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.

Refractory high-entropy alloys (RHEAs) have recently attracted much attention, primarily due to their mechanical properties at elevated temperatures. However, the equilibrium phase-stability of these alloy systems is not well established. The present investigation focuses on the phase stability of Al0.5NbTa0.8Ti1.5V0.2Zr RHEA at temperatures ranging from 600 to 1200 °C. The detailed phase characterization involves coupling of scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The stable phases present at these temperatures are (i) 1200 °C—body-centered cubic (BCC) matrix with nano-B2 precipitates; (ii) 1000 °C and 800 °C—a BCC matrix phase with Al–Zr rich hexagonal closed packed intermetallic precipitates and, (iii) 600 °C—a BCC + B2 microstructure, comprising a continuous BCC matrix with discrete B2 precipitates. These results highlight the substantial changes in phase stability as a function of temperature in RHEAs, and high-entropy alloys in general, and also the importance of accounting for these changes especially while designing alloys for high temperature applications.

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.

A recent structural model reconciles apparently conflicting features of randomness, short-range order, and medium-range order that coexist in metallic glasses. In this efficient cluster packing model, short-range order can be described by efficiently packed solute-centered clusters, producing more than a dozen established atomic clusters, including icosahedra. The observed preference for icosahedral short-range order in metallic glasses is consistent with the theme of efficient atomic packing and is further favored by solvent-centered clusters. Driven by solute—solute avoidance, medium-range order results from the organization in space of overlapping, percolating (via connected pathways), quasi-equivalent clusters. Cubic-like and icosahedral-like organization of these clusters are consistent with measured medium-range order. New techniques such as fluctuation electron microscopy now provide more detailed experimental studies of medium-range order for comparison with model predictions. Microscopic free volume in the efficient cluster packing model is able to represent experimental and computational results, showing free volume complexes ranging from subatomic to atomic-level sizes. Free volume connects static structural models to dynamic processes such as diffusion and deformation. New approaches dealing with “free” and “anti-free” microscopic volume and coordinated atomic motion show promise for modeling the complex dynamics of structural relaxations such as the glass transition. Future work unifying static and dynamic structural views is suggested.

Correlations between three characteristic temperatures: glass transition, Tg, Kauzmann, Tk, and Vogel-Fulcher-Tammann, To, were identified from the analysis of more than 60 metallic and non-metallic glass-forming materials. It was found that Tg ≥ Tk ≥ To and Tk is the geometric mean of Tg and To. The relation Tk ≥ To indicates that the excess total entropy of a super-cooled liquid ΔS approaches zero at a higher temperature than the configurational entropy ΔSconf, and such behavior was explained by the stronger temperature dependence of the excess vibrational entropy of the liquid, ΔSvib, than that of the corresponding glass, . A relationship between the fragility index m, reduced excess heat capacity ΔCp(Tg)/Sm, and reduced glass transition temperature, Trg, was identified using the found correlation between the characteristic temperatures.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Polycrystalline NiAl exhibits <001> slip at temperatures below about 450°C. In Fe-Al alloys there is a transition in the Burgers vector from a/2<111> at low temperatures to a<001> at intermediate temperatures. The transition temperature is dependent upon composition.

A transition in the Burgers vector from a/2<111> to a<001> has been found to exist as a function of composition and temperature in the B2 pseudo-binary (Fe,Ni)-40Al (at%) system. At room temperature, this transition occurs at a composition between Fe-40Al-10Ni, which exhibits <111> slip, and Fe-40Al-3ONi, which exhibits <001> slip. At 400°C this transition shifts further toward the iron-rich compositions. Fe-40Al-10Ni exhibits both <111> and <001> slip, whereas Fe-40Al-20Ni exhibits <001> slip.

Structural intermetallic materials have undergone an evolutionary process whereby some of the materials could provide major payoffs in gas turbine engines. This maturation of select intermetallic systems has provided significant hope for making still greater advances in turbine performance through further developments in other intermetallic materials. The same maturation process has highlighted specific limitations and requirements which are key to the utilization of intermetallic systems. This paper briefly reviews some critical ground rules for high temperature intermetallics development and identifies research directions being pursued by the Air Force for advancing intermetallic materials.