High Entropy Materials

Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (Cr, Mo, W) refractory elements, can be potentially new generation high-temperature materials.However, most existing RHEAs lack room-temperature ductility, similar to conventional refractory
metals and alloys. Here, we propose an alloy design strategy to intrinsically ductilize RHEAs based on the electron theory and more specifically to decrease the number of valence electrons through
controlled alloying. A new ductile RHEA, Hf0.5Nb0.5Ta0.5Ti1.5Zr, was developed as a proof of concept, with a fracture stress of close to 1 GPa and an elongation of near 20%. The findings here will shed light on the development of ductile RHEAs for ultrahigh-temperature applications in aerospace and power-generation industries.

Since the Bronze Age, humans have been altering the properties of materials by adding alloying elements. For example, a few percent by weight of copper was added to silver to produce sterling silver for coinage a thousand years ago, because pure silver was too soft. Examples from the modern era include steels that consist primarily of iron, to which elements such as carbon and chromium are added for strength and corrosion resistance, respectively, and copper alloyed with beryllium to make it strong and non-sparking for use in explosive environments. With few exceptions, the basic alloying strategy of adding relatively small amounts of secondary elements to a primary element has remained unchanged over millennia. It is even reflected in the way alloys are named after their principal constituent: ferrous alloys, aluminium alloys, titanium alloys, nickel alloys and so on. However, such a primary-element approach drastically limits the total number of possible element combinations and, therefore, alloys, most of which have been identified and exploited. New approaches are needed if the compositional space to explore is to be significantly enlarged. One such approach is based on mixing together multiple principal elements in relatively high (often equi-atomic) concentrations. This approach stands in sharp contrast to the traditional practice and has, therefore, attracted much attention. The related surge in research activity, especially during the past 5 years, can be traced back to the publication of two seminal papers 1,2 in 2004. Two groups independently proposed the study of a new class of alloys containing multiple elements in near-equiatomic concentrations. It was subsequently pointed out that conventional alloys tend to cluster around the corners or edges of phase diagrams, where the number of possible element combinations is limited, and that vastly more numerous combinations are available near the centres of phase diagrams, especially in quaternary, quinary and higher-order systems 3. Owing to their sheer numbers, little is known about concentrated, multi-component alloys but, by the same token, because there are so many possible combinations, the concept offers promise to discover interesting new alloys with useful properties in their midst. Jien-Wei Yeh and co-workers 1 provided an additional intriguing rationale for investigating these alloys: they hypothesized that the presence of multiple (five or more) elements in near-equiatomic proportions would increase the configurational entropy of mixing by an amount sufficient to overcome the enthalpies of compound formation, thereby deterring the formation of potentially harmful intermetallics. This was a counter-intuitive notion because the conventional view-likely based on binary phase diagrams in which solid solutions are typically found at the ends and compounds near the centres-was that the greater the number of elements in concentrated alloys, the higher the probability that some of the elements would react to form compounds. But Yeh and colleagues reasoned that, as the number of elements in an alloy increased, the entropic contribution to the total free energy would overcome the enthalpic contribution and, thereby, stabilize solid solutions (Box 1; Fig. 1). They coined a catchy new name, high-entropy alloys (HEAs), for this Abstract | Alloying has long been used to confer desirable properties to materials. Typically , it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking.

This paper reviews the recent research and development of highentropy alloys (HEAs). HEAs are loosely defined as solid solution alloys that contain more than five principal elements in equal or near equal atomic percent (at.%). The concept of high entropy introduces a new path of developing advanced materials with unique properties, which cannot be achieved by the conventional micro-alloying approach based on only one dominant element. Up to date, many HEAs with promising properties have been reported, e.g., high wear-resistant HEAs, Co 1.5 CrFeNi 1.5 Ti and Al 0.2 Co 1.5 CrFeNi 1.5 Ti alloys; high-strength body-centered-cubic (BCC) AlCoCrFeNi HEAs at room temperature, and NbMoTaV HEA at elevated temperatures. Furthermore, the general corrosion resistance of the Cu 0.5 NiAlCoCrFeSi HEA is much better than that of the conventional 304-stainless steel. This paper first reviews HEA formation in relation to thermodynamics, kinetics, and processing. Physical, magnetic, chemical, and mechanical properties are then discussed. Great details are provided on the plastic deformation, fracture, and magnetization from the perspectives of crackling noise and Barkhausen noise measurements, and the analysis of serrations on stress-strain curves at specific strain rates or testing temperatures, as well as the serrations of the magnetization hysteresis loops. The comparison between conventional and high-entropy bulk metallic glasses is analyzed from the viewpoints of eutectic composition, dense atomic packing, and entropy of

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 highthroughput predictive computational modeling.

High-entropy alloys (HEAs), although often presumed to be random solid solutions, have recently been shown to display nanometer-scale variations in the arrangements of their multiple chemical elements. Here, we study the effects of this compositional heterogeneity in HEAs on their mechanical properties using in situ compression testing in the transmission electron microscope (TEM), combined with molecular dynamics simulations. We report an anomalous size effect on the yield strength in HEAs, arising from such compositional heterogeneity. By progressively reducing the sample size, HEAs initially display the classical "smaller-is-stronger" phenomenon, similar to pure metals and conventional alloys. However, as the sample size is decreased below a critical characteristic length (~180 nm), influenced by the size-scale of compositional heterogeneity, a transition from homogeneous deformation to a heterogeneous distribution of planar slip is observed, coupled with an anomalous "smaller-is-weaker" size effect. Atomic-scale computational modeling shows these observations arise due to compositional fluctuations over a few nanometers. These results demonstrate the efficacy of influencing mechanical properties in HEAs through control of local compositional variations at the nanoscale.

In this study, (Al 0.5 CrFeNiTi 0.25)N x high-entropy films are prepared by a reactive direct current (DC) magnetron sputtering at different N 2 flow rates on silicon wafers. It is found that the structure of (Al 0.5 CrFeNiTi 0.25)N x high-entropy films is amorphous, with x = 0. It transforms from amorphous to a face-centered-cubic (FCC) structure with the increase of nitrogen content, while the bulk Al 0.5 CrFeNiTi 0.25 counterpart prepared by casting features a body-centered-cubic (BCC) phase structure. The phase formation can be explained by the atomic size difference (δ). Lacking nitrogen, δ is approximately 6.4% for the five metal elements, which is relatively large and might form a BCC or ordered-BCC structure, while the metallic elements in this alloy system all have a trend to form nitrides like TiN, CrN, AlN, and FeN. Therefore, nitride components are becoming very similar in size and structure and solve each other easily, thus, an FCC (Al-Cr-Fe-Ni-Ti)N solid solution forms. The calculated value of δ is approximately 23% for this multicomponent nitride solid solution. The (Al 0.5 CrFeNiTi 0.25)N x films achieve a pronounced hardness and a Young's modulus of 21.45 GPa and 253.8 GPa, respectively, which is obviously much higher than that of the as-cast Al 0.5 CrFeNiTi 0.25 bulk alloys.

The Fe x Co 1-x NiMnGa (FeNiMnGa, Fe 0.5 Co 0.5 NiMnGa, and CoNiMnGa) alloys are designed based on the high entropy effect and the model of the Ni 2 MnGa alloy in searching for the potential functional magnetic alloys with improved mechanical properties. Although the substitution of Ni by Fe or Co can reduce the ordering of the phases due to the high mixing of entropy, these three alloys are composed of ordered phases, which lead the alloys to show the brittle deformation behavior with the combined intergranular and cleavage fracture mechanisms. These alloys exhibit excellent soft magnetic properties with very low saturation magnetostriction coefficients. For CoNiMnGa alloy, the saturation magnetization can be 115.92 emu/g, and the Curie temperature is about 657.3 K. The addition of Co enhances the exchange interaction and makes the alloy demonstrate potential applications at higher temperatures. In addition, the single disordered solid-solution phase-formation rule of high entropy alloys is evaluated, which can shed light on the HEAs design.