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Topic Review
Additive Manufacturing of High Entropy Alloys
Alloying has been very common practice in materials engineering to fabricate metals of desirable properties for specific applications. Traditionally, a small amount of the desired material is added to the principal metal. However, a new alloying technique emerged in 2004 with the concept of adding several principal elements in or near equi-atomic concentrations. These are popularly known as high entropy alloys (HEAs) which can have a wide composition range.
  • 1.6K
  • 15 Mar 2022
Topic Review
Properties of Wire Arc-Sprayed Fe-Based Coatings
Among different thermal spraying methods, arc-spraying has been widely used due to its low operating costs and high deposition efficiency. The rapid progress of cored wire technology in arc-spraying has increased possibilities for the preparation of new Fe-based coating materials with enhanced properties by adding reinforcement particles and alloying elements to suit the different applications. 
  • 938
  • 09 Mar 2022
Topic Review
Ni-Base Superalloys
Ni-base superalloys are materials largely used in aero-space and energy production sectors, in particular for manufacturing engine parts (e.g. blades, rotors, turbine disks etc.) of aircrafts and aerospace vehicles and parts of power plants (e.g. extraction of oil and gas, nuclear reactors, etc.). At high temperature they exhibit an exceptional combination of high mechanical strength and excellent corrosion resistance. Ni-base superalloys are considered materials of strategic importance and a lot of metallurgical research has been devoted for optimizing their microstructure and improving mechanical properties so that they can operate at ever higher temperature in conditions of safety and reliability. Ni-base superalloys are strengthened by the precipitation of the ordered γ' phase, L12 Ni3(Al,Ti), crystallographically coherent to the f.c.c. γ matrix and their unique mechanical properties at high temperature result from the great microstructure stability. The volume fraction of γ' phase varies from 25% to 50% in polycrystalline superalloys and reaches about 70% in the most modern single crystal superalloys used for the first stage of aeronautical turbine blades. In order to reduce as much as possible the strain misfit between coherent γ and γ' phases (less than 0.4%) they are designed by an accurate tailoring of the chemical composition and a strict control of the process parameters; the resulting interface energy (20-30 mJ/m2) guarantees an excellent stability of the microstructure at high temperature. Other phases such as carbides, borides, γ'', η, δ, σ, µ and Laves phases may be also present with various effects on the mechanical properties; for instance, the topological closed-packed (TCP) σ, µ and Laves phases are undesirable because reduce the ductility.  In spite of the fact that Ni-base superalloys cost from 3 to 5 times the Fe-base ones, their use is expanding especially in gas turbine components for the production of energy because higher temperature of the thermal cycle guarantees greater efficiency and reduction of polluting emission. The demand of Ni-base superalloys is expected to expand also for the energy production through conventional steam turbine plants for achieving super-critical conditions with a predicted increase of efficiency to ~ 60% and reduction of CO2 to about 0.7 ton/kWatth while current sub-critical power plants have an efficiency of ~ 35% and produce 1.2 ton/kWatth of CO2. Of course, higher operating temperature involves more severe degradation of mechanical properties owing to these factors: (i) microstructure evolution including formation of undesired phases, coalescence of γ' precipitates, degeneration of carbides due to fatigue and creep exposure etc.; ii) the formation of cracks. Three topics of great industrial relevance will be discussed hereinafter: (i) microstructural stability; (ii) manufacturing parts of complex geometry; (iii) welding of superalloys. 
  • 863
  • 16 Feb 2022
Topic Review
Metallurgical Coke Structures
The structure of coke affects its reactivity and strength, which directly influences its performance in the blast furnace.
  • 1.8K
  • 11 Feb 2022
Topic Review
Cadmium Recovery from Spent Ni-Cd Batteries
The significant increase in the demand for efficient electric energy storage during the last decade has promoted an increase in the production and use of Cd-containing batteries. On the one hand, the amount of toxic Cd-containing used batteries is growing, while on the other hand, Cd is on a list of critical raw materials (for Europe). Both of these factors call for the development of effective technology for Cd recovery from spent batteries. Alkaline nickel-cadmium (Ni-Cd) batteries are widely used as autonomous sources of industrial and household current (power banks) due to a successful combination of feasibility studies and achieved sustainable electrical characteristics. In recent decades, the market of secondary current sources for portable equipment has undergone significant changes, which leads to an intensive replacement of Ni-Cd batteries with lithium-ion (LIB) and nickel-metal-hydride.
  • 739
  • 07 Feb 2022
Topic Review
Development of Bottom-Blowing Copper Smelting Technology
Bottom-blowing copper smelting technology was initiated and developed in China in the 1990s. Injection of oxygen-enriched high-pressure gas strongly stirs the molten bath consisting of matte and slag. Rapid reaction at relatively lower temperatures and good adaptability of the feed materials are the main advantages of this technology. Development and optimisation of bottom-blowing copper smelting technology were supported by extensive studies on the thermodynamics of the slag and the fluid dynamic of the molten bath.
  • 2.7K
  • 26 Jan 2022
Topic Review
Biobeneficiation of PGMs
Conventional beneficiation of the Platinum Group of Metals (PGMs) relies on the use of inorganic chemicals. With the depreciation of high grade deposits, these conventional processes are becoming less economically viable. Furthermore, the use of chemicals has serious negative impacts on the environment. To address the challenges of conventional PGM beneficiation, biobeneficiation has been proposed. Bio-beneficiation is the concentration of mineral species by employing microorganisms that interact with either the gangue or the valuable mineral species. Bio-beneficiation can also be described as the use of microorganisms to interact with minerals to subsequently induce processes such as magnetic separation, flotation, and flocculation.
  • 1.1K
  • 19 Jan 2022
Topic Review
Influence of Cold Spray Parameters on Bonding Mechanisms
The cold spray process is governed by the impact of high velocity feedstock particles onto a substrate without melting. Hence, the bulk material properties are retained. However, it is challenging to achieve good adhesion strength. The adhesion strength depends on factors such as the cold spray process parameters, substrate conditions, coating/substrate interactions at the interface and feedstock material properties. This entry examines fundamental studies concerning the adhesion mechanisms of cold spray technology and considers the effect of cold spray input parameters such as temperature, stand-off-distance, pressure, process gas, spray angle, and traverse speed of the cold spray torch on the bonding mechanism and adhesion strength. Furthermore, the effects of substrate conditions such as temperature, hardness, roughness and material on the adhesion mechanism are highlighted. The effect of feedstock properties, such as feed rate, shape and size are summarized. Understanding the effect of these parameters is necessary to obtain the optimal input parameters that enable the best interfacial properties for a range of coating/substrate material combinations. It is expected that feedstock of spherical morphology and small particle size (<15 μm) provides optimal interfacial properties when deposited onto a mirror-finished substrate surface using high pressure cold spray. Deep insights into each parameter exposes the uncovered potential of cold spray as an additive manufacturing method.
  • 706
  • 11 Jan 2022
Topic Review
Abrasive Wear of Cermets
Abrasive wear occurs when hard particles or sometimes hard protuberances on a counterface are forced against and are moved along the surface. The amount of material removed depends on the normal load pressing particles against the surface and the sliding distance. A distinction is usually made between the two-body and the three-body abrasive wear and between low-stress (abrasive particles remain unbroken during abrasion) and high-stress (abrasive particles are broken during the wear process) abrasion. WC-based hardmetals (cemented carbides) are employed widely as wear-resistant ceramic-metal composites for tools and wear parts. Raw materials supply, environmental concerns and some limitations of hardmetals have directed efforts toward development of alternative wear-resistant composites-cermets. Cermets consist primarily of ceramic particles such as titanium carbonitride (Ti(C,N)), titanium carbide (TiC), and chromium carbide (Cr3C2) bonded with alloys of Ni, Co or Fe. Cermets as resistant to abrasive wear materials demonstrate their potential mainly in environmentally severe wear conditions – at elevated temperatures and corrosive envronments.
  • 741
  • 10 Jan 2022
Topic Review
Erosive Wear of Cermets
Solid-particle erosion occurs when discrete solid particles strike a surface. It differs from three-body abrasion primarily in the origin of forces between the particles and the wearing surface. In erosion, the extent of wear depends on the number and mass of individual particles striking the surface and on their impact velocity [8]. The difference of erosion from the abrasive wear lies in its fluid contribution to the mechanical action producing material removal. Solid-particle erosion is common in any system in which a gas stream carries abrasive particles. If loose abrasive particles are carried by a liquid, the wear is termed as slurry erosion.  WC-based hardmetals (cemented carbides) are employed widely as wear-resistant ceramic-metal composites for tools and wear parts. Raw materials supply, environmental concerns and some limitations of hardmetals have directed efforts toward development of alternative wear-resistant composites-cermets. Cermets consist primarily of ceramic particles such as titanium carbonitride (Ti(C,N)), titanium carbide (TiC), and chromium carbide (Cr3C2) bonded with alloys of Ni, Co or Fe. Cermets as resistant to solid particle erosion materials demonstrate their potential primarily in environmentally severe wear conditions – at elevated temperatures and corrosive environments.
  • 495
  • 10 Jan 2022
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