Thermodynamic descriptions have become available for a large number of alloy systems and allow the calculation of the phase diagrams of multicomponent alloys. This article begins with a discussion on three laws of thermodynamics: the Law of Conservation of Energy, the Second Law of Thermodynamics, and the Third Law of Thermodynamics. It informs that for transformations that occur at a constant temperature and pressure, the relative stability of the system is determined by its Gibbs free energy. The article describes the Gibbs free energy of a single-component unary system and the Gibbs free energy of a binary solution. It schematically illustrates the structure of a binary solid solution with interatomic bonds and shows how the equilibrium state of an alloy can be obtained from the free-energy curves at a given temperature. The article concludes with information on the construction of eutectic and binary phase diagrams from Gibbs free-energy curves.

This article discusses the two extremes of solute redistribution, equilibrium solidification and nonequilibrium Gulliver-Scheil solidification, for which solid redistribution of solute within the primary solid phase is the distinguishing parameter. The process and material parameters that control microsegregation are discussed in relation to the manifestations of microsegregation in simple and then increasingly complex alloy systems. The measurement and kinetics of microsegregation are discussed for the binary isomorphous systems: titanium-molybdenum; binary eutectic systems: aluminum-copper and aluminum-silicon; binary peritectic systems: copper-zinc; multicomponent eutectic systems: Al-Si-Cu-Mg; and for systems with both eutectic and peritectic reactions: Fe-C-Cr and nickel-base superalloy.

This article begins with the one-component, or unary, diagram for magnesium. The diagram shows what phases are present as a function of the temperature and pressure. When two metals are mixed in the liquid state to produce a solution, the resulting alloy is called a binary alloy. The article describes the various types of solid solutions such as interstitial solid solutions and substitutional solid solutions. Free energy is important because it determines whether or not a phase transformation is thermodynamically possible. The article discusses the thermodynamics of phase transformations and free energy, as well as kinetics of phase transformations. It concludes with a description of solid-state phase transformations that occur when one or more parent phases, usually on cooling, produces a phase or phases.

The term isomorphous refers to metals that are completely miscible in each other in both the liquid and solid states. This article discusses the construction of simple phase diagrams by using the appropriate points obtained from time-temperature cooling curves. It describes the two methods to determine a phase diagram with equilibrated alloys: the static method and the dynamic method. The article illustrates the construction of phase boundaries according to the Gibbs' phase rule and describes the calculation methods that allow the prediction of the phases present, the chemical compositions of the phases present, and the amounts of phases present. Phase diagrams provide useful information for understanding alloy solidification. The article provides two simple models that can describe the limiting cases of solidification behavior.

In some phase diagrams, the appearance of several reactions is the result of the presence of intermediate phases. These are phases whose chemical compositions are intermediate between two pure metals, and whose crystalline structures are different from those of the pure metals. This article describes the order-disorder transformation that typically occurs on cooling from a disordered solid solution to an ordered phase. It provides a table that lists selected superlattice structures and alloy phases that order according to each superlattice. The article informs that spinodal decomposition has been particularly useful in the production of permanent magnet materials, because the morphologies favor high magnetic coercivities. It also describes the theory of spinodal decomposition with a simple binary phase diagram.

This article begins with a schematic illustration of a eutectic system in which the two components of the system have the same crystal structure. Eutectic systems form when alloying additions cause a lowering of the liquidus lines from both melting points of the pure elements. The article describes the aluminum-silicon eutectic system and the lead-tin eutectic system. It discusses eutectic morphologies in terms of lamellar and fibrous eutectics, regular and irregular eutectics, and the interpretation of eutectic microstructures. The article examines the solidification of a binary alloy of exactly eutectic composition. It concludes with a discussion on terminal solid solutions.

This article presents the binary eutectic phase diagram to understand the various structures that evolve in a binary eutectic system during solidification. It describes the various classifications and solidification principles of the eutectic structures. The formation of halos in eutectic microstructures of most alloy systems is also discussed.

The physical and mechanical properties of aluminum alloy can be improved by strengthening mechanisms such as strain hardening used for non-heat treatable aluminum alloy and precipitation hardening used for heat treatable aluminum alloy. This article focuses on the effect of strengthening mechanisms on the physical and mechanical properties of non-heat treatable and heat treatable aluminum alloys. It describes the use of the aluminum alloy phase diagram in determining the melting temperature, solidification path, equilibrium phases, and explains the effect of alloying element in phase formation.

The application of phase diagrams is instrumental in solid-state transformations for the processing and heat treatment of alloys. A unary phase diagram plots the phase changes of one element as a function of temperature and pressure. This article discusses the unary system that can exist as a solid, liquid, and/or gas, depending on the specific combination of temperature and pressure. It describes the accomplishment of conversion between weight percentage and atomic percentage in a binary system by the use of formulas. The article analyzes the effects of alloying on melting/solidification and on solid-state transformations. It explains the construction of phase diagrams by the Gibbs phase rule and the Lever rule. The article also reviews the various types of alloy systems that involve solid-state transformations. It concludes with information on the sources of phase diagram.

This article discusses the application of thermodynamic in the form of phase diagrams for visually representing the state of a material and for understanding the solidification of alloys. It presents the derivation of the relationship between the Gibbs energy functions and phase diagrams, which forms the basis for the calculation of phase diagrams (CALPHAD) method. The article also discusses the calculation of phase diagrams and solidification by using the Scheil-Gulliver equation.

Alloy phase diagrams are useful for the development, fabrication, design and control of heat treatment procedures that will produce the required mechanical, physical, and chemical properties of new alloys. They are also useful in solving problems that arise in their performance in commercial applications, thus improving product predictability. This article describes different equilibrium phase diagrams (unary, binary, and ternary) and microstructures, description terms, and general principles of reading alloy phase diagrams. Further, the article discusses plotting schemes; areas in a phase diagram; and the position and shapes of the points, lines, surfaces, and intersections, which are controlled by thermodynamic principles and properties of all phases that comprise the system. It also illustrates the application of the stated principles with suitable phase diagrams.

This article presents conservation equations for heat, species, mass, and momentum to predict transport phenomena during solidification processing. It presents transport equations and several examples of their applications to illustrate the physics present in alloy solidification. The examples demonstrate the utility of scaling analysis to explain the fundamental physics in a process and to demonstrate the limitations of simplifying assumptions. The article concludes with information on the solidification behavior of alloys as predicted by full numerical solutions of the transport equations.

This article reviews the fundamental solidification concepts for understanding microstructural evolution in fusion welds. The common concepts, namely, nucleation, competitive grain growth, constitutional supercooling, solute redistribution, and rapid solidification, depend on the solidification parameters during welding, are discussed. The article discusses important solidification parameters, including temperature gradient, solid/liquid interface growth rate, and cooling rate.

This article provides a thorough review of the physical metallurgy of aluminum alloys and its role in determining the properties and from a design and manufacturing perspective. And its role in include the effects of composition, mechanical working, and/or heat treatment on structure and properties. This article focuses on the effects of alloying and the metallurgical factors on phase constituents, structure, and properties of aluminum alloys. Effects from different combinations of alloying elements are described in terms of relevant alloy phase diagrams. The article addresses the underlying alloying and structural aspects that affect the properties and possible processing routes of aluminum alloys. It provides information on the heat treatment effects on the physical properties of aluminum alloys and the microstructural effects on the fatigue and fracture of aluminum alloys. The important alloying elements and impurities are listed alphabetically as a concise review of major effects.