solid solution phases. The primary dendrite of Sn40Bi40Pb alloy is composed of larger fishbone-like Sn-(Bi, Pb) solid solution phase and smaller size island-like Bi-(Pb) solid solution phase; the primary dendrite of Sn50Bi30Pb alloy is composed of relatively small cross-shaped Sn-(Bi, Pb) solid solution phase and larger size massive Bi-(Pb) solid solution phase, and the content of primary dendritic crystals was reduced. While Sn58Bi22Pb alloy may be due to the content of Bi element is much higher than that of Pb element, and (Bi+Pb) eutectic is relatively less, resulting in larger size of incipient dendrites consisting of irregularly shaped Sn-(Bi, Pb) solid solution and extremely large size of massive Bi-(Pb) solid solution.

Fig. 7

Organization of Sn-Bi-Pb low melting point alloy: (a)–(c) Sn40Bi40Pb, (d)–(f) Sn50Bi30Pb, and (g)–(i) Sn58Bi22Pb.

Figure 8 shows the SEM image of the microstructure and morphology of Sn50Bi30Pb alloy and EDS elements mapping, in which the gray-white area should be the enriched area of Bi and Pb elements, and the gray-black area is the enriched area of Sn elements. In the EDS elemental mapping analysis, the distribution areas of Pb and Bi elements are basically the same, and segregation occurs. Combined with the mapping scan distribution of Sn element in Fig. 8(d), it can be seen that the small black phase in the gray-white area is also Sn-rich phase. This is because during the solidification process, most of the Sn-rich phase first solidifies into a large black block, while the remaining Sn-rich phase remains in the molten state. With the further decrease of temperature, the fine black Sn-rich phase began to precipitate and formed eutectic structure with Pb₇Bi₃, corresponding to the hypoeutectic reaction: L → (Pb₇Bi₃/Sn)_eutectic.

Fig. 8

SEM images (a) and EDS element mapping of Sn50Bi30Pb low melting point alloy: (b) Pb, (c) Bi, and (d) Sn.

According to the analytical results of EDS point analysis in Fig. 9, it can be seen that point A is Sn-(Bi, Pb) solid solution phase and point B is Bi-(Pb) solid solution phase, which is consistent with the analytical results of the XRD diffraction pattern in Fig. 6, but the atomic ratio of Bi to Pb at point C is close to 6:4, and the results of the EDS analysis are inconsistent with the results of the XRD phase analysis. Therefore, it is assumed that the uneven distribution of Pb atoms in the gray-white region is due to the low Pb content and uneven distribution of the alloy, and most of the Pb atoms are polarized with Bi atoms to form Pb₇Bi₃ compounds, but there are still a few Pb atoms as solid solute atoms solidly dissolved in the Bi solid solution. Combined with the DSC curve, the first smooth small heat absorption peak should be the melting process of the Pb₇Bi₃ phase, which can prove that the C point is the Pb₇Bi₃ compound phase. In summary, the alloy can be considered as a three-phase sub-eutectic alloy, but Sn50Bi30Pb is not a three-phase sub-eutectic alloy in the traditional sense, and the alloy is actually a two-phase alloy because it conforms to the growth mechanism of eutectic coupling, and therefore the third phase, i.e., the Pb₇Bi₃ compound phase, is, in fact, a precipitation phase of the Bi-(Pb) solid solution phase [19].

Fig. 9

SEM images (a) and EDS results of region A (b), region B (c), and region C (d) in Sn50Bi30Pb low melting point alloy.

The secondary electron image of the microstructure of Sn-Bi-Pb alloy with different contents of Bi and Pb elements are given in Fig. 10, based on which the area fractions occupied by the Bi-(Pb) phase and Pb₇Bi₃, as well as the dimensions of the Bi-(Pb) phase in each alloy, were calculated using ImageJ software, and the results are shown in Fig. 11. With the increase of Bi content and the decrease of Pb elemental content, the area occupied by Pb₇Bi₃ in the alloy shows a trend from a slow decrease to a sharp decrease, while the trend of the increase of the area occupied by the Bi-(Pb) solid solution phase is relatively gentle, but the increase of the size of the Bi-(Pb) phase changes drastically, and the size of Bi-(Pb) phase in Sn40Bi40Pb alloy is increased by an average of 3.9 µm, as compared with that in Sn58Bi22Pb alloy. size increases from an average of 3.9 µm to an average of 29.2 µm, and significant coarsening occurs.

Fig. 10

SEM images of Sn-Bi-Pb low melting point alloy: (a)–(b) Sn40Bi40Pb, (c)–(d) Sn50Bi30Pb, and (e)–(f) Sn58Bi22Pb.

Fig. 11

Area fraction of Bi-(Pb) Phase and Pb₇Bi₃ (a) and size of Bi-(Pb) Phase (b) in Sn-Bi-Pb alloys.

Figure 12 shows the secondary electron image of the microscopic morphology of Sn50Bi30Pb and Sn58Bi22Pb low melting point alloys, in which the gray-black region is the Sn-(Bi, Pb) phase, and in the Sn-(Bi, Pb) phase region can be seen in the white dots or lumps, due to the Sn-(Bi, Pb) phase from the melting point down to the room temperature in the process of part of the elements of Bi and Pb will be desolvated from the supersaturated solid solution. While the Pb₇Bi₃ compound phase and the Bi-(Pb) phase are located in the off-white region, showing a reticular organization structure, it can be seen from the enlarged image that the Bi-(Pb) phase behaves smoother visually, and the Pb₇Bi₃ compound phase resembles a granular structure with more micro-interfaces. Due to the small size of the Pb₇Bi₃ compound and its close proximity to the Bi-(Pb) phase region in the SEM image, it is difficult to observe a clear demarcation between the Pb₇Bi₃ compound phase and the Bi-(Pb) phase in the gray-white region. As can be seen in the figure, the Sn-Bi-Pb alloy is different from the conventional alloy in morphology, which consists of a large number of Sn, Bi, and Pb micro-layered lamellar tissues inside the individual grains, and the grain boundaries are not clearly discernible due to the influence of the three-phase interlayers.

Fig. 12

High magnification morphology and phase micromorphology of low melting point alloy: (a) Sn50Bi30Pb and (b) Sn58Bi22Pb.

Intermetallic compounds and solid solutions are formed in Sn-Bi-Pb alloys, which have different solidification and crystallization modes. The classification of the eutectic crystallization mode can be judged by the entropy of dissolution, which crystallizes in a small planar phase when its value is higher than 23 J/(mol·K), and in a non-small planar phase when its value is lower than 23 J/(mol·K) [20]. The entropy of dissolution for the Pb₇Bi₃ compounds is higher than 23 J/(mol·K), whereas the rest of the metal monomers’ entropy of dissolution is less than 23 J/(mol·K). Therefore, due to the different crystallization modes of the Pb₇Bi₃ compound phase and the other solid solution phases, the microstructure morphology of the Sn-Bi-Pb alloy presents a complex and regular morphology as shown in Fig. 12.

3.4 Mechanical property

Figure 13(a) shows the stress-strain curves of Sn-Bi-Pb low melting point alloy. From the figure, it can be seen that the simultaneous increase of Bi element and decrease of Pb element content to Sn-Bi-Pb alloys significantly increased their tensile strength, but the elongation showed a slow decrease to a sharp decrease. Among them, Sn40Bi40Pb alloy has the best elongation of 64.32% and strength of 34.92 MPa; Sn58Bi22Pb alloy has the greatest tensile strength of 43.93 MPa, which is 25.8% higher than that of Sn40Bi40Pb alloy, but the elongation deteriorates to 26.81%; and Sn50Bi30Pb alloy is the most balanced. The Sn50Bi30Pb alloy is the most balanced, in the strength has obvious enhancement at the same time still has excellent elongation, strength can reach 38.59 MPa, compared with Sn40Bi40Pb alloy to enhance the 10.5%, elongation slightly decreased can reach 61.13%, in addition, it has the highest yield strength of the three in the three alloys show more excellent comprehensive mechanical properties.

Fig. 13

Stress-strain curves (a) and ultimate tensile strength and elongation (b) of the Sn-Bi-Pb low melting point alloys.

The trend presented by the elongation in Fig. 13(b) is close to the trend of the area occupied by Pb₇Bi₃ in the alloy above, and it can be reasonably hypothesized that the mechanical properties of the alloy are related to the size of the Bi-(Pb) phase in the alloy and the contents of the Bi-(Pb) and Pb₇Bi₃ phases. As the Bi content increases and the Pb content decreases, the Pb₇Bi₃ phase content in the alloy decreases and the Bi-(Pb) phase content increases, in addition to a dramatic increase in the size of the Bi-(Pb) phase. In the alloy in the stretching process, dislocations will be interrupted in the movement to the interface, dislocations in the movement between the phase and the phase there is an obstacle to the phase interface should be dislocation plugging accumulation of stress concentration occurs, easy to produce the initial crack, and when the Pb₇Bi₃ phase content decreases, the micro-interfacial content decreases as well, it will further enhance the alloy as a whole the ability of plastic deformation, so that the strength rises, such as the alloy Sn50Bi30Pb. In addition, Sn-Bi-Pb is a triphase alloy with large differences in strength and hardness between the three phases, its deformation is mainly concentrated in the soft phase, while the hard phase is almost no deformation, Bi-(Pb) phase is hard and brittle phase, the alloy deformation of the Bi-(Pb) phase of the degree of plastic deformation is small, for the alloy to form a diffuse reinforcement to improve the strength of the alloy, but due to the brittleness of the Bi-(Pb) phase is larger, the crack extension occurs preferentially in the Sn-(Bi, Pb) phase and Pb₇Bi₃ phase, so the content is too high and the size of the Bi-(Pb) phase will seriously affect the plasticity of the alloy, such as Sn58Bi22Pb.

Through the analysis of the melting properties and mechanical properties of Sn-Bi-Pb system low melting point alloys, it is found that the solid phase line temperature of Sn50Bi30Pb alloy is close to 94.0°C and the melting range is only 6.6°C, and it has better comprehensive mechanical properties.

Figure 14 shows the fracture morphology of Sn-Bi-Pb low melting point alloys, it can be seen that with the increase of Bi content and the decrease of Pb content, the fracture of the alloy from the ductile fracture mode to the brittle fracture mode. Among them, Sn40Bi40Pb alloy fracture has more large and deep dimples, the overall toughness fracture morphology, at this time the alloy plasticity reaches the best. When the Bi and Pb content continues to change, Sn50Bi30Pb alloy tensile fracture shows mixed fracture morphology characteristics, the alloy fracture at the larger dimples gradually disappears, the dimples become small and shallow, the interface locally begins to appear deconstructed surfaces and part of the tearing ridges. When Sn58Bi22Pb alloy, the fracture has been completely transformed into a brittle fracture, at this moment the alloy fracture surface has no tough nests, and the overall presence of more facets and a rock-like pattern, from the figure can be seen at this time the fracture mode for the fracture along the crystal, and can be observed in the magnified picture of some of the microcracks.

Fig. 14

Fracture morphology in Sn-Bi-Pb low melting point alloy: (a) Sn40Bi40Pb, (b) Sn50Bi30Pb, and (c) Sn58Bi22Pb.

The backscattered electron image of the high magnification fracture morphology of the Sn50Bi30Pb low melting point alloy is shown in Fig. 15. As can be seen in Fig. 15(a), the fracture shows a large deconvoluted facet and a river pattern formed by deconvoluted steps, due to the different deformation coordination of Sn-(Bi, Pb) and Bi-(Pb) phases, there are obvious cracks on the Bi-(Pb) phase, and the Sn-(Bi, Pb) phase shows a fracture morphology of the raised structure, and a large number of ribs can be seen on the fracture after magnification, and the ligamentous fossae and pores can be seen locally, and at the same time, more small facets and rock-like patterns are observed on the fracture surface. The fracture of Sn50Bi30Pb low melting point alloy is a mixed fracture of quasi-cleavage and along-crystal fracture, which can be observed at the same time, and the fracture mechanism of Sn50Bi30Pb low melting point alloy belongs to the mixed fracture of quasi-cleavage and along-crystal fracture.

Fig. 15

SEM image of fracture in (a)–(c) Sn50Bi30Pb low melting point alloy.

4. Conclusion

1. (1)    Through the JMatPro phase diagram calculation software, it can be seen that with the increase of the Bi content and the decrease of the Pb content, the solid-phase line temperature of the Sn-Bi-Pb alloy is almost unchanged, and the melting interval of the alloy becomes narrower and then wider. The results of the TG-DSC data show that the melting interval of the Sn50Bi30Pb alloy is 94.0°C∼100.6°C, and the melting range is 6.6°C. Pb and Bi in Sn50Bi30Pb alloy will form the intermetallic compound Pb₇Bi₃, which consumes part of the high melting point Bi elements, resulting in the Sn-Bi sub-eutectic system shifting to the eutectic system, close to the eutectic system, and therefore the melting point is close to the eutectic point, which then narrows down the melting temperature interval, and the melting characteristics of Sn50Bi30Pb are optimal.
2. (2)    The Sn-Bi-Pb alloy is a triphase eutectic alloy containing the Sn-(Bi, Pb) phase, the Bi-(Pb) phase, and the Pb₇Bi₃ compound phase, but the alloy is actually dual-phases because it conforms to the eutectic-coupled growth mechanism, and thus the third phase, the Pb₇Bi₃ compound phase, is actually the precipitation phase of the Bi-(Pb) solid solution phase. With the increase of Bi content and the decrease of Pb elemental content, the content of the Pb₇Bi₃ phase in the alloy shows a decreasing trend, while the content of Bi-(Pb) solid solution phase increases gently, but the increase of its phase size varies drastically, and the size of the Bi-(Pb) phase in the Sn40Bi40Pb alloy increases from an average of 3.9 µm to an average of 29.2 µm, as compared to that in the Sn58Bi22Pb alloy, a significant coarsening occurred.
3. (3)    The simultaneous increase of Bi element and decrease of Pb element content to Sn-Bi-Pb alloy obviously improved its tensile strength, but the elongation showed a decreasing trend. Among them, Sn50Bi30Pb alloy has the most excellent mechanical properties, its tensile strength is 38.60 MPa, elongation is 61.13%, the fracture as a whole is a large cleavage faceted and through the cleavage steps of the formation of the river pattern, zoom in to find that a large number of tearing ridges on the fracture, the local appearances of the dimples and holes, and at the same time, there are a large number of small facets and rocky pattern on the fracture surface, the tensile rupture mechanism belongs to the quasi-cleavage and mixed type of fracture along the crystal. The tensile fracture mechanism is a mixed type of quasi-cleavage and along-crystal fracture.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 52274333).

Song Zhuofei (Corresponding Author): Conceptualization, Funding Acquisition, Resources, Supervision, Writing - Review & Editing.

REFERENCES

• 1)   Y.  Maruya,  H.  Hata,  I.  Shohji and  S.  Koyama: Bonding Characteristics of Sn-57Bi-1Ag Low-Temperature Lead-Free Solder to Gold-Plated Copper, Procedia Eng. 184 (2017) 223–230. doi:10.1016/j.proeng.2017.04.089
• 2)   H.R.  Kotadia,  P.D.  Howes and  S.H.  Mannan: A review: On the development of low melting temperature Pb-free solders, Microelectron. Reliab. 54 (2014) 1253–1273. doi:10.1016/j.microrel.2014.02.025
• 3)   T.  Li,  Y.-G.  Lv,  J.  Liu and  Y.-X.  Zhou: A powerful way of cooling computer chip using liquid metal with low melting point as the cooling fluid, Forsch. Ingenieurwes. 70 (2006) 243–251. doi:10.1007/s10010-006-0037-1
• 4)   A.G.  Wee,  R.L.  Schneider and  S.A.  Aquilino: Use of low fusing alloy in dentistry, J. Prosthet. Dent. 80 (1998) 540–545. doi:10.1016/S0022-3913(98)70029-0
• 5)   A.G.  Fernández,  S.  Ushak,  H.  Galleguillos and  F.J.  Pérez: Development of new molten salts with LiNO₃ and Ca(NO₃)₂ for energy storage in CSP plants, Appl. Energy 119 (2014) 131–140. doi:10.1016/j.apenergy.2013.12.061
• 6)   A.G.  Fernández,  H.  Galleguillos,  E.  Fuentealba and  F.J.  Pérez: Corrosion of stainless steels and low-Cr steel in molten Ca(NO₃)₂–NaNO₃–KNO₃ eutectic salt for direct energy storage in CSP plants, Sol. Energy Mater. Sol. Cells 141 (2015) 7–13. doi:10.1016/j.solmat.2015.05.004
• 7)   H.  Ge,  H.  Li,  S.  Mei and  J.  Liu: Low melting point liquid metal as a new class of phase change material: An emerging frontier in energy area, Renew. Sustain. Energy Rev. 21 (2013) 331–346. doi:10.1016/j.rser.2013.01.008
• 8)   A.  Lipchitz,  G.  Harvel and  T.  Sunagawa: Determination of Specific Heat of Eutectic Indium – Bismuth-Tin Liquid Metal Alloys as a Test Material for Liquid Metal - Cooled Applications, Appl. Mech. Mater. 420 (2013) 185–193. doi:10.4028/www.scientific.net/AMM.420.185
• 9)   A.M.  Bolind,  A.P.  Shivprasad,  D.  Frazer and  P.  Hosemann: Essential aspects of controlling the oxygen content of molten tin in engineering applications, Mater. Des. 52 (2013) 168–178. doi:10.1016/j.matdes.2013.04.096
• 10)   V.I.  Subbotin,  M.N.  Arnol’dov,  F.A.  Kozlov and  A.L.  Shimkevich: Liquid-Metal Coolants for Nuclear Power, At. Energy 92 (2002) 29–40. doi:10.1023/A:1015050512710
• 11)   W.-D.  Steinmann,  D.  Laing and  C.  Odenthal: Development of the CellFlux Storage Concept for Sensible Heat, J. Sol. Energy Eng. 136 (2014) 011011. doi:10.1115/1.4024921
• 12)   A.  Fritsch,  J.  Flesch,  V.  Geza,  Cs.  Singer,  R.  Uhlig and  B.  Hoffschmidt: Conceptual Study of Central Receiver Systems with Liquid Metals as Efficient Heat Transfer Fluids, Energy Procedia 69 (2015) 644–653. doi:10.1016/j.egypro.2015.03.074
• 13)   S.  Li,  X.  Wang,  Z.  Liu,  Y.  Jiu,  S.  Zhang,  J.  Geng,  X.  Chen,  S.  Wu,  P.  He and  W.  Long: Corrosion behavior of Sn-based lead-free solder alloys: a review, J. Mater. Sci. Mater. Electron. 31 (2020) 9076–9090. doi:10.1007/s10854-020-03540-2
• 14)   J.H.  Lau: State of the Art of Lead-Free Solder Joint Reliability, J. Electron. Packag. 143 (2021) 020803. doi:10.1115/1.4048037
• 15)   Y.  Shi,  Y.  Yan,  J.  Liu,  Z.  Xia,  Y.  Lei,  F.  Guo and  X.  Li: Constitutive modeling on creep deformation for a SnPb-based composite solder reinforced with microsized Cu particles, Microelectron. Reliab. 50 (2010) 2020–2025. doi:10.1016/j.microrel.2010.07.008
• 16)  T. Xu: Ab initio Molecular Dynamics Study on Structure, Kinetics and Thermodynamics of Liquid Ti-Al Alloys, Master Thesis, Shandong University, (2022).
• 17)   A.  Takeuchi and  A.  Inoue: Classification of Bulk Metallic Glasses by Atomic Size Difference, Heat of Mixing and Period of Constituent Elements and Its Application to Characterization of the Main Alloying Element, Mater. Trans. 46 (2005) 2817–2829. doi:10.2320/matertrans.46.2817
• 18)  Q. Wang: Design and performance of Sn-Bi-Pb-Zn based high temperature heat transfer and heat storage materials, Doctor, Wuhan University of Technology, (2019).
• 19)  K. Zhou: Composition, microstructure and basic physicochemical properties of multi-component low-melting-point eutectic alloys, Master Thesis, Dalian University of Technology, (2016).
• 20)   Z.  Ling,  S.  Ling and  H.  Fu: Directional Solidification and Growth of Faceted Phases, Foundry Technol. 31 (2010) 1151–1155.