a substitutional solid solution
A solid solution is a solid that is genuinely a mixture at the atomic level — two kinds of atom sharing one crystal, the way salt dissolves invisibly into water but frozen into a single solid lattice. In a SUBSTITUTIONAL solid solution, the guest (solute) atoms take the place of host (solvent) atoms on the ordinary lattice sites: same lattice, same sites, but a random sprinkling of them now holds the foreign element. It stays one crystal structure throughout — not two phases side by side, but a single homogeneous crystal of mixed composition.
Whether two metals form a substitutional solid solution, and over what range, is governed by the Hume-Rothery rules. Roughly: the atomic radii should differ by no more than about 15 percent, the two elements should have the same crystal structure, similar electronegativity, and similar valence. Meet all four well and the two can mix in any proportion (complete solid solubility). Copper and nickel are the storybook example: both face-centred cubic, radii within 3 percent, close in electronegativity and valence — so Cu and Ni dissolve into each other across the entire range from pure copper to pure nickel, every composition a single substitutional solid solution.
Substitutional solid solutions are the workhorses of alloy design. Because each substituted atom strains the lattice a little, they strengthen the metal (solid-solution strengthening) while keeping it a single, easily worked phase. Brass (zinc in copper), bronze (tin in copper), and the nickel-rich superalloys all rely on substitutional dissolving to tune strength, ductility, and corrosion resistance without forming brittle second phases — at least until the solubility limit is exceeded and the extra solute is forced to precipitate out.
The copper-nickel system shows complete solid solubility: at 50-50 you have a single face-centred-cubic crystal in which copper and nickel atoms randomly share every lattice site. There is no boundary, no second phase — just one substitutional solid solution whose lattice parameter smoothly interpolates between pure Cu (0.3615 nm) and pure Ni (0.3524 nm).
Cu-Ni: both FCC, similar size and chemistry, so they mix in all proportions as one substitutional solid solution.
Complete solubility (any proportion) is the exception, not the rule; most substitutional pairs dissolve only up to a limit set by size, valence, and electronegativity mismatch, beyond which a second phase appears. Meeting the Hume-Rothery rules is necessary but not a guarantee.