Chapter 9: Phase Diagrams

Phase diagrams serve as fundamental tools in materials science for predicting the stable structures of materials across varying temperatures, compositions, and pressures. This chapter establishes core concepts including components as elemental or compound constituents, phases as homogeneous regions possessing distinct properties, and microstructure as the spatial arrangement and relative amounts of phases present. The solubility limit represents the maximum concentration of solute atoms that can dissolve in a solvent phase before a new phase emerges, while phase equilibrium describes the thermodynamically stable state defined by free energy minimization. The chapter begins with unary phase diagrams exemplified by water, illustrating critical features such as the triple point where solid, liquid, and vapor states coexist in equilibrium. Binary phase diagrams, plotting temperature against composition at constant pressure, enable prediction of alloy microstructures and reveal key boundaries through liquidus, solidus, and solvus lines. Isomorphous systems such as copper-nickel demonstrate complete solubility in both solid and liquid states, with tie lines and the lever rule providing quantitative methods for determining phase compositions and their relative proportions. Equilibrium solidification contrasts sharply with nonequilibrium cooling, which produces segregated cored structures that require homogenization heat treatments. Eutectic systems like lead-tin exhibit characteristic transformations where a liquid phase decomposes into two solid phases at specific compositions and temperatures, generating distinctive lamellar microstructures. The chapter addresses intermediate phases and intermetallic compounds alongside invariant reactions including eutectoid transformations where one solid phase converts into two distinct solids, and peritectic reactions combining liquid and solid phases to form different solid products. The Gibbs phase rule establishes the theoretical framework for calculating the number of phases capable of coexisting at equilibrium. The iron-iron carbide system forms the culminating application, underpinning steel and cast iron metallurgy through phases including ferrite with body-centered cubic structure and low carbon solubility, austenite with face-centered cubic arrangement and higher carbon capacity, and cementite as an intermetallic compound. The eutectoid reaction at 0.76 weight percent carbon produces pearlite, a lamellar microconstituent governing steel mechanical properties, while compositional ranges define hypoeutectoid and hypereutectoid steel classifications. Alloying element additions systematically shift eutectoid composition and temperature, enabling engineering of strength, toughness, and heat-treatability for diverse applications.