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This book is a reprint of a special issue of Metals (ISSN 2075-4701), titled High Entropy Materials: Challenges and Prospects. It is a compilation of nine articles from different aspects of high-entropy materials. The book primarily focuses on high-entropy alloys, the first emergent high-entropy materials, but also covers high-entropy ceramics and high-entropy composites, which are the extensions of high-entropy alloys. The articles on high-entropy alloys cover some important facets in the field such as phase structures, mechanical properties, laser beam welding, design of soft magnetic alloys, and potential as biomaterials. In addition, there are one article introducing the potential of using high-entropy carbides as hard metals for machining, and one another on high-entropy composite studying the microstructures and tribological properties of the FeCoNiCuAl-TiC composite. The goal of this reprinted book is essentially two-fold. In the first place, it offers a platform for researchers in the broad field of high-entropy materials to communicate their views and recent research on the subject. Next, it reports challenges in the sub-fields of high-entropy materials and inspires researchers to continue to practice diligence to resolve these challenges and advance high-entropy materials solidly. We hope that readers in the field feel encouraged, inspired, and challenged by the book, and readers outside the field can grasp some basic ideals of high-entropy materials and their potential to the society as a family of novel materials.

high-entropy alloys --- intermetallic --- alloy design --- phase stability --- high-entropy alloy --- soft magnetic properties --- mechanical properties --- saturation magnetostriction coefficient --- face-centered cubic (FCC) structure --- high entropy alloys --- laser beam welding --- microstructure --- TiC --- tribological properties --- wear mechanism --- refractory metals --- bulk metallic glass (BMG) --- fatigue behavior --- industrial-grade zirconium raw material --- carbide --- high-entropy carbides --- binders --- high-entropy hardmetals --- biomedical materials --- n/a

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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.

History of engineering & technology --- high-entropy alloy --- laser cladding --- microstructure --- slurry erosion --- Nb/SiC composite material --- hot pressing sintering --- mechanical property --- corrosion --- surface degradation --- wear --- high entropy alloys --- complex concentrated alloys --- potentiodynamic polarization --- erosion-corrosion --- slurry-erosion --- oxidation wear --- highly wear resistant coatings --- multi-principal element alloys --- computational models --- first-principles calculations --- molecular dynamics --- phases --- properties --- dislocation nucleation --- activation volume --- activation energy --- nano-indentation --- high/medium entropy alloys --- spark plasma sintering --- pressure --- mechanical properties --- high-entropy --- high pressure --- high pressure torsion --- diamond anvil cells --- CoCrFeMnNi high entropy alloys --- additive manufacturing --- corrosion behavior --- non-equilibrium microstructure --- micro-pores --- high-entropy alloys --- corrosion resistance --- wear resistance --- serrated flow --- thermal coarsening --- actuators --- phase transformation --- nanoporous metals and alloys --- AlCoCrFeNi2.1 --- CCA --- HEA --- aging --- precipitates --- tribology --- creep --- stress exponent --- data analysis --- n/a

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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.

History of engineering & technology --- high-entropy alloy --- laser cladding --- microstructure --- slurry erosion --- Nb/SiC composite material --- hot pressing sintering --- mechanical property --- corrosion --- surface degradation --- wear --- high entropy alloys --- complex concentrated alloys --- potentiodynamic polarization --- erosion-corrosion --- slurry-erosion --- oxidation wear --- highly wear resistant coatings --- multi-principal element alloys --- computational models --- first-principles calculations --- molecular dynamics --- phases --- properties --- dislocation nucleation --- activation volume --- activation energy --- nano-indentation --- high/medium entropy alloys --- spark plasma sintering --- pressure --- mechanical properties --- high-entropy --- high pressure --- high pressure torsion --- diamond anvil cells --- CoCrFeMnNi high entropy alloys --- additive manufacturing --- corrosion behavior --- non-equilibrium microstructure --- micro-pores --- high-entropy alloys --- corrosion resistance --- wear resistance --- serrated flow --- thermal coarsening --- actuators --- phase transformation --- nanoporous metals and alloys --- AlCoCrFeNi2.1 --- CCA --- HEA --- aging --- precipitates --- tribology --- creep --- stress exponent --- data analysis

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In recent years, people have tended to adjust the degree of order/disorder to explore new materials. The degree of order/disorder can be measured by entropy, and it can be divided into two parts: topological disordering and chemical disordering. The former mainly refers to order in the spatial configuration, e.g., amorphous alloys which show short-range ordering but without long-range ordering, while the latter mainly refers to the order in the chemical occupancy, that is to say, the components can replace each other, and typical representatives are high-entropy alloy (HEAs). HEAs, in sharp contrast to traditional alloys based on one or two principal elements, have one striking characteristic: their unusually high entropy of mixing. They have not received much noticed until the review paper entitled “Microstructure and Properties of High-Entropy Alloys” was published in 2014 in the journal of Progress in Materials Science. Numerous reports have shown they exhibit five recognized performance characteristics, namely, strength–plasticity trade-off breaking, irradiation tolerance, corrosion resistance, high-impact toughness within a wider temperature range, and high thermal stability. So far, the development of HEAs has gone through three main stages: 1. Quinary equal-atomic single-phase solid solution alloys; 2. Quaternary or quinary non-equal-atomic multiphase alloys; 3. Medium-entropy alloys, high-entropy fibers, high-entropy films, lightweight HEAs, etc. Nowadays, more in-depth research on high-entropy alloys is urgently needed.

Research & information: general --- high-entropy alloys --- alloys design --- lightweight alloys --- high entropy alloys --- elemental addition --- annealing treatment --- magnetic property --- microhardness --- in situ X-ray diffraction --- grain refinement --- thermoelectric properties --- scandium effect --- HEA --- high-entropy alloy --- CCA --- compositionally complex alloy --- phase composition --- microstructure --- wear behaviour --- metal matrix composites --- mechanical properties --- high-entropy films --- phase structures --- hardness --- solid-solution --- interstitial phase --- transmission electron microscopy --- compositionally complex alloys --- CrFeCoNi(Nb,Mo) --- corrosion --- sulfuric acid --- sodium chloride --- entropy --- multicomponent --- differential scanning calorimetry (DSC) --- specific heat --- stacking-fault energy --- density functional theory --- nanoscaled high-entropy alloys --- nanodisturbances --- phase transformations --- atomic-scale unstable --- mechanical alloying --- spark plasma sintering --- nanoprecipitates --- annealing --- phase constituent --- ion irradiation --- hardening behavior --- volume swelling --- medium entropy alloy --- high-pressure torsion --- partial recrystallization --- tensile strength --- high-entropy alloys (HEAs) --- phase constitution --- magnetic properties --- Curie temperature --- phase transition --- precipitation --- strengthening --- coherent microstructure --- conventional alloys --- nanocrystalline materials --- high entropy alloy --- sputtering --- deformation and fracture --- strain rate sensitivity --- liquid phase separation --- immiscible alloys --- HEAs --- multicomponent alloys --- miscibility gaps --- multi-principal element alloys --- MPEAs --- complex concentrated alloys --- CCAs --- electron microscopy --- plasticity methods --- plasticity --- serration behavior --- alloy design --- structural metals --- CALPHAD --- solid-solution alloys --- lattice distortion --- phase transformation --- (CoCrFeNi)100−xMox alloys --- corrosion behavior --- gamma double prime nanoparticles --- elemental partitioning --- atom probe tomography --- first-principles calculations --- bcc --- phase stability --- composition scanning --- laser cladding --- high-entropy alloy coating --- AZ91D magnesium alloy --- wear --- kinetics --- deformation --- thermal expansion --- diamond --- composite --- powder metallurgy --- additive manufacturing --- low-activation high-entropy alloys (HEAs) --- high-temperature structural alloys --- microstructures --- compressive properties --- heat-softening resistance --- tensile creep behavior --- microstructural evolution --- creep mechanism --- first-principles calculation --- maximum entropy --- elastic property --- mechanical property --- recrystallization --- laser metal deposition --- elemental powder --- graded material --- refractory high-entropy alloys --- elevated-temperature yield strength --- solid solution strengthening effect --- bulk metallic glass --- complex stress field --- shear band --- flow serration --- deformation mechanism --- ab initio --- configuration entropy --- matrix formulation --- cluster expansion --- cluster variation method --- monte carlo --- thermodynamic integration --- (AlCrTiZrV)-Six-N films --- nanocomposite structure --- refractory high entropy alloys --- medium entropy alloys, mechanical properties --- thin films --- deformation behaviors --- nanocrystalline --- coating --- interface --- mechanical characterization --- high pressure --- polymorphic transition --- solidification --- eutectic dendrites --- hierarchical nanotwins --- precipitation kinetics --- strengthening mechanisms --- elongation prediction --- welding --- Hall–Petch (H–P) effect --- lattice constants --- high-entropy ceramic --- solid-state diffusion --- phase evolution --- mechanical behaviors --- high-entropy film --- low-activation alloys

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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.

aluminum --- yttrium oxide (yttria) --- mechanical alloying --- microwave sintering --- microstructure and mechanical properties --- half-Heusler alloys --- Mössbauer spectroscopy --- metal hydrides --- hydrogen storage --- hydriding kinetics --- surface modification --- refractory --- high entropy alloy --- phase transformation --- mechanical properties --- reactive black 5 --- decolorization --- UV-visible spectrophotometry --- LC-MS analysis --- austenitic alloys --- high-nitrogen steels --- atomic redistribution --- point defects --- microstructure --- Fe based alloys --- nanocrystalline (NC) alloy --- microcrystalline (MC) alloy --- ball-milling --- oxidation resistance --- n/a --- Mössbauer spectroscopy --- Technology.