Recent discoveries of multicomponent concentrated solid-solution alloys hold promise for enhanced properties—such as enhanced mechanical properties, radiation tolerance, high temperature strength, corrosion resistance and some novel functional properties, provide a new strategy for alloy design using extreme disorder. Yet, deep understanding of these intriguing properties is complicated by the very effects of disorder that make them interesting. All the desirable properties of these alloys ultimately originate from the disorder-induced properties of underlying electronic structure, lattice dynamics, and thermodynamics. Therefore, understanding the disorder-induced fundamental physical properties is prerequisite for the science-based design of this class of alloys for practical applications. Here, we elucidate the role of extreme (maximal) substitutional disorder plays in the fundamental physics of disordered alloys and review the recently developed theoretical methodologies in modeling the basic physical properties, particularly electronic structure, magnetism, electrical transport, and lattice vibrations in multicomponent concentrated solid-solution alloys.

High-entropy and multiprincipal element alloys present exciting opportunities and challenges for computational modeling of their structure and phase stability. Recent interest has catalyzed rapid development of techniques and equally rapid growth of new results. This review surveys the essential concepts of thermodynamics and total energy calculation, and the bridge between them provided by statistical mechanics. Specifically, we review the electronic density functional theory of alloy total energy as applied to supercells and special quasirandom structures. We contrast these with the coherent potential approximation and semi-empirical approximations. Statistical mechanical approaches include cluster expansions, hybrid Monte Carlo/molecular dynamics simulations, and extraction of entropy from correlation functions. We also compare first-principles approaches with Calculation of Phase Diagrams (CALPHAD) and highlight the need to augment experimental databases with first-principles derived data. Numerous example applications are given highlighting recent progress utilizing the concepts and methods that are introduced.

This work presents a critical review of the application of CALPHAD techniques in the development of high entropy alloys (HEAs) and complex concentrated alloys (CCAs). This assessment covers three major themes: thermodynamics of mixtures and stability, retrospective thermodynamics, and predictive thermodynamics. Based on statistical and thermodynamic analysis, we assess the concept of entropic stabilization. A brief description of the major accomplishments of the CALPHAD technique applied to explain the stability and microstructure of HEAs and CCAs is presented. We describe the role of CALPHAD and its integration with other design tools, such as physicochemical criteria, data mining, and optimization techniques, to accelerate the discovery of new materials. Finally, we recommend future efforts in the development of the next generation of HEAs and CCAs in connection with the design of their microstructures, with an emphasis on precipitation strengthening and twinning-induced or transformation-induced plasticity (TWIP, TRIP).

Classical alloy design strategies often aim to benefit from metastability. Examples are numerous: metastable transformation- and twinning-induced plasticity steels, cobalt or titanium based alloys, age hardenable aluminum alloys, and severe plastic deformed nanostructured copper. In each of these cases, superior engineering property combinations are achieved by exploring limits of stability. For the case of high-entropy alloys (HEAs), on the other hand, majority of present research efforts focus on exploring compositions that would yield stable single-phase structures. HEA metastability and its effects on microstructure and property development constitute only a relatively small fraction of ongoing work. To help motivate and guide a corresponding shift in HEA research efforts, here in this paper, we provide an overview of the research activities on metastability in HEAs. To this end, we categorize the past research on the topic into two groups based on their focus, namely, compositional and structural stability, and discuss the most relevant and exciting findings.

High-entropy alloys (HEAs) consisting of multiprincipal elements have demonstrated many interesting structural, physical, and chemical properties for a wide range of applications. This article is a review of the current theoretical research on the elastic parameters of HEAs. The performance of various ab initio-based computational models (effective medium and supercell approaches) is carefully analyzed. Representative theoretical elastic parameters of different HEAs, including single-crystal elastic constants, polycrystalline elastic moduli, elastic anisotropy, and Debye temperature, are presented and discussed. For comparison, simple mixtures of the elastic moduli of pure elements are calculated and contrasted with the ab initio results. The present work provides a reference for future theoretical investigation of the micromechanical properties of systems based on HEAs.

One of the founding concepts of the high-entropy alloy (HEA) field was that the lattice structures of multicomponent solid solution phases are highly distorted. The displacement of the constituent atoms, away from their ideal locations (local lattice strain), has been widely cited as the reason for a number of the observed physical and mechanical properties. However, very little data directly characterizing these lattice distortions exist and, thus, the validity of this hypothesis remains an open question. Here, the concept is reviewed by considering the underlying principles of the lattice distortions, the suitability of different assessment methods, and the direct experimental data currently available. It is found that, at present, there is no clear evidence that the lattice distortions in HEAs are significantly greater than those of conventional alloys. However, so few alloys have been appropriately characterized that this conclusion cannot be considered overarching and further research is required.

High-entropy alloys (HEAs), originally introduced to the literature due to their ability to stabilize a single phase across large temperature ranges, have recently demonstrated to display multiphase systems undergoing a variety of strengthening mechanisms. Previous reports have focused on solid solution strengthening and precipitation hardening; however, other hardening mechanisms such as twinning and martensite formation have been reported to play a key role in controlling their mechanical behavior. Such deformation mechanisms display significant variations with temperature and strain rate. The present contribution provides an outline of the various hardening mechanisms reported in the literature for HEAs. For each mechanism, a modeling strategy is proposed to describe the associated mechanical behavior. The mechanisms are combined into a single framework to discover new HEAs of improved mechanical behavior. A strategy for HEA design is presented, and the advantages of adopting additive layer manufacturing to improve mechanical behavior are discussed.

Advanced alloys with both high strength and ductility are highly desirable for a wide range of engineering applications. Conventional alloy design strategies based on the single-principle element are approaching their limits in further optimization of their performances. Precipitation-hardened high-entropy alloys (HEAs), especially those strengthened by coherent L12-nanoparticles, have received considerable interest in recent years, enabling a new space for the development of advanced structural materials with superior mechanical properties. In this review, we highlight recent important advances of the newly developed L12-strengthened HEAs, including the aspects of computation-aided alloy design, unique properties, atomic-level characterization, phase evolution, and stability. In particular, we focus our attention on elucidating fundamental scientific issues involving the alloying effects, precipitation behaviors, mechanical performances, and the corresponding deformation mechanisms, all of which provide a comprehensive metallurgical understanding and guidance for the design of this new class of HEAs. Finally, future research directions and prospects are also critically assessed.

The basic principle of high-entropy alloys (HEAs) is that high mixing entropies of solid-solution phases enhance the phase stability, which renders us a new strategy on alloy design. The current research of HEAs mostly emphasizes mechanical behavior at room and higher temperatures. Relatively fewer papers are focused on low-temperature behaviors, below room temperature. However, based on the published papers, we can find that the low-temperature properties of HEAs are generally excellent. The great potential for cryogenic applications could be expected on HEAs. In this article, we summarized and discussed the mechanical behaviors and deformation mechanisms, as well as stacking-fault energies, of HEAs at low temperatures. The comparison of low-temperature properties of HEAs and conventional alloys will be provided. Future research directions will be suggested at the end.

As high-entropy alloys (HEAs) are being actively explored for next-generation structural materials, gaining a comprehensive understanding of their creep, fatigue, and fracture behaviors is indispensable. These three aspects of mechanical properties are particularly important because (i) creep resistance dictates an alloy’s high-temperature applications; (ii) fatigue failure is the most frequently encountered failure mode in the service life of a material; (iii) fracture is the very last step that a material loses its load-carrying capability. In consideration of their importance in designing HEAs toward applicable structural materials, this article offers a comprehensive review on what has been accomplished so far in these three topics. The sub-topics covered include a comparison of different creep testing methods, creep-parameter extraction, creep mechanism, high-cycle fatigue S–N relation, fatigue-crack-growth behavior, fracture toughness, fracture under different loading conditions, and fractography. Directions for future efforts are suggested in the end.

In the past decade, nanomechanical techniques have become ubiquitous for mechanical measurement concurrently with the discovery of high-entropy alloys (HEAs). Different from large-scale testing, small-scale measurements offer quantitative details about mechanical behavior of materials at the micro/nanoscale, presenting new opportunities to probe fundamental nature of HEAs. This article will review the literature on using versatile nanomechanical tools for HEA studies, including nanoindentation, microcompression, high-temperature deformation, fracture measurement, and in situ electron microscopy. With these approaches, many interesting phenomena and properties of HEAs have been unveiled, for example, properties about incipient plasticity, strain-rate sensitivity, creep, diffusion, size-dependent strength, and fracture, which are difficult, or impossible, to be measured in macroscopic experiments. Despite current literature only focusing on a few HEA compositions and several methods, as nanomechanics and HEAs are developing rapidly, a new avenue of research is to be exploited. The article concludes with perspectives about future directions in this field.

The current review outlines the size-dependent plastic behavior of high-entropy alloys (HEAs) and the underlying deformation mechanisms. Particular focus is laid upon the influence of microstructural design on the small-scale deformation characteristics. The role of defect types as carriers of plasticity is appraised and correlated with the frequently observed mechanical behavior peculiar to the breed of HEAs. Deformation response is classified on the basis of mechanical testing techniques probing intrinsic (nanoindentation techniques) as well as extrinsic size (micro/nanopillar compression) effects. The mechanisms of incipient plasticity and serrated flow behavior in HEAs are discussed. Furthermore, the role of interfaces between crystallographically dissimilar lattices on small-scale deformation behavior in these alloys is assessed. The article provides a clear overview of the existing HEA research in this avenue as well as the critical knowledge gaps that need to be addressed.

Single-phase concentrated solid solution alloys (SP-CSAs) are newly emerging advanced structural materials, which are defined as multiprincipal element solid solutions. SP-CSAs with more than four components in equimolar or near-equimolar ratios are also referred to as high-entropy alloys due to their high configurational entropy. SP-CSAs are potential structural materials in advanced nuclear energy systems due to their attractive mechanical properties. Therefore many investigations have been carried out to study the irradiation-induced structural damage and defect behavior in SP-CSAs. This paper reviews recent experimental results on the irradiation responses of various SP-CSAs, focusing on the accumulation of irradiation-induced structural damage, void swelling resistance, and solute segregation behavior. In addition, the characteristic defect behavior in SP-CSAs derived from ab initio and molecular dynamics simulations, as well as the challenges in the applications of SP-CSAs for the nuclear energy systems are briefly discussed.

Open literature publications, in the period from 2010 to the end of January 2018, on refractory high entropy alloys (RHEAs) and refractory complex concentrated alloys (RCCAs) are reviewed. While RHEAs, by original definition, are alloys consisting of five or more principal elements with the concentration of each of these elements between 5 and 35 at.%, RCCAs can contain three or more principal elements and the element concentration can be greater than 35%. The 151 reported RHEAs/RCCAs are analyzed based on their composition, processing methods, microstructures, and phases. Mechanical properties, strengthening and deformation mechanisms, oxidation, and corrosion behavior, as well as tribology, of RHEA/RCCAs are summarized. Unique properties of some of these alloys make them promising candidates for high temperature applications beyond Ni-based superalloys and/or conventional refractory alloys. Methods of development and exploration, future directions of research and development, and potential applications of RHEAs are discussed.

The concept of high-entropy alloys has been extended to ceramics, polymers, and composites. “High-entropy materials (HEMs)” are named to cover all these materials. Recently, HEMs has become a new emerging field through the collective efforts of many researchers. Basically, high mixing entropy can enhance the formation of solution-type phases for alloys, ceramics, and composites at high temperatures, and in general leads to simpler microstructure. Large degrees of freedom in composition design as well as process design have been found to provide a wide range of microstructure and properties for applications. There are many opportunities for HEMs to overcome the bottlenecks of conventional materials. In this article, several possible breakthrough applications are pointed out and emphasized for turbine blades, thermal spray bond coatings, high-temperature molds and dies, sintered carbides for cutting tools, hard coatings for cutting tools, hardfacings, and radiation-damage resistant materials. In addition, more possible breakthrough examples are briefly described.

While most papers on high-entropy alloys (HEAs) focus on the microstructure and mechanical properties for structural materials applications, there has been growing interest in developing high-entropy functional materials. The objective of this paper is to provide a brief, timely review on select functional properties of HEAs, including soft magnetic, magnetocaloric, physical, thermoelectric, superconducting, and hydrogen storage. Comparisons of functional properties between HEAs and conventional low- and medium-entropy materials are provided, and examples are illustrated using computational modeling and tuning the composition of existing functional materials through substitutional or interstitial mixing. Extending the concept of high configurational entropy to a wide range of materials such as intermetallics, ceramics, and semiconductors through the isostructural design approach is discussed. Perspectives are offered in designing future high-performance functional materials utilizing the high-entropy concepts and high-throughput predictive computational modeling.

High-entropy alloys (HEAs) with multiple principal elements open up a practically infinite space for designing novel materials. Probing this huge material universe requires the use of combinatorial and high-throughput synthesis and processing methods. Here, we present and discuss four different combinatorial experimental methods that have been used to accelerate the development of novel HEAs, namely, rapid alloy prototyping, diffusion-multiples, laser additive manufacturing, and combinatorial co-deposition of thin-film materials libraries. While the first three approaches are bulk methods which allow for downstream processing and microstructure adaptation, the latter technique is a thin-film method capable of efficiently synthesizing wider ranges of composition and using high-throughput measurement techniques to characterize their structure and properties. Additional coupling of these high-throughput experimental methodologies with theoretical guidance regarding specific target features such as phase (meta)stability allows for effective screening of novel HEAs with beneficial property profiles.

The atomic structure, electronic structure, and physical properties of (TiZrNbCu)1−xNix (x ≤ 0.5) metallic glasses (MGs) were studied in both the high-entropy (0 < x < 0.35) and the higher Ni concentration range (x ≥ 0.35). Atomic structure studies performed with X-ray diffraction and synchrotron powder diffraction provided average atomic volumes, structure factors, radial distribution functions, coordination numbers, and packing densities. Electronic structure studies performed using photoemission spectroscopy and low-temperature specific heat provided information about the electronic density of states within the valence band and at the Fermi level and also about interatomic bonding and atomic vibrations [from the Debye temperature and the boson peak (BP)]. Variations of both atomic structure and electronic structure with x showed a clear change for x ≥ 0.35, which corresponds to a valence electron number ≥7.4. All physical properties, namely, thermal stability parameters, Debye temperatures, BPs, magnetic, elastic, and electronic transport properties, change their concentration-dependence for x ≥ 0.35. The results are compared with those for binary and ternary MGs of the same elements.