a binary isomorphous system
/ eye-so-MOR-fus /
Isomorphous means same form. This is the friendliest phase diagram there is: two metals that dissolve in each other completely, in any proportion, both as liquid and as solid, the way alcohol and water mix to any ratio. Copper and nickel do exactly this. The diagram has only a liquid region, a solid region, and a lens-shaped two-phase region between them.
More precisely, a binary isomorphous system has two components fully soluble in each other in the solid state, forming a single solid solution across the whole composition range from 0 to 100 percent. Complete solid solubility demands that the Hume-Rothery rules be well satisfied: similar atomic size, the same crystal structure, and similar electronegativity and valence. Cu and Ni are both FCC and nearly the same size, so they share one FCC lattice at every composition. Only two lines appear, the liquidus on top and the solidus below, meeting to close the lens.
To read it, pick a composition and temperature; if you land between the two lines, draw a tie line to read the two coexisting phase compositions and apply the lever rule for their amounts. On real, non-equilibrium cooling the solid cores, meaning each grain ends up layered from a high-melting-metal-rich center to a low-melting-metal-rich rim, because solid-state diffusion is too slow to even it out. A homogenizing anneal repairs this.
Cu-Ni is the textbook isomorphous system, the basis of monel and cupronickel coinage alloys.
One FCC solid solution spans every composition, so only liquidus and solidus appear.
Complete solid solubility is actually rare; it needs the Hume-Rothery rules met. Most metal pairs have limited solubility and instead form eutectic or more complex diagrams.