Silver-copper alloys, phase diagram

Fig. 2. Liquidus of isotherms of gold-copper-silver alloys and phase diagrams of the binary constituents (83).

P. R. Subramanian and J. H. Perepezko, Ag-Cu (silver-copper) system, in Phase Diagrams of Binary Copper Alloys (P. R. Subramanian, D. J. Chakrabarti and D. E. Laughlin, eds.). Materials Park, OH ASM International, 1994. 

Consider a binary alloy of copper and silver. At a temperature near 850 °C and an alloy composition near 80% Cu-20% Ag, the phase diagram says that there should be a copper-rich solid and a liquid. Considering the pressure to be fixed at one atmosphere but allowing temperature variations,... 

Figure 13.2 shows two temperamre-composition phase diagrams with single eutectic points. The left-hand diagram is for the binary system of chloroform and carbon tetrachloride, two liquids that form nearly ideal mixmres. The solid phases are pure crystals, as in Fig. 13.1. The right-hand diagram is for the silver-copper system and involves solid phases that are solid solutions (substimtional alloys of variable composition). The area labeled s is a solid solution that is mostly silver, and s is a solid solution that is mostly copper. Tie lines in the two-phase areas do not end at a vertical line for a pure solid component as they do in the system shown in the left-hand diagram. The three phases that can coexist at the eutectic temperature of 1,052 K are the melt of the eutectic composition and the two solid solutions.

As silver is added to copper, the temperature at which the alloys become totally liquid decreases along the liqnidns line, line AE thus, the melting temperature of copper is lowered by silver additions. The same may be said for silver the introduction of copper reduces the temperature of complete melting along the other liquidus line, FE. These liquidus lines meet at the point E on the phase diagram, which point is designated by composition and temperature Te, for the copper-silver system, the values for these two parameters are 71.9 wt% Ag and 779°C (1434°F), respectively. It should also be noted there is a horizontal isotherm at 779°C and represented by the line labeled BEG that also passes through point E. 

In a eutectic reaction, as found in some alloy systems, a hquid phase transforms isothermally into two different solid phases upon cooUng (i.e., L + /3). Such a reaction is noted on the copper-silver and lead-tin phase diagrams (Figures 9.7 and 9.8, respectively). 

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