= Binary alloy
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 $x_0$ lies between equilibrium phase compositions $x_\alpha$ and $x_\beta$, conservation of material gives the phase fraction
$$
f_\alpha=\frac{x_\beta-x_0}{x_\beta-x_\alpha},\qquad f_\beta=1-f_\alpha.
$$
The 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.
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