Eutectic reactions

Al—Li. Ahoys containing about two to three percent lithium [7439-93-2] Li, (Fig. 15) received much attention in the 1980s because of their low density and high elastic modulus. Each weight percent of lithium in aluminum ahoys decreases density by about three percent and increases elastic modulus by about six percent. The system is characteri2ed by a eutectic reaction at 8.1% Li at 579°C. The maximum soHd solubiHty is 4.7% Li. The strengthening precipitate in binary Al—Li ahoys is metastable Al Li [12359-85-2] having the cubic LI2 crystal stmcture, and the equhibrium precipitate is complex cubic... 

The copper-antimony phase diagram contains two eutectic reactions and one eutectoid reaction. For each reaction ... 

The cloudiness of ordinary ice cubes is caused by thousands of tiny air bubbles. Air dissolves in water, and tap water at 10°C can - and usually does - contain 0.0030 wt% of air. In order to follow what this air does when we make an ice cube, we need to look at the phase diagram for the HjO-air system (Fig. 4.9). As we cool our liquid solution of water -i- air the first change takes place at about -0.002°C when the composition line hits the liquidus line. At this temperature ice crystals will begin to form and, as the temperature is lowered still further, they will grow. By the time we reach the eutectic three-phase horizontal at -0.0024°C we will have 20 wt% ice (called primary ice) in our two-phase mixture, leaving 80 wt% liquid (Fig. 4.9). This liquid will contain the maximum possible amount of dissolved air (0.0038 wt%). As latent heat of freezing is removed at -0.0024°C the three-phase eutectic reaction of... 

DEF. A eutectic reaction is a three-phase reaction, by which, on cooling, a liquid transforms into two solid phases at the same time. It is a phase reaction, of course, but a special one. If the bottom of a liquid-phase field closes with a V, the bottom of the V is a eutectic point. 

Not all alloys in the lead-tin system show a eutectic pure lead, for example, does not. Examine the Pb-Sn phase diagram and list the composition range for which a eutectic reaction is possible. 

At 577°C the eutectic reaction takes place the liquid decomposes into solid (Al) mixed with solid Si, but on a finer scale than before (bottom of Fig. A1.32). This intimate mixture of secondary (Al) with secondary Si is the eutectic structure. 

Cast irons, although common, are in fact quite complex alloys. The iron-carbon phase diagram exhibits a eutectic reaction at 1 420 K and 4-3 wt.<7oC see Fig. 20.44). One product of this eutectic reaction is always austenite however, depending on the cooling rate and the composition of the alloy, the other product may be cementite or graphite. The graphite may be in the form of flakes which are all interconnected (although they appear separate on a... 

The peritectic transformation generally has little effect on the structure, properties or corrosion resistance of steels at room temperature an exception to this occurs in the welding of certain steels, when 6-ferrite can be retained at room temperature and can affect corrosion resistance. Furthermore, since most steels contain less than about 1 -0 oC (and by far the greatest tonnage contains less than about 0-3%C) the eutectic reaction is of relevance only in relation to the structure and properties of cast irons, which generally contain 2-4%C. This discussion, therefore, will be limited to the eutectoid reaction that occurs when homogeneous austenite is cooled. 

Eutectic reactions, liquid —a + f), can result in several geometric configurations of a and /3. When the volumes of both phases are nearly equal, the most common morphology is lamellar. This is true of the Cu-Ag and Pb-Sn eutectics. If the amount of one phase is much less than the other, the eutectic is likely to be rods of one phase surrounded by the other phase (e.g., NiAl-Cr, TaC-Ni... 

This is called the phase reaction for the eutectic reaction. On cooling at the eutectic point the reaction will proceed to die right. On heating the reaction will proceed to the left. Solids produced by the eutectic reaction is in general a fine grain mixture of A and B. 

On further cooling to just below the eutectic temperature, the remaining liquid which has the eutectic composition will freeze immediately according to the eutectic reaction. The solid structure will thus be the mixture of the primary phase of A and the eutectic structure which is the fine mixture of A and B. 

At the temperature Te, the remaining liquid (fraction =cf/ce) undergoes die eutectic reaction. 

The liquid composition at the eutectic reaction is represented by e. This liquid is transformed into solid A and solid B by the eutectic reaction. 

At point c, both A and B co-crystallise forming eutectic structure until all liquid is consumed. The temperature remains constant during this eutectic reaction ... 

During the eutectic reaction, the mean composition of the solid phases changes along the line yq. The solid composition reaches point q when the liquid is completely consumed by the eutectic crystallisation. 

At g, a eutectic reaction takes place until the last portion of liquid is completely consumed three solid phases AC, BC and B crystallise out together to form a eutectic structure. 

During the eutectic reaction at g, the mean solid composition changes from n to... 

The end of solidification, as defined here, is denoted the solidus temperature and is strongly dependent on the cooling rate. It was particularly difficult to determine the solidus temperature by thermal analysis in steels with a high carbon content. This is a result of their wide solidification ranges and very low growth rates near the end of solidification. Furthermore, eutectic reactions occurred at the end of solidification over a large temperature range which led to poorly defined minima in the derivative. 

As shown in figures 6.3 and 6.4, steels 303, 304 and 309 had very low solidus temperatures explained by eutectic carbide precipitation. The pseudobinary equilibrium phase diagrams for Fe — 5Cr — C and Fe — 13Cr - C, figures 3.1 and 3.2, indicate a eutectic reaction for carbon contents of 1,2 and 0,8% respectively. The appearance of carbide eutectics at much lower carbon concentrations is a result of microsegregation. In figure 6.4 the solidus lines have been interrupted between steels 308 and 309, as it is certain that eutectic precipitation of carbides will take place in steels with carbon contents lower than that of steel number 309. 

Obviously, Scheil s equation (predicting an infinite solute concentration when the solid fraction becomes 1) is invalid above a certain system-specific liquid concentration, where the remaining liquid solidifies by some eutectic reaction. 

A B-rich liquid forms at the reaction interface with the B4C as a result of this temperature rise. The first B-rich liquid in the system outside of molten boron occurs at 2165°C from eutectic reaction of B4C, C, and ZrCl (l. On further local heating (T > 2220°C), a continuous B-rich liquid can form between pure boron and the eutectic liquid. The exact composition of the B-rich liquid is unknown, but it appears to lie somewhere within the triangle formed by B4C, C, and ZrB2. 

The eutectically solidified ceramics represent a large group of ceramic systems with microstructures that can be tailored during cooling. Since the eutectic solidification is based on the simultaneous crystallization of two solids from one liquid, many complex arrangements of the eutectic phases have been observed [93]. A schematic representation of a eutectic reaction is as follows ... 

Figure 10.15 Determining a binary phase diagram containing a eutectic reaction by DSC or DTA. (Reproduced with kind permission of Springer Science and Business Media from M. Brown, Introduction to Thermal Analysis, Kluwer Academic Publishers, Dordrecht. 2001 Springer Science.)...

On production of FGM by the eutectic bonding method, a metal and an intermetallic compound are combined using a eutectic reaction which exists between both the substances. An importcint advantage in this combination of the substances is that relieving thermal stress is easy. Because the difference in thermal expansion coefficients between metals and intermetallic compounds is much smaller than that between metals and ceramics. 

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