Search Results for solid-state transformation structures

This article provides a review of metallographic procedures and techniques for analyzing the microstructure of fusion welded joints. It discusses sample preparation, the use of backing plates, and common sectioning methods. It identifies the various types of defects that can occur in arc welded metals, organizing them according to the sectioning method by which they are observed. It describes the relationship between weld bead morphology and sectioning direction and its effect on measurement error. The article examines micrographs from stainless steel, aluminum, and titanium alloy joints, highlighting important details such as solidification and solid-state transformation structures and what they reveal about the welding process. Besides arc welding, it also discusses laser and electron beam welding methods, resistance and spot welding, and the welding of dissimilar metals.

This introductory article provides basic information on the various aspects of solid-state transformation: multiphase microstructures, substructures, and crystallography, which assist in characterizing the morphology of phase transformations. It contains a flowchart that illustrating the classification of transformations by growth processes.

Solid-state transformations from invariant reactions are of three types: eutectoid, peritectoid, and monotectoid transformations. This article focuses on structures from eutectoid transformations with an emphasis on the classic iron-carbon system of steel. It illustrates the morphology of a pearlite nodule and the effect of various substitutional alloy elements on the eutectoid transformation temperature and effective carbon content, respectively. Peritectic and peritectoid phase equilibria are very common in several binary systems. The article reviews structures from peritectoid reactions and details the formation of peritectic structures that can occur by at least three mechanisms: peritectic reaction, peritectic transformation, and direct precipitation of beta from the melt.

This article introduces basic physical metallurgy concepts that may be useful for understanding and interpreting variations in metallographic features and how processing affects microstructure. It presents some basic concepts in structure-property relationships. The article describes the use of equilibrium binary phase diagrams as a tool in the interpretation of microstructures. It reviews an account of the two types of solid-state phase transformations: isothermal and athermal. The article discusses isothermal transformation and continuous cooling transformation diagrams which are useful in determining the conditions for proper heat treatment (solid-state transformation) of metals and alloys. The influence of the mechanisms of phase nucleation and growth on the morphology, size, and distribution of grains and second phases is also described.

Eutectoid and peritectoid transformations are classified as solid-state invariant transformations. This article focuses primarily on the structures from eutectoid transformations with emphasis on the classic iron-carbon system of steel. It reviews peritectoid phase equilibria that are very common in several binary systems. The addition of substitutional alloying elements causes the eutectoid composition and temperature to shift in the iron-carbon system. The article graphically illustrates the effect of various substitutional alloying elements on the eutectoid transformation temperature and effective carbon content. The partitioning effect of substitutional alloying elements, such as chromium, manganese, and silicon, in pearlitic steel is also illustrated.

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 introduces the mechanism of diffusion and the common types of heat treatments such as annealing and precipitation hardening, which are applicable to most ferrous and nonferrous systems. Three distinct processes occur during annealing: recovery, recrystallization, and grain growth. The article also describes the various types of solid-state transformations such as isothermal transformation and athermal transformation, resulting from the heat treatment of nonferrous alloys. It provides information on the homogenization of chemical composition within a cast structure.

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.

Electroslag welding (ESW) and electrogas welding (EGW) are two related procedures that are used to weld thick-section materials in the vertical or near-vertical position between retaining shoes. This article discusses the fundamentals of the electroslag process in terms of heat flow conditions and metal transfer and weld pool morphology. It presents constitutive equations for welding current, voltage, and travel rate for ESW. The article describes the metallurgical and chemical reactions in terms of fusion zone compositional effects, weld metal inclusions, solidification structure, and solid-state transformations. It describes the electroslag process development and the applications of electroslag and electrogas processes. The article concludes with a discussion on weld defects, such as temper embrittlement, hydrogen cracking, and weld distortion.

This article provides information on the general structural features and origins of metals. The characteristic structural features of single-phase metals and alloys, such as grain structure and substructure, are discussed. The article also describes the major types of multiphase structures and macrostructure of metals and alloys.

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 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.

Spinodal transformation is a phase-separation reaction that occurs from kinetic behavior. This article discusses the theory of spinodal decomposition, and outlines the methods used in the characterization of spinodal structures in metal matrices.

Precipitation reactions occur in many different alloy systems when one phase transforms into a mixed-phase system as a result of cooling from high temperatures. This article discusses the homogenous and heterogeneous nucleation and growth of coherent and semicoherent precipitates. It describes two precipitation modes, namely, general or continuous precipitation and cellular or discontinuous precipitation. The article also provides information on the precipitation sequences in aluminum alloys.

This article provides a detailed account of extended x-ray absorption fine structure (EXAFS). It begins with a description of the fundamentals of EXAFS, providing information on the physical mechanism, single-scattering approximation, and multiple-scattering effects. This is followed by a discussion on the use of synchrotron radiation as an X-ray source for EXAFS. Data-reduction procedures used to extract EXAFS signals are then described. The article also provides information on the analysis of x-ray absorption near-edge structure spectrum and ends with a discussion on the unique features and applications of EXAFS.

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.

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 provides an introduction to extended x-ray absorption fine structure (EXAFS). It describes the fundamentals of EXAFS with an emphasis on the physical mechanism, the single-scattering approximation, and multiple-scattering effects. The article discusses the use of synchrotron radiation as the x-ray source for EXAFS experiments. It also describes the typical EXAFS data analysis of pure nickel at 90 K, and explains the near-edge structure analysis of vanadium. The article presents a discussion on the unique features and applications of EXAFS.

Fusion welding induces residual stresses and distortion, which may result in loss of dimensional control, costly rework, and production delays. In thermal analysis, conductive heat transfer is considered through the use of thermal transport, heat-input, and material models that provide values for the applied welding heat input. This article describes how the solid-phase transformations that occur during the thermal cycle produced by welding lead to irreversible plastic deformation known as transformation plasticity. Residual stress and welding distortion are also discussed.