The gas-tungsten arc welding (GTAW) process is performed using a welding arc between a nonconsumable tungsten-base electrode and the workpieces to be joined. The arc discharge requires a flow of electrons from the cathode through the arc column to the anode. This article discusses two cases of electron discharge at the cathode: thermionic emission and nonthermionic emission, also called cold cathode, or field emission. It schematically illustrates relative heat transfer contributions to workpiece in the GTAW process. The article provides information on the effects of cathode tip shape and shielding gas composition in the GTAW process.

This article provides the basic physics of the two most widely used arc welding processes: gas tungsten arc welding and gas metal arc welding. It describes the various control parameters of these processes such as arc length control, voltage control, heat input control, and metal-transfer control.

Plasma melting is a material-processing technique in which the heat of thermal plasma is used to melt a material. This article discusses two typical design principles of plasma torches in the transferred mode: the tungsten tip design and the hollow copper electrode design. It describes the sources of atmospheric contamination in plasma melting furnaces and their control measures. The equipment used in plasma melting furnaces are also discussed. The article provides a detailed discussion on various plasma melting processes, such as plasma consolidation, plasma arc remelting, plasma cold hearth melting, and plasma casting.

The scanning electron microscope (SEM) is one of the most versatile instruments for investigating the microscopic features of most solid materials. The SEM provides the user with an unparalleled ability to observe and quantify the surface of a sample. This article discusses the development of SEM technology and operating principles of basic systems of SEM. The basic systems covered include the electron optical column, signal detection and display equipment, and the vacuum system. The processes involved in the preparation of samples for observation using an SEM are described, and the application of SEM in fractography is discussed. The article covers the failure mechanisms of ductile failure, brittle failure, mixed-mode failure, and fatigue failure. Lastly, image dependence on microscope type and operating parameters is also discussed.

This article provides detailed information on the instrumentation and principles of the scanning electron microscope (SEM). It begins with a description of the primary components of a conventional SEM instrument. This is followed by a discussion on the advantages and disadvantages of the SEM compared with other common microscopy and microanalysis techniques. The following sections cover the critical issues regarding sample preparation, the physical principles regarding electron beam-sample interaction, and the mechanisms for many types of image contrast. The article also presents the details of SEM-based techniques and specialized SEM instruments. It ends with example applications of various SEM modes.

Fusion-based additive manufacturing (AM) processes rely on the formation of a metallurgical bond between a substrate and a feedstock material. Energy sources employed in the fusion AM process include conventional arcs, lasers, and electron beams. Each of these sources is discussed, with an emphasis on their principles of operation, key processing variables, and the influence of each source on the transfer of heat and material. Common energy sources used for metals AM processes, particularly powder-bed fusion and directed-energy deposition, are also discussed. Brief sections at the end of the article discuss the factors dictating the choice of each of these energy sources and provide information on alternative sources of AM.

The scanning electron microscopy (SEM) is one of the most versatile instruments for investigating the microstructure of metallic materials. This article highlights the development of SEM technology and describes the operation of basic systems in an SEM, including the electron optical column, signal detection and display equipment, and vacuum system. It discusses the preparation of samples for observation using an SEM and describes the application of SEM in fractography. If the surface remains unaffected and undamaged by events subsequent to the actual failure, it is often a simple matter to determine the failure mode by the use of an SEM. In cases where the surface is altered after the initial failure, the case may not be so straightforward. The article presents typical examples that illustrate these points. Image dependence on the microscope type and operating parameters is also discussed.

This article discusses the general principles, optical systems, and emission sources of optical emission spectroscopy for elemental analysis. Changes in the energy of the valence or outer shell electrons result in the atomic lines used in emission spectroscopy. Each possible combination of electron configurations produces a spectroscopic term that describes the state of the atom. Atomic emission is analytically useful only to the extent that the emission from one atomic species can be measured and its intensity recorded independent of emission from other sources. Emission sources are often designed to minimize molecular emission. Each of the four types of emission sources; arcs, high-voltage sparks, glow discharges, and flames; has a set of physical characteristics with accompanying analytical assets and liabilities. The article also discusses the applications of each type of emission source.

Transmission electron microscopy (TEM) approach enables essentially simultaneous examination of microstructural features through imaging from lower magnifications to atomic resolution and the acquisition of chemical and crystallographic information from small regions of the thin specimen. This article discusses fundamentals of the technique, especially for solving materials problems. Background information is provided to help understand basic operations and principles, including instrumentation, the physics of signal generation and detection, image formation, electron diffraction, and spectrometry techniques with data analysis.

This article is a detailed account of optical emission spectroscopy (OES) for elemental analysis. It begins with a discussion on the historical background of OES and development trends in OES methods. This is followed by a description of the general principles and optical systems of OES, along with various types of emission sources commonly used for OES. Some of the processes involved in calibration and quantification of OES for direct solids analysis by the ratio method are then described. The article ends with a discussion on the applications of each type of emission sources.

Power sources are apparatuses that are used to supply current and voltages that are suitable for particular welding processes. This article describes power sources for arc welding, resistance welding, and electron-beam welding. The more-common welding processes that use constant-current and constant-voltage power sources are listed in a table. The article describes the open-circuit voltage characteristics and power source control methods. The control methods employ either pulse width modulation (PWM) or frequency modulation (FM).

Rare earth metals belong to Group IIIA of the periodic table that includes scandium, yttrium, and the lanthanide elements which are lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. This article classifies the rare earth metals based on their purity level, which are designated as research grades (>99.8% pure) and commercial grades (95% - 98% pure), and describes the preparation and purification, including solid-state electrolysis. It further discusses physical, mechanical, and chemical properties; electronic configurations; crystal structures, and explains the alloy forming characteristics of rare earth elements. The article concludes by describing the various applications of commercial-grade rare earth elements and commercial alloys, which incorporates rare earth elements as additives.

Very high density materials are used for such applications as counterweights and radiation shields. This article focuses on the metallurgy, processing, properties, fabrication, design considerations, health and safety considerations, and applications of the most commonly used very high density materials: depleted uranium and tungsten and their alloys.

This article presents a general description of pulsed-laser deposition. It describes the components of pulsed-laser deposition equipment. The article also discusses the effects of angular distribution of materials. Finally, the article reviews the characteristics of high-temperature superconductors and ferroelectric materials.

This article outlines the beam/sample interactions and the basic instrumental design of a scanning electron microscopy (SEM), which include the electron gun, probeforming column (consisting of magnetic electron lenses, apertures, and scanning coils), electron detectors, and vacuum system. It discusses the contrasts mechanisms used for imaging and analyzing materials in the SEM. These include the topographic contrast, compositional contrast, and electron channeling pattern and orientation contrast. Special instrumentation and accessory equipment used at elevated pressures and during the X-ray microanalysis are reviewed. The article also provides information on the sample preparation procedure and the materials applications of the SEM.

Sputtering is a nonthermal vaporization process in which the surface atoms are physically ejected from a surface by momentum transfer from an energetic bombarding species of atomic/molecular size. It uses a glow discharge or an ion beam to generate a flux of ions incident on the target surface. This article provides an overview of the advantages and limitations of sputter deposition. It focuses on the most common sputtering techniques, namely, diode sputtering, radio-frequency sputtering, triode sputtering, magnetron sputtering, and unbalanced magnetron sputtering. The article discusses the fundamentals of plasma formation and the interactions on the target surface. A comparison of reactive and nonreactive sputtering is also provided. The article concludes with a discussion on the several methods of process control and the applications of sputtered films.

This article explains cooling mechanisms involving saltwater solutions used as quenchants. The analyses of cooling power include studies of cooling curves, heat-transfer coefficients, and cooling rates. The influence of other bath parameters, such as temperature and agitation, is also discussed. The article discusses solute additions and several factors impacting quenching.

Scanning electron microscopy (SEM) has shown various significant improvements since it first became available in 1965. These improvements include enhanced resolution, dependability, ease of operation, and reduction in size and cost. This article provides a detailed account of the instrumentation and principles of SEM, broadly explaining its capabilities in resolution and depth of field imaging. It describes three additional functions of SEM, including the use of channeling patterns to evaluate the crystallographic orientation of micron-sized regions; use of backscattered detectors to reveal grain boundaries on unetched samples and domain boundaries in ferromagnetic alloys; and the use of voltage contrast, electron beam-induced currents, and cathodoluminescence for the characterization and failure analysis of semiconductor devices. The article compares the features of SEM with that of scanning Auger microscopes, and lists the applications and limitations of SEM.

This article discusses the basic principles of and chemical effects in Auger electron spectroscopy (AES), covering various factors affecting the quantitative analyses of AES. The discussion covers instrumentation and sophisticated electronics typically used in AES for data acquisition and manipulation and various limitations of AES. Various examples highlighting the capabilities of the technique are also included.