5 a l l o y i n g b e h av i o r

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1 has dissolved in A to form a substitutional solid solution (igure 5.1b). It can be said also that this is a single-phase alloy. The word phase simply implies that the crystal structure and the alloy composition in terms of the amounts of Metal A and Metal B are the same throughout the crystal. In some cases, it is possible to substitute every atom of A with an atom of B without changing the basic crystal structure until the structure of Metal B is obtained (igure 5.1c) and we say that Metals A and B can form a complete series of solid solutions. This can only happen if both metals have the same type of crystal lattice. Only the lattice parameter will change because of a difference in the sizes of atoms of A and B. Remember, this is a stylized two-dimensional representation in one plane of an f.c.c. lattice but the same is happening three-dimensionally on every plane and orientation within the crystal. One binary alloy system that shows the above behavior of a complete series of solid solutions happens to be the gold silver system at all temperatures from the start of complete solidification down to room temperature and below. It can happen also for the gold-copper system but only at temperatures from the melting range down to 410 C (770 ). In other systems, there may be a limit to the number of atoms of B that can be dissolved in the lattice of A and vice-versa. One of several alternative actions may then take place. or instance, as the amount of Metal B exceeds the limit of solid solubility of B in A, a second solid solution phase based on the lattice of Metal B with some of its atoms substituted by atoms of A will appear. aid another way, the microstructure of the alloy may be a twophase structure consisting of crystals of both types of solid solutions (igure 5.2). A good example of this behavior is in the silvercopper binary alloy system. All three binary igures 5.1a 5.1c A igure 5.1a A B igure 5.1b B igure 5.1c systems mentioned in this paragraph have great theoretical and practical relevance and importance for silversmiths, goldsmiths, and jewelers as we shall see later. Other alternatives are that intermediate phases may be produced that are different in structure and alloy composition to either of the terminal solid solutions based on the lattices of A or B. These intermediate phases may exist over a range of composition and [ 57 ]

2 I n t r o d u c t i o n t o p r e c i o u s m e ta l s igure 5.2 tructure of a two-phase alloy. Note the different lattice structures of the two phases. temperature. It may also happen that some intermediate phases exist only at certain set atomic ratios. In practice, these may show a narrow range of composition. ain, this is of great importance for the gold-copper alloys at certain narrow composition ranges as it affects the mechanical properties and workability of the red karat golds. Pure metals melt and solidify at one temperature characteristic for that particular metal. This can be shown in a cooling curve for a pure metal that has been melted in a crucible containing a temperature measuring device such as a thermocouple. On removing the heat source used for melting, the fall in temperature is measured against time (say, at 10 second intervals), until solidification is complete and beyond. The resulting plot of temperature versus time in seconds is the characteristic cooling curve (igure 5.3). Temperature remains constant during the time that the metal solidifies (or conversely melts) as is shown by the plateau on the curve in spite of the fact that the heat source has been removed. The reason is that heat is evolved when a substance transforms from the melt to the solid and this is known as the latent heat of transformation. The point denotes the start of solidification and the completion. The larger the mass of metal involved, the greater is the length of the plateau. Once solidification is complete, the temperature continues to fall as would be expected. The vast majority of alloys melt and solidify over a range of temperatures, which is known as the melting or freezing range. There are some exceptions that we shall Melting Point Temperature Time igure 5.3 Characteristic cooling curve for a pure metal. [ 58 ]

3 deal with later. igure 5.4 shows a series of cooling curves for pure gold, pure silver, and three gold-silver alloys. ain, points denote the start of solidification and points the completion. You will notice that the freezing ranges of the alloys are between the melting points of the pure metals for this alloy system although this need not be necessarily true for all alloy systems. Binary Alloy Phase Equilibrium Diagrams Metallurgists have a convenient way of displaying alloying behavior between metals in terms of alloy composition, melting and solidification temperatures, solid solution formation, and intermediate phases. Data from the cooling curves can be plotted as temperature versus composition. or example, if the points on the cooling curves shown in igure 5.4a are plotted against composition expressed in terms of weight percentage of silver in the alloy, the ºC 1047ºC 1037ºC Liquid L Temperature (ºC) % Au 20% 50% 1015ºC 1000ºC olidus 984 C 975 C 962 C Liquidus % 100% olid α (a) Time (b) Au Composition Wt% igure 5.4 Construction of the binary gold-silver phase diagram from cooling curves. a) Cooling curves (left) b) Phase diagram (right) [ 59 ]

4 I n t r o d u c t i o n t o p r e c i o u s m e ta l s Gold Atomic Percent Liquid L º, 80.1% olid olution (Au, Cu) º, 75.6% 385º AuCu 285 AuCu 3 390º, 50.8% Au Cu Gold Weight Percent igure 5.5 The gold-copper binary phase diagram. upper line in the diagram shown in igure 5.4b, can be drawn by connecting the points. This line is known as the liquidus and any alloy in the system at a temperature above it will be completely liquid (the phase field L). imilarly, the points can be plotted to produce the lower line known as the solidus. We know that gold and silver form a complete series of solidification and these will be completely solid below the solidus. Therefore, this region in the diagram is a single phase field. By convention, letters of the Greek alphabet are used to label phase fields and so this is the α-phase. The region between the liquidus and the solidus is the two-phase field L+α. This type of diagram is an example of binary alloy phase equilibrium diagram and it describes the constitution of any alloy in the system from 100% Au to 100%, and at any temperature. The inclusion of the word equilibrium implies that the data have been collected under as near equilibrium conditions as possible. That is, slow heating or cooling conditions and uniform or homogeneous alloy compositions. In practice, such as in production of castings or quenching rapidly from annealing temperatures non-equilibrium conditions may apply and the diagrams may need to be interpreted with caution. Nevertheless, they do provide valuable information on alloying behavior. Another binary system of great importance in the metallurgy of karat gold alloys is the gold-copper (Au-Cu) system. As mentioned earlier, this system also displays a complete series of α-phase solid solutions from 0% to 100% Cu. However, this only occurs above 410 C (770 ) (igure 5.5). The upper and lower lines at the top of the diagram are the liquidus and solidus lines, [ 60 ]

5 COOL HEAT Cu Atom ABOVE 410 BELOW 410 Au Atom a) Disordered b) Ordered igure 5.6 The disorder-order transformation. respectively. It so happens that the liquidus and the solidus meet at a composition of 80.1 wt.% Au and a temperature of 911 C (1672 ). There are some who think that this is a eutectic alloy (see the -Cu system later), but it is not, because solidification does not follow the eutectic solidification reaction. Except to note that Au-Cu alloys have a very narrow solidification range, this is not of major importance. At temperatures below 410 C (770 ) and in certain compositions ranges, the α-phase transforms to intermediate phases centered on AuCu and AuCu3. You will notice that the composition is expressed in terms of weight percentage of copper on one scale and in terms of atomic percentage of copper on the top scale, i.e., the percentage of atoms in the alloy that are copper. The weight percent and atomic percent scales do not coincide because the atomic weights of the two metals are different. The phase AuCu is centered on 50 atomic% Cu, i.e., a 50:50 ratio of Au to Cu atoms or 1:1, meaning one atom of gold to one atom of copper which can be written as AuCu. imilarly, the phase AuCu3, or one atom of gold to three atoms of copper is centered on 25 at.% Au-75 at.% Cu. These phases have a tremendous bearing on the behavior of karat gold alloys in the manufacture of jewelry, as we shall see later in Chapter 9. These two phases are of a special type known as ordered phases where the gold and copper atoms take up certain set positions in the crystal lattice. The α-phase solid solution from which they form on cooling below 410 C or 390 C (770 or 734 ) is disordered, which is to say that the gold and copper atoms are randomly distributed throughout the f.c.c. lattice. The transformation on cooling is a disorder-to-order transformation. These transformations occur at compositions where the atom ratios are relatively simple, such as 1:1 and 1:3, as is the case with AuCu and AuCu3. Another characteristic of ordered phases is that in their formation, the crystal lattice of the disordered parent phase becomes severely distorted particularly [ 61 ]

6 I n t r o d u c t i o n t o p r e c i o u s m e ta l s Y D A Liquid L L + ß 800 α B L + α E 779 C C 92.0 ß Temperature C 600 olid α+β G Composition weight, percent copper X Cu igure 5.7 The silver-copper binary phase diagram. in one crystal lattice direction. It changes from f.c.c. for the disordered phase to face centered tetragonal (f.c.t.) for the ordered phase AuCu (igure 5.6). This has the effect of considerably increasing strength and hardness but decreasing the ductility and workability of the alloy. A further point to note from the two composition scales in igure 5.5 is that 50 atomic weight percent is virtually coincident with 75 wt.% Au 25 wt.% Cu, which is an 18k red gold. It should be said that this coincidence only applies to the Au-Cu system and not to other gold alloy systems. The relationship between atomic weight percent and weight percent depends on the atomic weights of the metals concerned. It is possible to convert one scale to the other from a knowledge of the atomic weights, but with the exception of appreciating the importance of AuCu and AuCu3 in understanding the metallurgy of some of the karat gold alloys, it is not necessary for silversmiths and jewelers to use the atomic percent scale. Therefore, for the remainder of this book, alloy compositions will be given in weight percent unless stated otherwise. Let us now consider a diagram that shows limited solid solubility of Metal A in Metal B and vice-versa. ortunately, there is an alloy system that is very relevant to silversmiths and jewelers. The silver-copper (-Cu) system, the binary phase diagram for which is shown in igure 5.7. There are a number of important features in this diagram. The liquidus is denoted by the upper lines AE and ED and the solidus by the sequence ABECD. ingle-phase liquid exists above the liquidus but, as mentioned earlier, silver and copper do not form a complete series of solid solutions in spite [ 62 ]

7 Au % Au (22kt) 40 14kt Composition Percent ilver 60 9kt X Y 75% Au(18kt) 60 Composition Percent Gold Cu Composition Percent Copper igure 5.8 Horizontal composition base for the Au--Cu ternary phase diagram. of the fact that they are both f.c.c. metals. Instead, the silver lattice can only dissolve a relatively small number of copper atoms forming a partial solid solution labeled α and the extent of this phase field is shown by the lines AB. The limit of solubility increases with temperature up to 8.8% Cu at point B (779 C or 1434 ). imilarly, copper can only dissolve up to a maximum of 8.0% in its lattice as shown by the lines DCG. This phase field is the partial solid solution β. The solidus happens to be at a constant temperature of 779 C between points B and C. Alloys having compositions between these points, i.e., % Cu, will solidify to form a mixture of two types of crystal, namely, the α- + β- phases or α+β (see igure 5.2). It is a general rule of binary phase diagrams that single-phase fields are separated by two-phase fields. The liquidus and the solidus come together at point E. This denotes a particular type of alloy structure known as a eutectic alloy. The name comes from the Greek word eutektos meaning easily melted. The fixed eutectic temperature is 779 C (1434 ) and the eutectic composition is 71.9% -28.1% Cu. The eutectic temperature is lower than the melting points of the parent metals. Binary eutectic reactions occur in many alloy systems and always take the form liquid L > two solid phases on solidification. urthermore, because they occur at the lowest liquidus temperature, they are often used as the basis for solder alloys. [ 63 ]