Eutectic Phase

Diagram

sugar in coffee. Consequently, only one phase (sweet coffee) is present. (The analogue is true for the -phase.) In the   re-gion, two phases are present, comparable to a mixture of blue and red marbles. The implications of this mixture of two phases to the strength of materials will be discussed later.

We consider now a silver alloy containing 28.1% copper called the eutectic composition (from Greek eutektos, “easy melting”).

We notice that this alloy solidifies at a lower temperature (called the eutectic temperature) than either of its constituents. (This phe-nomenon is exploited for many technical applications, such as for solder made of lead and tin or for glass-making.) Upon slow cooling from above to below the eutectic temperature, two solid phases (the - and the -phases) form simultaneously from the liquid phase according to the three-phase reaction equation:

L28.1% Cu씮8.8% Cu 92% Cu. (5.2) This implies that, for this specific condition, three phases (one liq-uid and two solid) are in equilibrium. The phase rule, F
C  P 1 [Eq. (5.1)] teaches us that, for the present case, no degree of free-dom is left. In other words, the composition as well as the temper-ature of the transformation are fixed as specified above. The eutec-tic point is said to be an invariant point. The alloy therefore remains at the eutectic temperature for some time until the energy differ-ence between solid and liquid (called the latent heat of fusion,

Hf) has escaped to the environment. This results in a cooling curve which displays a thermal arrest (or plateau) quite similar to that of pure metals where, likewise, no degree of freedom remains dur-ing the coexistence of solid and liquid. A schematic cooldur-ing curve for a eutectic alloy is depicted in Figure 5.8.

1000

FIGURE5.7. Binary copper–silver phase diagram containing a eutectic transfor-mation.

FIGURE5.9. (a) Schematic representation of a lamellar or platelike mi-crostructure as typically observed in eutectic alloys. (b) Photomicro-graph of a eutectic alloy, 180 (CuAl2–Al). Reprinted with permission from Metals Handbook, 8th Edition, Vol. 8 (1973), ASM International, Materials Park, OH, Figure 3104, p. 156.

The microstructure, observed by inspecting a eutectic alloy in an optical microscope, reveals a characteristic platelike or lamel-lar appearance; see Figure 5.9. Thin  and  layers (several crometers in thickness) alternate. They are called the eutectic mi-croconstituent. (A microconstituent is a phase or a mixture of phases having characteristic features under the microscope.) This configuration allows easy interdiffusion of the silver and the cop-per atoms during solidification or during further cooling.

Alloys which contain less solute than the eutectic composition are called hypoeutectic (from Greek, “below”). Let us assume a Ag–20% Cu alloy which is slowly cooled from the liquid state. Upon crossing the liquidus line, initially two phases ( and liquid) are pre-sent, similar as in an isomorphous alloy. Thus, the same consider-ations apply, such as a successive change in composition during

Temperature

Time FIGURE5.8.Schematic

representa-tion of a cooling curve for a eu-tectic alloy (or for a pure metal).

The curve is experimentally ob-tained by inserting a thermome-ter (or a thermocouple) into the liquid alloy and reading the tem-perature in periodic time inter-vals as the alloy cools.





  

(a) (b)

cooling, dendritic growth, and the lever rule. When the eutectic tem-perature (780°C) has been reached, the remaining liquid transforms eutectically into - and -phases. Thus, the microstructure, as ob-served in an optical microscope, should reveal the initially formed -solid-solution (called primary , or proeutectic constituent) inter-spersed with lamellar eutectic. Indeed, the micrographs depicted in Figure 5.10 contain gray, oval-shaped  areas as well as alternating black () and white () plates in between. A schematic cooling curve for a Ag–20% Cu alloy which reflects all of the features just dis-cussed is shown in Figure 5.11(a). For comparison, the cooling curve for an isomorphous alloy is depicted in Figure 5.11(b).

Silver alloys containing less than 8.8% Cu solidify similar to an isomorphous solid solution. In other words, they do not con-tain any eutectic lamellas. However, when cooled below the solvus line, the phase precipitates and a mixture of - and -phases is formed, as described previously in Section 5.2.1.

Hypereutectic alloys (from Greek, “above”) containing, in the present example, between 28.1 and 92% Cu in silver, behave quite analogous to the hypoeutectic alloys involving a mixture of pri-mary-phase (appearing dark in a photomicrograph), plus plate-shaped eutectic microconstituents.

FIGURE5.10.(a) Schematic representation of a microstructure of a hy-poeutectic alloy revealing primary  particles in a lamellar mixture of  and microconstituents. (b) Microstructure of 50/50 Pb-Sn as slowly so-lidified. Dark dendritic grains of lead-rich solid solution in a matrix of lamellar eutectic consisting of tin-rich solid solution (white) and lead-rich solid solution (dark) 400 , etched in 1 part acetic acid, 1 part HNO₃, and 8 parts glycerol. Reprinted with permission from Metals Handbook, 8th Ed. Vol 7, page 302, Figure 2508, ASM International, Materials Park, OH (1972).

(a)

(b)

The eutectoid reaction involves a transformation between three solid phases. Specifically, at the eutectoid point, a solid phase (say) decomposes into two other solid phases upon cooling, ac-cording to the reaction equation:

 씮  , (5.3)

and as schematically depicted in Figure 5.12. As for the eutectic reaction described in Section 5.2.2, no degree of freedom is avail-able at the eutectoid point, which requires that the transformation must take place at a fixed temperature and a fixed composition.

5.2.4

FIGURE5.12.Schematic represen-tation of a eutectoid three-phase reaction in a hypothetical binary phase diagram consisting of ele-ments A and B. The high-temper-ature region is not shown for clarity and emphasis.

5.2.3 Eutectoid Transformation

A process that involves the reaction between a solid and a liquid from which eventually a new solid phase emerges is called peri-tectic (from Greek peritekto, “melting the environment”). The equation is, for example:

 L 씮 , (5.4)

as schematically depicted in Figure 5.13. When a peritectic A–4%

B alloy is cooled from the liquid, the solidification starts by form-ing an -phase containing only about 1% B. During further