Search Results for Phase transformations

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.

This article describes the integration of thermodynamic modeling, mobility database, and phase-transformation crystallography into phase-field modeling and its combination with transformation texture modeling to predict phase equilibrium, phase transformation, microstructure evolution, and transformation texture development during heat treatment of multicomponent alpha/beta and beta titanium alloys. It includes quantitative description of Burgers orientation relationship and path, discussion of lattice correspondence between the alpha and beta phases, and determination of the total number of Burgers correspondence variants and orientation variants. The article also includes calculation of the transformation strain with contributions from defect structures developed at alpha/beta interfaces as a precipitates grow in size. In the CALculation of PHAse Diagram (CALPHAD) framework, the Gibbs free energies and atomic mobilities are established as functions of temperature, pressure, and composition and serve directly as key inputs of any microstructure modeling. The article presents examples of the integrated computation tool set in simulating microstructural evolution.

In the case of steels, heat treatment plays a fundamental role because no other process step can manipulate the microstructure in order to fulfill such a wide variety of possible in-service conditions. This article addresses heat treatment with regard to hardening and subsequent tempering of steel components in order to optimize tribological properties. It focuses on the heat treatment of tempering and bearing steels and on volume changes that take place due to phase transformations. Plastic deformations that occur due to shrinking and phase transformation are also discussed. The article also describes the generation of thermal, transformation, and hardening residual stresses.

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.

This article provides a discussion on the analytical modeling and simulation of residual stress states developed in steel parts and the reasons for these varied final stress states. It illustrates how the metallurgical phase transformation of steel alloys can be applied in the simulation of induction hardening processes and the role of these phase transformations in affecting stress and distortion. Emphasis is placed on induction surface hardening, which is the main application of induction heating in steel heat treatment. The article concludes with examples of induction surface-hardened shafts and through-hardened shafts made of plain carbon steel, alloy steel, and limited hardenability steel.

Of the various thermal processing methods for steel, heat treating has the greatest overall impact on control of residual stress and on dimensional control. This article provides an overview of the effects of material- and process-related parameters on the various types of failures observed during and after heat treating of quenched and tempered steels. It describes phase transformations of steels during heating, cooling of steel with and without metallurgical transformation, and the formation of high-temperature transformation products on the surface of a carburized part. The article illustrates the use of carbon restoration on decarburized spring steels. Different geometric models for carbide formation are shown schematically. The article also describes the different microstructural features such as grain size, microcracks, microsegregation, and banding.

This article reviews the concepts considered important for an understanding of the processes used for preparing cobalt-chromium alloy implants, the microstructures resulting from this processing, and the resulting material properties. The review includes solidification of alloys, diffusionless (martensitic) phase transformation as occurs with face-centered cubic to hexagonal close-packed transformation in cobalt-chromium alloys, and stacking faults and twins and their role in this transformation. It also discusses the strengthening mechanisms that are responsible for the mechanical properties of cast and wrought cobalt alloys. The article contains tables that list the commonly used cobalt alloys and their biomedical applications and chemical compositions. It discusses the mechanical and corrosion properties of cobalt alloys, and provides a description of the microstructure of cobalt alloys.

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.

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.