A binary alloy is a metallic material whose specified alloy system has two elemental components. “Binary” counts components, not phases: one composition may form a single solid solution, while another or a different temperature can produce two coexisting phases.
If an overall component fraction lies between equilibrium phase compositions and , conservation of material gives the phase fractionThe fractions must use a consistent mass- or mole-based convention. A binary phase diagram therefore relates composition and temperature to equilibrium phase assemblages; it does not say that every alloy consists of separate pure-metal grains.
