Materials and Structure

Copper and Nickel form a substitutional solid solution over the complete range of possible compositions of the alloy. The equilibrium "phase diagram" opposite shows the dependence of the alloys melting point on its composition, and the existence of the single solid phase below the solidus line indicates the complete solid state miscibility of the two elements. The phase diagram shows that the melting point of each of the pure elements is a unique value: 1357 K for copper and 1728 K for nickel.

From: Newey and Weaver, "Materials Principles and Practice," Butterworths (1990)

For any other composition the alloy has a range of melting temperatures. The alloy indicated by the vertical green line has the composition Cu - 30 wt% Ni. This alloy starts to solidify at 1513 K, the temperature where the composition line crosses the liquidus line. The first solid has the composition: Cu - 48 wt % Ni (red line) the point at which the isotherm crosses the solidus line. The alloy then enters a range of temperatures where solid and liquid are in equilibrium but there compositions change, the liquid moving down the liquidus line and the solid down the solidus line. As the temperature is reduced the quantity of liquid decreases and the quantity of solid increases. Solidification is complete at the temperature where the composition line crosses the solidus line. The final liquid has the composition Cu - 14 wt % Ni. The final solid has the composition Cu - 30 wt % Ni as atoms must be conserved.