Nonplanar microstructures form most frequently during the solidification of alloys, and play a crucial role in governing the properties of the solidified material. This article emphasizes the basic ideas, characteristic lengths, and the processing conditions required to control the columnar and equiaxed microstructures. The formation of cellular and dendritic structures in one- and two-phase structures is presented with emphasis on the effect of processing conditions and composition on the selection of microstructure and microstructure scales.

This article reviews the various aspects of the simulation of solidification microstructures and grain textures. It describes the grain structures and morphology of dendrites or eutectics that compose the internal structure of the grains. A particular emphasis has been put on the simulation of defects related to grain textures and microstructures. The article provides information on the application of the most important simulation approaches and the status of numerical simulation.

The most common aluminum alloy systems are aluminum-silicon, aluminum-copper, and aluminum-magnesium. This article focuses on the grain structure, eutectic microstructure, and dendritic microstructure of these systems. It provides information on microsegregation and its problems in casting of alloys. The article also illustrates the casting defects such as macroporosity, microshrinkage, and surface defects, associated with the alloys.

This article discusses the characterization of gray iron structures, following the sequence of structure formation, as it applies to unalloyed or low-alloyed gray iron. Austenite grains are the basic crystallographic entities of the metallic matrix in gray cast iron precipitated from the liquid melt. The article describes the macrostructure and dendrite morphology of primary austenite. Eutectoid transformation in the solid state causes the transformation of austenite to pearlite and/or ferrite, producing the as-cast structure. The article discusses the observations of the graphite and ferritic/pearlitic structure in as-cast gray iron.

This article provides information on four different length scales at which surface morphology can be viewed: macro, meso, micro and nanoscale. Elementary thermodynamics demonstrates that a liquid cannot solidify unless some undercooling below the equilibrium (melting) temperature occurs. The article details five types of solidification undercooling, namely, kinetic, thermal, constitutional (solutal), curvature, and pressure undercooling. It explains the types of nucleation which occur in the melt during solidification. The effects of local instabilities at the solid/liquid interface during growth are illustrated. The article also describes the solidification structures of pure metals, solid solutions, eutectics, peritectics, and monotectics.

Computational modeling assists in addressing the issues of solid/liquid interface dynamics at the microlevel. It also helps to visualize the grain length scale, fraction of phases, or even microstructure transitions through microstructure maps. This article provides a detailed account of the general capabilities of the various models that can generate microstructure maps and thus transform the computer into a dynamic microscope. These include standard transport models, phase-field models, Monte Carlo models, and cellular automaton models.

The ferrous metals are the most significant class of commercial alloys. This article describes the solidification structures of plain carbon steel, low-alloy steel, high-alloy steel, and cast iron, with illustrations. The formation of nonmetallic inclusions in the liquid before and during solidification is also discussed.

This article describes the development of heat-resistant titanium-base alloys and their classification into several microstructure categories based on their strengthening mechanisms. It explains the phase transformation in titanium-aluminum-base alloys and two peritectic reactions that take place in the titanium-aluminum system. The article also describes two approaches for controlling the orientation of the high-temperature alpha phase to achieve the required lamellar orientation by directional solidification in order to improve the strength and ductility of titanium-aluminum alloys. One approach is by seeding the alpha phase in the alloys, and the other is without seeding, by controlling the solidification path of alloys through appropriate alloying. The article discusses the grain refinement technique used to improve the ductility of cast titanium-aluminum alloys to a level of above 1" at room temperature and reasonable room temperature ductility in the as-cast condition. Finally, it provides information on the microstructures produced through various near-net shape manufacturing processes.

Solidification processing is one of the oldest manufacturing processes, because it is the principal component of metal casting processing. This article discusses the fundamentals of solidification of cast iron. Undercooling is a basic condition required for solidification. The article describes various undercooling methods, including kinetic undercooling, thermal undercooling, constitutional undercooling, and pressure undercooling. For solidification to occur, nuclei must form in the liquid. The article discusses the various types of nucleation: homogeneous nucleation, heterogeneous nucleation, and dynamic nucleation. It reviews the classification of eutectics based on their growth mechanism: cooperative growth and divorced growth. The article concludes with a discussion on the solidification structures of peritectics.

This article begins with a description of indirect and direct semisolid metalworking processes. It then provides information on alloy compositions of common aluminum semisolid metalworking alloys and primary die-cast magnesium alloys in a tabular form. The article describes the macroscopic examination of defects, which occur in semisolid metalworking with illustrations. It discusses the macroscopic examination of gating systems and semisolid feedstocks. The article also provides information on feedstock microstructures, direct semisolid metalworking component microstructures, and indirect semisolid metalworking component microstructures of series 300 aluminum casting alloys and magnesium die-casting alloys.

For a wide range of new or better products, solidification processing of metallic materials from the melt is a step of uppermost importance in the industrial production chain. This article discusses the casting and solidification of molten metallic alloy along with the application of low-gravity platforms and facilities for solidification processing. It provides a description of dendritic growth studies and electromagnetic levitation. The article concludes with information on the in situ and real-time monitoring of solidification processing.

This article focuses on the intermediate length scales, where transport phenomena govern the spatial and temporal evolution of a structure. It presents the cellular automaton (CA) and phase field (PF) methods that represent the state of the art for modeling macrostructure and microstructure. The article describes the principles of the PF method and provides information on the applications of the PF method. The CA model is introduced as a computationally efficient method to predict grain structures in castings using the mesoscopic scale of individual grains. The article discusses the coupling of the CA to macroscopic calculation of heat, flow, and mass transfers in castings and applications to realistic casting conditions.

Modeling of structure formation in casting of alloys involves several length scales, ranging from the atomic level to macroscopic scale. Intermediate length scales are used to define the microstructure of the growing phases and the grain structure. This article discusses the principles and applications of the phase field method and the cellular automaton method for modeling the direct evolution of structure at the intermediate length scales, where transport phenomena govern the spatial and temporal evolution of the structure that involves nucleation and growth.

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 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.

Semisolid metal (SSM) processing is a special die casting process, where partially solidified metal slurry is injected into a die cavity to form die cast components. This article discusses the flow behavior of an SSM slurry with emphasis on viscosity, rheological behavior, and yield stress. It illustrates the microstructural formation of semisolids under forced convection. The article concludes with an illustration that compares two SSM processes, namely, thixocasting and rheocasting.