This article describes the metallurgy and microstructure of high-performance cobalt-base alloys. It discusses metallographic preparation procedures, including sectioning, mounting, grinding, polishing, etching, staining, and heat tinting. It examines the microstructure of cobalt alloys in cast, wrought, and powder metal forms, including magnetic alloys as well as several cobalt-base superalloys.

Superalloys are nickel, iron-nickel, and cobalt-base alloys generally used for high-temperature applications. Superalloys are used in aircraft, industrial, marine gas turbines, nuclear reactors, spacecraft structures, petrochemical production, orthopedic and dental prostheses, and environmental protection applications. This article discusses the material characteristics, phases, structures, and systems of superalloys. It describes the processing of superalloys, including primary and secondary melting, deformation processing (conversion), powder processing, investment casting, and joining methods. The article also describes the properties, microstructure, and thermal exposure of superalloys. It further discusses the effects of environmental factors on superalloys, including oxidation and hot corrosion. Protective coatings are also discussed. The article provides information on the mechanical properties and chemical composition of nickel, iron, and cobalt-base superalloys in both the cast and wrought forms.

Cobalt is used as an alloying element in alloys for various applications. This article provides a detailed account of the metallurgy of cobalt-base alloys. It focuses on the compositions, properties, and applications of cobalt-base alloys, which include wear-resistant cobalt alloys, heat-resistant cobalt alloys, and cobalt-base corrosion-resistant alloys. The article also describes the heat treatments such as annealing and aging, for these alloys.

Pack cementation is the most widely employed method of diffusion coating. This article briefly reviews pack cementation processes of aluminizing, chromizing, and siliconizing. It contains tables that list typical characteristics of pack cementation processes and commercial applications of pack cementation aluminizing, which is used to improve the performance of steels in high-temperature corrosive environments.

The initial cast superalloy developments in the United States centered on cobalt-base materials. Nickel-base and nickel-iron-base superalloys owe their high-temperature strength potential to their gamma prime content. For polycrystalline superalloy components, high-temperature strength is affected by the condition of the grain boundaries and, in particular, the grain-boundary carbide morphology and distribution. Vacuum induction melting offers more control over alloy composition and homogeneity than all other vacuum melting processes. The primary purification reaction occurring in the process is the removal of melt contained oxygen by means of a reaction with carbon to form carbon monoxide. A number of casting processes can provide near-net shape superalloy cast parts, but essentially all components are produced by investment casting. The solidification of investment cast superalloy components is precisely controlled so that the microstructure, which ultimately determines mechanical properties, remains consistent. Heat treating cast superalloys involves homogenization and solution heat treatments or aging heat treatments.

This article discusses the specimen preparation of three types of cast and wrought heat-resistant alloys: iron-base, nickel-base, and cobalt-base. Specimen preparation involves sectioning, mounting, grinding, polishing, and etching. The article illustrates the microstructural constituents of cast and wrought heat-resistant alloys. It describes the identification of ferrite by magnetic etching. The transmission electron microscopy examination of the fine strengthening phases in wrought alloys and bulk extraction in heat-resistant alloys are included. The article also reviews the gamma prime phase, gamma double prime phase, eta phase, laves phase, sigma phase, mu phase, and chi phase in wrought heat-resistant alloys.

This article reviews some of the trends in superalloy development as they relate to U.S. strategic materials availability and the aerospace industry. It discusses the supply sources and availability of strategic materials and summarizes the status of U.S. resources and reserves. The article presents a list of several superalloys that have been used in gas turbine engines or that are emerging as replacements because of the promise of increased operating temperatures and higher efficiencies for the aircraft of the future. It concentrates on the objectives, results, and methodology of the NASA Conservation of Strategic Aerospace Materials (COSAM) program.

This article focuses on the properties of conventional wrought superalloys based on nickel, iron, and cobalt, as well as on the properties of alloys produced from powder. The powder metallurgy (P/M) category includes alloys that were originally developed as casting alloys; new alloy compositions developed specifically to benefit from powder processing; and oxide dispersion strengthened alloys (particularly those produced by mechanical alloying). The article discusses some of the applications of superalloys and emphazises the interplay between chemical composition, microstructure, consolidation method, mechanical properties and surface stability of wrought nickel alloys. Vacuum melting processes are a necessity for many nickel- and iron-nickel-base alloys because of the presence of aluminum and titanium as solutes. Cobalt-base alloys do not usually contain these elements and may be melted in air. An appendix to this article presents the property data and corresponding information on a family of cobalt-chromium-tungsten-carbon alloys that use P/M processing.

Chemical vapor deposition (CVD) involves the formation of a coating by the reaction of the coating substance with the substrate. Serving as an introduction to CVD, the article provides information on metals, ceramics, and diamond films formed by the CVD process. It further discusses the characteristics of different pack cementation processes, including aluminizing, siliconizing, chromizing, boronizing, and multicomponent coating.

The corrosion issues in the compressor, combustor and turbine sections of industrial gas turbines used in steam production generally depend on the quality of the fuel, air, and water used in the engine than on the specific industrial application. This article focuses on the forms of corrosion and their preventive measures in the compressor, combustor and turbine sections of a steam turbine. The compressor section mainly suffers from aqueous corrosion; while in case of the combustor and turbine sections, which are made of nickel-base superalloys, high-temperature environmental attack in the form of high-temperature oxidation and hot corrosion are predominant. The effect of high-temperature oxidation and hot corrosion on the mechanical properties of superalloys is also discussed.

Cobalt finds its use in various applications owing to its magnetic properties, corrosion resistance, wear resistance, and its strength at elevated temperatures. This article discusses the mining and processing of cobalt and cobalt alloys. It describes the types of cobalt alloys, including wear-resistant alloys, high-temperature alloys, corrosion-resistant alloys, and special-purpose alloys. The article provides data on the chemical composition, mechanical properties, and physical properties of these alloys. Further, it provides information on the uses of cobalt in superalloys, cemented carbides, magnetic materials, low-expansion alloys, and high-speed tool steels.

This article focuses primarily on the various steps involved in the brazing of heat-resistant alloys (nickel- and cobalt-base alloys). The major steps include the selection of brazing filler metals, surface cleaning and preparation, brazing processes and their corresponding atmospheres, and fixturing. The article also provides an overview of the brazing of blow-alloy steels and tool steels and oxide dispersion-strengthened alloys.

This article is a comprehensive collection of tables that list the values for hardness of plastics, rubber, elastomers, and metals. The tables also list the tensile yield strength and tensile modulus of metals and plastics at room temperature. A comparison of various engineering materials, on the basis of tensile strength, is also provided.

This article describes the widespread use of diffusion coatings for elevated-temperature protection of the turbine components for aircraft engines and gas turbines. The principles of pack diffusion coating, namely, aluminizing, chromizing, and siliconizing, are discussed. The article presents information on the coating formation mechanism of superalloys and explains the steps involved in a typical pack cementation process. It concludes with information on the processing procedures and properties of pack aluminized steels.

This article provides a general overview of physical and mechanical properties, alloy compositions, applications, and product forms of cobalt-base alloys as wear-resistant, corrosion-resistant, and/or heat-resistant materials. The discussion is largely focused on cobalt-base alloys for wear resistance, as this is the single largest application area of cobalt-base alloys.

Precipitation hardening is a hardening mechanism found in various steels and alloy systems, such as nickel-, cobalt-, titanium-, copper-, and iron-base alloys. This article provides a brief description of precipitation hardening process, furnace equipment, surface-related problems, and protective atmospheres used in heat treatment of iron-base precipitation-hardenable (PH) superalloys. It focuses on various factors to be considered in heat treating of PH stainless steels: cleaning prior to heat treatment, furnace atmospheres, time-temperature cycles, variations in cycles, and scale removal after heat treatment. The article describes the mechanical properties, solution treatment, and aging treatment for many martensitic PH alloys, including: Alloy 17-4 PH, Alloy 13-8 Mo, Alloy 15-5 PH, Custom 450, and Custom 455; as well as semiaustenitic PH stainless steels such as Alloy 17-7 PH, Alloy PH 15-7 Mo, AM-350, Pyromet 350, AM-355, and Pyromet 355; austenitic PH stainless steel, A-286; cast PH stainless steels; and iron-nickel PH superalloys.

This article discusses the physical metallurgy of cast cobalt alloys with an emphasis on the crystallography, compositions, phases and microstructure, and properties. Cobalt alloys are cast by several different foundry methods. The article describes the argon-oxygen decarburization and continuous casting process. It provides information on castability and quality of the casted alloys. The article details the postcasting treatment, including heat treatment, hot isostatic pressing, and coatings. It summarizes the applications of cast cobalt alloys.