What information do tie lines convey on a binary phase diagram at a fixed temperature?

• A. They indicate the compositions of the two coexisting phases in equilibrium at that temperature
• B. They show the diffusion rates of each component
• C. They indicate the lattice parameters of each phase
• D. They mark the melting points of pure components

Answer: (A)

Tie lines at a fixed temperature connect the compositions of the two phases that coexist in equilibrium. The endpoints lie on the phase boundaries and give the compositions of each phase (Cα and Cβ); the overall alloy composition lies between them, with the phase fractions set by the lever rule. They don’t convey diffusion rates, lattice parameters, or melting points.