Alloying and Processing Effects on the Aqueous Corrosion Behavior of High-Entropy Alloys

The effects of metallurgical factors on the aqueous corrosion behavior of high-entropy alloys (HEAs) are reviewed in this article. Alloying (e.g., Al and Cu) and processing (e.g., heat treatments) effects on the aqueous corrosion behavior of HEAs, including passive film formation, galvanic corrosion, and pitting corrosion, are discussed in detail. Corrosion rates of HEAs are calculated using electrochemical measurements and the weight-loss method. Available experimental corrosion data of HEAs in two common solutions [sulfuric acid (0.5 M H$_{2}$SO$_{4}$) and salt water (3.5 weight percent, wt.%, NaCl)], such as the corrosion potential (E$_{corr}$), corrosion current density (i$_{corr}$), pitting potential (E$_{pit}$), and passive region (ΔE), are summarized and compared with conventional corrosion-resistant alloys. Possible directions of future work on the corrosion behavior of HEAs are suggested

The influence of incorporation of Mn on the pitting corrosion performance of CrFeCoNi High Entropy Alloy at different temperatures

The electrochemical behavior and susceptibility to pitting corrosion of CrFeCoNi and CrMnFeCoNi high entropy alloys were studied in a 0.1 M NaCl solution at temperatures ranging from 25 to 75 °C. Electrochemical measurements revealed that CrMnFeCoNi is more susceptible to oxide film breakdown and localized corrosion compared to CrFeCoNi. Post corrosion microscopic observations showed severe pitting corrosion for CrMnFeCoNi in higher temperatures compared to CrFeCoNi. Based on in-depth XPS profile measurements on the remaining oxide films, this behavior was attributed to the depletion of Cr in the oxide film and detrimental presence of Mn in the matrix solid solution of CrMnFeCoNi

High-Entropy Coatings (HEC) for High-Temperature Applications: Materials, Processing, and Properties

High-entropy materials (HEM), including alloys, ceramics, and composites, are a novel class of materials that have gained enormous attention over the past two decades. These multi-component novel materials with unique structures always have exceptionally good mechanical properties and phase stability at all temperatures. Of particular interest for high-temperature applications, e.g., in the aerospace and nuclear sectors, is the new concept of high-entropy coatings (HEC) on low-cost metallic substrates, which has just emerged during the last few years. This exciting new virgin field awaits exploration by materials scientists and surface engineers who are often equipped with high-performance computational modelling tools, high-throughput coating deposition technologies and advanced materials testing/characterisation methods, all of which have greatly shortened the development cycle of a new coating from years to months/days. This review article reflects on research progress in the development and application of HEC focusing on high-temperature applications in the context of materials/composition type, coating process selection and desired functional properties. The importance of alloying addition is highlighted, resulting in suppressing oxidation as well as improving corrosion and diffusion resistance in a variety of coating types deposited via common deposition processes. This review provides an overview of this hot topic, highlighting the research challenges, identifying gaps, and suggesting future research activity for high temperature applications

Mechanical Alloying: Processing and Materials

Mechanical alloying is a technique of producing alloys and compounds that permits the development of metastable materials (with amorphous or nanocrystalline microstructure) or the fabrication of solid solutions with extended solubility. The elements or compounds to be mixed (usually as powders) are introduced in jars usually under a controlled atmosphere. Regarding the scope of this book, advanced materials have been developed by mechanical alloying: Fe–X–B–Cu (X = Nb, NiZr) nanocrystalline alloys, mixtures of the binary Fe–Mn and Fe–Cr alloys with chromium and manganese nitrides, Mn–Al–Co and Mn–Fe alloys, non-equiatomic refractory high-entropy alloys, nanocrystalline Fe–Cr steels, nanaocrystalline Mn–Co–Fe–Ge–Si alloys, Al–Y2O3 nanocomposite, and hydride-forming alloys. Likewise, production conditions and ulterior treatments can provide readers interesting ideas about the procedure to produce alloys with specific microstructure and functional behavior (mechanical, magnetic, corrosion resistance, hydrogen storage, magnetocaloric effect, wastewater treatment, and so on). As an example, to obtain the improvement in the functional properties of the alloys and compounds, sometimes controlled annealing is needed (annealing provokes the relaxation of the mechanical-induced strain). Furthermore, the powders can be consolidated (press, spark plasma sintering,and microwave sintering) to obtain bulk materials

Magnesium Alloys Structure and Properties

Magnesium Alloys Structure and Properties is a comprehensive overview of the latest knowledge in the field of magnesium alloys engineering. Modern magnesium alloys are promising for a variety of applications in many branches of the industry due to their excellent mechanical properties, high vibration, damping capacity, and high dimensional stability. This book discusses the production, processing, and application of magnesium alloys. It includes detailed information on the impact of alloying additives and selected casting technologies, as well as modern manufacturing technologies based on powder metallurgy, the production of composites and nano-composites with metal matrixes, and methods for improving alloy properties

Design and Validation of Novel Potential High Entropy Alloys

The design approach and validation of single phase senary refractory high entropy alloys (HEAs) MoNbTaTiVW and HfNbTaTiVZr were presented in first part of this dissertation. The design approach was to combine phase diagram inspection of available binary and ternary systems and Calculation of Phase Diagrams (CALPHAD) prediction. Experiments using X-ray diffraction and scanning electron microscopy techniques verified single phase microstructure in body centered cubic lattice for both alloys. The observed elemental segregation agrees well with the solidification prediction using Scheil model. The lattice constant, density and microhardness were measured to be 0.3216 nm, 4.954 GPa and 11.70 g/cm3 for MoNbTaTiVW and 0.334 nm, 5.5 GPa and 9.36 g/cm3 for HfNbTaTiVZr. To elaborate the single-phase stability of HEAs, CrxMoNbTaVW was examined over a certain range of Cr content in the second part of this dissertation. The change in composition led to different BCC structures with different microstructures and physical properties. Microstructure characterizations were performed using X-ray diffraction and scanning electron microscopy. Chemical micro-segregation during solidification predicted using the Scheil model generally agrees with the experimental results. The lattice constant, density, and Vickers\u27 micro-hardness of the high-entropy alloy samples in the as-cast state are measured and discussed. For CrxMoNbTaVW, x=2.0 case appears exceeding the upper limit of maintaining a single BCC phase HEA, determined by the XRD patterns. The elemental dependence of the mixing thermodynamic properties (entropy, enthalpy and Gibbs energy) in BCC phase in the senary system is analyzed. The calculated entropy of mixing and enthalpy of mixing for CrMoNbTaVW are 14.7 J/K/mol and −662.5 J/mol respectively. Phase predictions and characterizations on as-solidified septenary refractory high-entropy alloy, CrMoNbReTaVW, are presented in the third part of the dissertation. The simulated solidification process predicts a single body-centered-cubic (BCC) crystal structure with the tendency of compositional segregation. X-ray diffraction results confirm the “single-phase-like” BCC structure, while further experimental characterizations reveal the existence of multiple grains with significantly different compositions yet the same crystal structure and similar lattice. For better understanding of corrosion properties of high entropy alloys, the CALPHAD method was further used to simulate the Pourbaix diagram and the corrosion layer evolutions under equilibrium conditions for CoCrFeNi based HEAs in the last part of the dissertation. The oxidation layer pitting and forming potential were calculated and compared favorably with published experimental results on CoCrFeNi, CoCrFeNiCu and CoCrFeNiAl0.5 HEAs

Complex Concentrated Alloys (CCAs)

This book is a collection of several unique articles on the current state of research on complex concentrated alloys, as well as their compelling future opportunities in wide ranging applications. Complex concentrated alloys consist of multiple principal elements and represent a new paradigm in structural alloy design. They show a range of exceptional properties that are unachievable in conventional alloys, including high strength–ductility combination, resistance to oxidation, corrosion/wear resistance, and excellent high-temperature properties. The research articles, reviews, and perspectives are intended to provide a wholistic view of this multidisciplinary subject of interest to scientists and engineers