imperfections and the potential for movement along slip planes is reduced. Such materials therefore tend to be relatively hard and brittle with low ductility.

Most alloys have properties which are specific to the particular system being considered. The general principles discussed for metals in the previous section, however, also hold true for most alloy systems. Hence, the grain size of alloys can be controlled by the rate of cooling from the melt. Alloys can be work hardened and they undergo recrystallization and grain growth under the correct conditions.

6.4 Cooling curves

Metals and alloys are sometimes characterised using cooling curves. The material is heated till molten then allowed to cool and a plot of temperature against time is recorded, as shown in Fig. 6.6. For a pure metal (Fig. 6.6a) the cooling curve displays a distinct plateau region at the melting point (Tm) indicating that temperature remains constant over a period of time during crystallization. With few exceptions, the cooling curves for alloys show no such plateau region (Fig. 6.6b). Crystallization begins at temperature T1, and is complete at temperature T2. Hence crystallization takes place over a range of temperatures.

For a binary solid solution alloy of two metals, A and B, in which the melting point of metal A is greater than that of metal B, the first material to crystallize, at just below temperature T1, will be rich in the higher melting point metal A, whilst the last material to crystallize, at a temperature just above T2, is rich in the lower melting point metal B. Each alloy grain can be envisaged as having a concentration gradient of metals; the higher melting metal being concentrated close to the nucleus and the lower melting metal close to the grain boundaries.

The material is said to have a cored structure. Such coring may influence corrosion resistance since electrolytic cells may be set up on the surface of the alloy between areas of different alloy composition.

If a series of cooling curves for alloys of different composition within a given alloy system are available a phase diagram can be constructed from which many important predictions regarding coring and other structural variations can be made.

Fig. 6.6 Cooling curves for (a) a pure metal, showing solidification at a fixed temperature and (b) an alloy showing solidification over a range of temperatures.

6.5 Phase diagrams

The temperature range over which an alloy crystallizes can readily be obtained from the cooling curve, as illustrated in Fig. 6.6b. If the temperatures T1 and T2 are obtained over a range of compositions for an alloy system and their values plotted against percentage composition, a useful graph emerges. This is illustrated in Fig. 6.7 for a hypothetical solid solution alloy of metals A and B. The melting points of the pure metals are indicated by the temperatures TmA and TmB. The upper and lower temperature limits of the crystallization range, T1 and T2, are shown for four alloys ranging in composition from 80% A/20% B to 20% A/80% B.

The phase diagram is completed by joining together all the T1 points and all the T2 points, together with the melting points of the pure metals, TmA and TmB. At temperatures in the region above the top line, known as the liquidus line, the alloy is totally liquid. At temperatures in the region below the bottom line, known as the solidus line, the alloy is totally solid. At temperatures in the region between the solidus and liquidus lines the alloy consists of a mixture of solid and liquid. The composition of the solid and liquid phases at any temperature between T1 and T2 can be predicted with the aid of the phase diagram.

Fig. 6.7 Phase diagram of a solid solution alloy constructed from a series of cooling curves (Fig. 6.6). The temperatures T1 and T2 are obtained from experiments using alloys of varying composition.

Fig. 6.8 Diagram illustrating how a solid solution phase diagram can be used to predict or explain certain alloy characteristics.

Solid solution phase diagrams

Figure 6.7 shows how the phase diagram for a binary solid solution alloy is constructed. The diagram is redrawn in Fig. 6.8 in order to illustrate how it may be used to predict some of the characteristics of the alloy. Consider, for example, an alloy of composition X (approximately 60% A and 40% B). This alloy may be rendered completely molten by heating it to a temperature above TL which represents the liquidus temperature for that particular composition. If the alloy is cooled from above TL it remains molten until the temperature TL is reached, when the first solid

begins to form. The composition of the first solid to form is given by drawing a horizontal line or tie line to intersect the solidus. In this case, drawing such a tie line reveals that the first solid to form has composition Z (approximately 90% A/10% B). As the alloy is cooled further, more crystallization occurs and between temperatures TL and TS a mixture of solid and liquid exists. Selecting one temperature, TSL, within this region, the composition of both solid and liquid can be predicted, by noting where the tie line intersects both solidus and liquidus. Thus, at temperature TSL the composition of the solid is Y (approximately 80% A/20% B) and the composition of the remaining liquid is W (approximately 75% B/25% A). On further cooling, the alloy becomes completely solid at temperature TS. The last liquid to crystallize has the composition V (approximately 80% B/20% A). This confirms the previous observation that for solid solution alloys a cored structure exists in which the first material to crystallize is rich in the metal with the higher melting point (A), whilst the last material to solidify is rich in the other metal (B). In the case of the alloy described, the variation in composition within the solidified alloy grains ranges from 90% A/10% B near the nuclei to 80% B/20% A at the grain boundaries. An indication of the degree of coring is given by the separation of the solidus and liquidus lines on the phase diagram. The potential for coring is greater when there is wide separation of solidus and liquidus lines as shown in Fig. 6.9.

The previous discussion describes what happens when a solid solution alloy is cooled rapidly, as occurs for example, during casting. With slow cooling the crystallization process is accompanied by diffusion and a random distribution of atoms results, with no coring. Rapid cooling quickly denies the alloy the energy and mobility required for diffusion of atoms to occur and the cored structure is ‘locked in’ at low temperatures. Reducing the cooling rate as a means of eliminating coring would be self-defeating since it would produce an alloy with large grain size which, of course, would have inferior mechanical properties.

The coring may markedly reduce the corrosion resistance of some alloys, a heat treatment is sometimes used to eliminate the cored structure. Such a heat treatment is termed a homogenization heat treatment. This involves heating the alloy to a temperature just below the solidus tempera-

ture for a few minutes to allow diffusion of atoms and the establishment of an homogeneous structure. The alloy is then normally quenched in order to prevent grain growth from occurring. An example of a solid solution alloy is the gold-silver system.

Eutectic phase diagrams

It has already been pointed out that some metals are completely insoluble in the solid phase and an alloy of such metals consists of a mixture of grains of the pure metal components. Phase diagrams may be constructed by exactly the same technique as that described for solid solutions – by constructing cooling curves and noting the temperatures at which crystallization commences and is complete. Such a phase diagram for a hypothetical alloy of two metals, A and B, is shown in Fig. 6.10. The liquidus line is given by joining points A, C and E, whilst the solidus is given by A B C D E. The temperatures TmA and TmB are again the melting points of the pure metals A and B. In the two triangular regions between the solidus and liquidus lines a mixture of solid and liquid exists. The solid is always one of the pure metals. To the right of point C it is always pure B and to the left of point C it is always pure A. The alloy with composition corresponding to point C is called the eutectic alloy and is of particular interest since it crystallizes at a given temperature and not over a range of temperatures. In this respect, the eutectic alloy behaves in a similar fashion to a pure metal. Alloys with composition close to the eutectic composition have narrow melting ranges and melting points considerably lower than those of the component pure metals. For these reasons they are often used as solders.

The eutectic phase diagram can be used to predict composition changes during crystallization in just the same way as the solid solution diagram was used. For the most simple case of the eutectic alloy, it solidifies at temperature TE to give a mixture of metals A and B. Figure 6.11 illustrates what happens during crystallization for an alloy, X, not having the eutectic composition. On cooling, crystallization begins at temperature TL. The intersection of the horizontal tie line with the solidus indicates that the first material to crystallize is pure metal A. At temperature T1 the alloy lies in the region between solidus and liquidus lines, indicating that a mixture of solid and liquid

exists. The compositions of solid and liquid are given by the points of intersection of the tie line with the solidus and liquidus lines. Thus it can be seen that at T1 the solid formed is still pure metal A whilst the remaining liquid has composition Y. On cooling further to temperature T2, the solid is still pure metal A but the remaining liquid has composition Z, very close to the eutectic composition (E). Finally, on reaching temperature TE the eutectic mixture of metals remaining in the liquid phase crystallize out.

Hence, a simplified explanation of what happens during cooling is that one of the pure metals crystallizes until the remaining mixture of molten metals has a composition equivalent to the eutec-

Fig. 6.9 Diagram to show how the extent of coring depends on the separation of solidus and liquidus line. (a) Solidus and liquidus widely separated. Extensive coring as illustrated by arrows. (b) Solidus and liquidus closer together resulting in less extensive coring.

tic. At this stage the remaining metals crystallize together.

The solidified alloy consists of a mixture of insoluble metals which often has inferior corrosion resistance due to the potential for the establishment of electrolytic cells on the surface of the alloy.

The situation of complete solid insolubility as described above is rarely met in practice. Much more common is the case of limited solubility. Where the solubility of one metal in the other in the solid state is low, the behaviour of the alloy is similar to that of a eutectic alloy except that instead of grains of pure metals being produced we get grains of two dilute solid solutions.