Which statement best describes what a binary phase diagram represents?

• A. A map of diffusion pathways for two components.
• B. A graph of phase stability and coexistence of two components as a function of composition and temperature.
• C. A plot of the mechanical properties of alloys versus temperature.
• D. A diagram showing the electronic structure of the two components.

Answer: (D)

A binary phase diagram maps which phases are stable for a two-component system across different compositions and temperatures. The horizontal axis shows composition from pure component A to pure component B, while the vertical axis shows temperature. Regions where only one phase is stable (single-phase regions) and regions where two phases coexist in equilibrium (two-phase regions) are delineated by phase boundaries, such as liquidus and solidus lines. The diagram also highlights invariant reactions at fixed temperatures where the phase assemblage changes abruptly (like eutectic or peritectic points). This description best captures what a binary phase diagram represents because it directly conveys phase stability and coexistence as composition and temperature vary. It does not depict diffusion pathways, mechanical property trends, or electronic structure.