Binary eutectic alloys

FIGURE 6.3 Example temperature and composition-induced phase transformations in the Cu-Ag binary eutectic alloy system. 

Normal alloys are associated with regular microstructures of a lamellar or rod form. Dining their solidification process, the constituent phases of a normal eutectic alloy grow into the liquid phase in a coupled or coordinated mechanism, which usually requires that the growth rates of the constituent phases are the same, and their growth mechanisms are non-faceting. For a normal binary eutectic alloy, two metals usually have similar melting points, and the two constituent phases have the same volume fraction. 

The addition of zinc results in a melting point (liquidus temperature) reduction. For example, the melting point of Sn-3.5Ag (binary eutectic alloy) is 221 °C, which is reduced to 217°C for Sn-3.5Ag-1.0Zn[16j. 

A series of experiments have been undertaken to evaluate the relevant thermodynamic properties of a number of binary lithium alloy systems. The early work was directed towards determination of their behavior at about 400 °C because of interest in their potential use as components in molten salt batteries operating in that general temperature range. Data for a number of binary lithium alloy systems at about 400 °C are presented in Table 1. These were mostly obtained by the use of an experimental arrangement employing the LiCl-KCl eutectic molten salt as a lithiumconducting electrolyte. 

Binary liquid metal systems were used in liquid-metal magnetohydrodynamic generators and liquid-metal fuel cell systems for which boiling heat transfer characteristics were required. Mori et al. (1970) studied a binary liquid metal of mercury and the eutectic alloy of bismuth and lead flowing through a vertical, alloy steel tube of 2.54-cm (1-in) O.D., which was heated by radiation in an electric furnace. In their experiments, both axial and radial temperature distributions were measured, and the liquid temperature continued to increase when boiling occurred. A radial temperature gradient also existed even away from the thin layer next to the... 

Fig. 3.12. Different types of surface scans a two-dimensional isoline representation of the Mn distribution on a steel surface measured by M-OES b the same Mn distribution in a three-dimensional representation (Danzer [1995]) c EPMA scan of Si in a binary eutectic Al-Si alloy...

Ref 1), The term is generally applied to alloys melting below 450°F(233°C). They can be binary, ternary, quaternary, or quinary mixts of Bi, Pb, Sn, Cd, Indium and less frequently other metals. Eutectic alloys are relatively few in number and are the particular compns that have definite and minimum mp as compared with other mixts of the same metals. Table I of Ref 2 lists 17 eutectics of mp s ranging from 46.89° to 248.0°C. Of these the Lipowitz s eutectic, melting at 70.0° is well known. It consists of Bt 50, -Pb 27, Sn 13 Cd 10%. Table II of Ref 2 lists 13 non-eu ectic alloys with mp s ranging from 64.0 to 1J9°C. Table III of Ref 2 lists eight common fusible alloys of mp s 70 to 138°. Of... 

Naturally, the fixed composition phase transformations treated in this section can be accompanied by local fluctuations in the composition field. Because of the similarity of Fig. 17.3 to a binary eutectic phase diagram, it is apparent that composition plays a similar role to other order parameters, such as molar volume. Before treating the composition order parameter explicitly for a binary alloy, a preliminary distinction between types of order parameters can be obtained. Order parameters such as composition and molar volume are derived from extensive variables any kinetic equations that apply for them must account for any conservation principles that apply to the extensive variable. Order parameters such as the atomic displacement 77 in a piezoelectric transition, or spin in a magnetic transition, are not subject to any conservation principles. Fundamental differences between conserved and nonconserved order parameters are treated in Sections 17.2 and 18.3. 

For a number of years, thallium sulfate had been used in rodenticides. Some use of thallium has been made in connection with alloys for low-temperature applications, particularly for switches, seals, and thermometers. The ternary eutectic mercury-thallium-indium alloy has a freezing point of —63.3 JCt while the binary eutectic mercury-thallium alloy has a freezing point of — 60°C. These freezing points are considerably lower than that of mercury usually used for similar applications at higher temperatures. Mercury freezes at —38.87JC. 

The volume fraction of reinforced phase in eutectics is 7.7 % and 31-wol.% for systems Ti-B and Ti-Si, respectively. The typical structures of eutectic alloys for Ti-Si system is shown in Fig. 2. According to binary diagrams of phase equlibria, an essential solubility of silicon in a- and 13-phases is observed, which is dependent on temperature there is an eutectoid transformation (in this respect diagram Ti-Si is similar to Fe-C diagram), but in system Ti-B essential solubility of boron in a- and 3- phases does not occur. For this reason the structure of composites of Ti-B system is more stable at temperature variation. 

It was noted above, that the as-cast eutectic alloys of binary Ti-Si system have no appreciable plasticity at room temperature. Alloying with aluminum, zirconium and use of various modifiers has also not allowed appreciable RT plasticity to be obtained. Data on temperature dependence of mechanical properties of deformed alloys of system Ti-3A1 6Zr-(2-6) Si show that, in contrast to as-cast alloys, deformed state with about 2 % Si demonstrate high plasticity ( 4%) reducing to 1.8 % in alloy with 6-wt.% Si. At the same time high-temperature strength of these alloys are practically the same, at 540-560 MPa level (600 °C). In such a way there is no reason to increase silicon content higher 2-wt.% in deformed state. 

As an instance, the partial Ti-Si-B phase diagram is presented in Fig. 1. Silicon is not practically dissolved in TiB, and is fully concentrated in the metal matrix and in a ternary boro-silicide when it forms. This ternary boro-silicide phase of unknown structure (designated here as the T-phase) was found to be present as a very fine dispersoid around 200 nm in diameter in the three-phase eutectic (Ti) + T + TiB (Fig. 2). The ternary eutectic alloy hardness vv temperature plot shows a great potential for strengthening up to 600°C in comparison with the binary (Ti) + TiB (Fig. 3). 

Figure 3. Temperature dependences of hardness (HV) for the ternary (Ti) + T +TiB and binary (Ti) +TiB eutectic alloys.

Sodium is miscible with the alkali metals below it in the periodic table (i.e., K, Rb, and Cs), and it forms a eutectic alloy with potassium (Na22 wt.% K78 wt.%) commercially known as NaK, which melts at -10°C. The eutectics formed in the Na-Rb and Na-Cs binary systems melt respectively at -4.5°C and -30°C. Sodium is the minor component with potassium and cesium of the ternary alloy Na-K-Cs. The composition of this ternary alloy is 3 wt.% Na, 24 wt.% K, and 73 wt.% Cs. This fluid is the lowest melting liquid alloy yet isolated, melting at -78°C. Sodium colors the flame of a Bunsen gas burner with a characteristic yellow color owing to the highly intense D line of its atomic spectra (589 nm). Sodium is ordinarily quite reactive with air, and its chemical reactivity is a function of the moisture content of air. 

Bi forms a binary eutectic with Sn in the proportions 58Bi 42Sn (m.p., 138 C). There are numerous bismuth alloys in use, many composed of two or more metals in addition to bismuth. The Sn-Bi eutectic, if contaminated with Pb, can be problematic as it is known to form a ternary alloy with a melting point of 96°C adversely affecting solder-joint fatigne characteristics. In some applications, solder joints will fall apart if the service temperatnre is high and the low melting point Sn-Bi-Pb alloy is formed. This becomes all the more critical with low-volume Sn-Bi solder joints. The Pb can come from solder predeposited on component leads, Sn-Pb hot-air solder leveled pads, or both. 

The most commonly used electrolytes are the LiCl KCl binary eutectic and the LiF-LiCl Lil ternary lithium halides. With Li-Al alloy anodes and FeSj positive electrodes the discharge occurs in several discrete steps ... 

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