High Entropy Alloys Research Papers (original) (raw)

Nanostructuring metals through nanograins and nanotwins is an efficient strategy for strength increase as the mean free path of dislocations is reduced. Yet, nanostructures are thermally often not stable, so that the material properties deteriorate upon processing or during service. Here, we introduce a new strategy to stabilize nanotwins by an interfacial nanophase design and realize it in an interstitial high-entropy alloy (iHEA). We show that nanotwins in a carbon-containing FeMnCoCrNi iHEA can remain stable up to 900 °C. This is enabled by co-segregation of Cr and C to nanoscale 9R structures adjacent to incoherent nanotwin boundaries, transforming the 9R structures into elongated nano-carbides in equilibrium with the nanotwin boundaries. This nanoscale 9R structures assisted nano-carbide formation leads to an unprecedented thermal stability of nanotwins, enabling excellent combination of yield strength (~1.1 GPa) and ductility (~21%) after exposure to high temperature. Stimulating the formation of nano-sized 9R phases by deformation together with interstitial doping establishes a novel interfacial-nanophase design strategy. The resulting formation of nano-carbides at twin boundaries enables the development of strong, ductile and thermally stable bulk nanotwinned materials.

Linear defects, referred to as dislocations, determine the strength, formability, and toughness of crystalline metallic alloys. The associated deformation mechanisms are well understood for traditional metallic materials consisting of one or two prevalent matrix elements such as steels or aluminum alloys. In the recently developed high-entropy alloys (HEAs) containing multiple principal elements, the relationship between dislocations and the mechanical behavior is less understood. Particularly HEAs with a hexagonal close-packed (hcp) structure can suffer from intrinsic brittleness due to their insufficient number of slip systems. Here we report on the surprisingly high formability of a novel high-entropy phase with hcp structure. Through in situ tensile testing and postmortem characterization by transmission electron microscopy we reveal that the hcp phase in a dual-phase HEA (Fe 50 Mn 30 Co 10 Cr 10 , at. %) activates three types of dislocations, i.e., hai, hci, and hc þ ai. Specifically, nonbasal hc þ ai dislocations occupy a high line fraction of ∼31% allowing for frequent double cross slip which explains the high deformability of this high-entropy phase. The hcp structure has a c=a ratio of 1.616, i.e., below the ideal value of 1.633. This modest change in the structure parameters promotes nonbasal hc þ ai slip, suggesting that ductile HEAs with hcp structure can be designed by shifting the c=a ratio into regimes where nonbasal slip systems are activated. This simple alloy design principle is particularly suited for HEAs due to their characteristic massive solid solution content which readily allows tuning the c=a ratio of hcp phases into regimes promoting nonbasal slip activation.

- by Dierk Raabe and +1
- •
- Microstructure, Mechanical Properties of Materials, High Entropy Alloys, Dislocations

The present work investigates the influence of micro-alloyed Mo on the corrosion behavior of (CoCrFeNi) 100−x Mo x high-entropy alloys. All of the (CoCrFeNi) 100−x Mo x alloys exhibit a single face-centered cubic (FCC) solid solution. However, the (CoCrFeNi) 97 Mo 3 alloy exhibits an ordered sigma (σ) phase enriched in Cr and Mo. With the increase of x (the Mo content) from 1 to 3, the hardness of the (CoCrFeNi) 100−x Mo x alloys increases from 124.8 to 133.6 Vickers hardness (HV), and the compressive yield strength increases from 113.6 MPa to 141.1 MPa, without fracture under about a 60% compressive strain. The potentiodynamic polarization curve in a 3.5% NaCl solution indicates that the addition of Mo has a beneficial effect on the corrosion resistance to some certain extent, opposed to the σ phase. Furthermore, the alloys tend to form a passivation film in the 0.5 M H 2 SO 4 solution in order to inhibit the progress of the corrosion reaction as the Mo content increases.

- by Qing Peng
- •
- Corrosion Science, Corrosion Engineering, Corrosion, High Entropy Alloys

The study was to evaluate the influence of temperature and pH on corrosion
resistance of Ni-Cr and Co-Cr dental alloys, in order to characterize the physical and
mechanical properties of corrosion resistance property of the Ni-Cr and Co-Cr and
investigate the correlations between corrosion and biocompatibility of dental alloys and to
interpret the results by comparison with ion concentrations found in necessary food

- by Tbrah Saleh and +1
- •
- Materials Science, Dentistry, High Entropy Alloys

A non-equiatomic AlCoCr0.75Cu0.5FeNi alloy has been identified as a potential high strength alloy, whose microstructure and consequently properties can be widely varied. In this research, the phase structure, hardness, and magnetic properties of AlCoCr0.75Cu0.5FeNi alloy fabricated by laser powder bed fusion (LPBF) are investigated. The results demonstrate that laser power, scanning speed, and volumetric energy density (VED) contribute to different aspects in the formation of microstructure thus introducing alterations in the properties. Despite the different input parameters studied, all the as-built specimens exhibit the body-centered cubic (BCC) phase structure, with the homogeneous elemental distribution at the micron scale. A microhardness of up to 604.6 ± 6.8 HV0.05 is achieved owing to the rapidly solidified microstructure. Soft magnetic behavior is determined in all as-printed samples. The saturation magnetization (Ms) is dependent on the degree of spinodal decomposition, i.e., the higher degree of decomposition into A2 and B2 structure results in a larger Ms. The results introduce the possibility to control the degree of spinodal decomposition and thus the degree of magnetization by altering the input parameters of the LPBF process. The disclosed application potentiality of LPBF could benefit the development of new functional materials.

- by Mika Salmi
- •
- Engineering, Materials Engineering, Mechanical Engineering, Materials Science

Brasses belong to a group of copper-based alloys with a wide variety of important applications. Traditionally, lead was a necessary alloying element in brasses due to its ability to improve machining in terms of time and quality. Additional effect of lead is a wear reduce of machining tools. The main risk of using lead-containing brasses is lead’s toxicity for humans. Various stages of producing and machining of brasses may be harmful, so certain safety regulations are normally required, which is time and
money expensive[1].
That’s why alternative technologies of producing lead-free brasses, using graphite, are of great interest. Carbon Nano-tubes (CNTs) can be considered as ideal reinforcements, due to their high specific strength, thermal, electrical and mechanical characteristics.
In the present study brass (Cu-Zn)/carbon nanotubes (CNT) composites were produced by injection of CNTs in brass using high pressure die casting method. Current
investigation demonstrates that more than 50% efficiency of direct injection of CNT in the brass has been achieved. The inserted CNTs have been partially dispersed in the metal matrix. Furthermore the friction coefficient significantly decreases with an increase of the amount of CNT content in the examined Cu-Zn brass alloy. Thus, the machinability improvement as a result of CNT performance in the examined Cu-Zn brass alloy is expected.

- by Yong Zhang and +1
- •
- High Entropy Alloys

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.

- by Sheng Guo and +2
- •
- High Entropy Alloys, Refractory Materials, High Entropy Alloy, High Entropy Materials

We introduce a new class of high-entropy alloys (HEAs), i.e., quinary (five-component) dual-phase (DP) HEAs revealing transformation-induced plasticity (TRIP), designed by using a quantum mechanically based and experimentally validated approach. Ab initio simulations of thermodynamic phase stabilities of Co 20 Cr 20 Fe 40-x Mn 20 Ni x (x ¼ 0e20 at. %) HEAs were performed to screen for promising compositions showing the TRIP-DP effect. The theoretical predictions reveal several promising alloys, which have been cast and systematically characterized with respect to their room temperature phase constituents, microstructures, element distributions and compositional homogeneity, tensile properties and deformation mechanisms. The study demonstrates the strength of ab initio calculations to predict the behavior of multi-component HEAs on the macroscopic scale from the atomistic level. As a prototype example a non-equiatomic Co 20 Cr 20 Fe 34 Mn 20 Ni 6 HEA, selected based on our ab initio simulations, reveals the TRIP-DP effect and hence exhibits higher tensile strength and strain-hardening ability compared to the corresponding equiatomic CoCrFeMnNi alloy.

Metastable solid solutions can form preferably over intermetallic compounds, in cast high-entropy alloys or multi-component alloys with equi- or nearly equi-atomic compositions, due to the entropy contribution at elevated temperatures. Meanwhile, the high mixing entropy also favors the amorphous phase formation. The phase selection between solid solutions and the amorphous phase upon alloying in highentropy alloys is intriguing. A two-parameter physical scheme, utilizing the atomic size polydispersity and mixing enthalpy, is found to be capable of capturing this phase selection mechanism.

- by Sheng Guo
- •
- High Entropy Alloys, Phase Stability

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.

- by Dierk Raabe and +1
- •
- Thermodynamics, Classical Thermodynamics, Phase Transformations, Microstructure

Alloying greatly expands the amount of available materials beyond the naturally existing ones, and more importantly offers the material scientists opportunities to initiatively control the... more

Alloying greatly expands the amount of available materials beyond the naturally existing ones, and more importantly offers the material scientists opportunities to initiatively control the composition-structure-property relationship in materials. Since commonly used metallic materials are
mostly multi-component alloys, the know-how of alloying through compositional control, certainly plays a critical role in designing materials with desired structure and properties. However , alloying in multi-component alloys is an extremely complicated issue, as the alloyed products could be the amorphous phase, various solid solutions and intermetallic compounds containing two or more alloy components. By narrowing down the scope of the multi-component alloys to those with equiatomic or
close-to-equiatomic compositions only, and also aiming at framing out the rules that govern the phase selection upon alloying in multi-component alloys in a broad sense, we have identified here a simple and easily executable two-parameter scheme that can effectively predict the formation of the amorphous phase, solid solutions and intermetallic compounds, in multi-component alloys, simply from the given alloy compositions. We believe this scheme reveals a clear physical scenario governing the phase selection in multi-component alloys, helps to simplify the alloy design, and benefits the future development of advancedmetallic alloys like bulkmetallic glasses andhigh entropy alloys.

- by Sheng Guo
- •
- High Entropy Alloys, Multicomponent Alloys
- by Yong Zhang
- •
- High Entropy Alloys

Al–Cr–Fe–Mn–Ni and Al–Cr–Cu–Fe–Mn–Ni high entropy alloy thin films were prepared by potentiostatic
electrodeposition and the microstructure of the deposits was investigated. The thin films were
co-deposited in an electrolyte based on a DMF (N,N-dimethylformamide)-CH3CN (acetonitrile) organic
compound. The energy dispersive spectrometry investigation (EDS) indicated that all the five respectively
six elements were successfully co-deposited. The scanning electron microscopy (SEM) analysis
revealed that the film consists of compact and uniform particles with particle sizes of 500 nm to 4 m.
The X-ray diffractometry (XRD) patterns indicated that the as-deposited thin films were amorphous.
Body-centered-cubic (BCC) structures were identified by XRD after the films were annealed at various
temperatures under inert Ar atmosphere. The alloys adhesion on the substrate was determined by the
scratch-testing method, with higher values obtained for the Al–Cr–Cu–Fe–Mn–Ni alloy.

- by Viorel Badilita and +1
- •
- High Entropy Alloys

- by Robert Ritchie and +1
- •
- High Entropy Alloys, High Entropy Alloy

The results of studies on the regularities of structure formation and properties of high-entropy alloys prepared by different methods are analyzed and generalized. Specific features of the synthesis of nitride coatings based on multicomponent alloys are considered. Main physicomechanical properties of the coatings of different elemental composition deposited in different manner were compared.

- by Береснев Вячеслав and +3
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- Thin Films and Coatings, Review, High Entropy Alloys

- by Robert O Ritchie and +1
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- High Entropy Alloys, High Entropy Materials
- by Jien-wei Yeh and +1
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- Materials Engineering, Condensed Matter Physics, Metallurgy, Deformation and strain

High-entropy alloys (HEAs) are alloys with five or more principal elements. Due to the distinct design concept, these alloys often
exhibit unusual properties. Thus, there has been significant interest in these materials, leading to an emerging yet exciting new field. This paper briefly reviews some critical aspects of HEAs, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors. Current challenges and important future directions are also pointed out.

- by Ming-Hung Tsai
- •
- High Entropy Alloys

High-entropy alloys (HEAs), also known as multi-principal element alloys or multi-component alloys, have been the subject of numerous investigations since they were first described in 2004. One of the earliest HEAs was the equiatomic CrMnFeCoNi “Cantor” alloy, which has been one of the most studied, but HEAs now encompass a broad class of metallic and ceramic systems based on the same design principle. The original concept, of utilizing the high entropy of mixing to develop stable multi-element alloys, has produced extraordinary mechanical properties in specific HEAs, mainly CrCoNi-based alloys, associated with their continuous work-hardening rate that is sustained to large plastic strains. The enhanced work-hardening ability in these face-centered cubic HEAs, which is maintained up to tensile strains of ~0.5 and at low temperatures, in combination with the high frictional forces on dislocations and a propensity for twinning, leads to outstandingly high fracture toughness values and resistance to localization of deformation and shear-band formation under dynamic loading. The fracture toughness of some of the CoCrNi-based alloys is on the order of, and can even exceed, 200 MPa.m1/2, with yield strengths that range from 200 MPa to close to 1000 MPa, depending on a number of intrinsic parameters including grain size. The critical shear strain for the onset of adiabatic shear band formation is ~7 for the Cantor alloy. This is much higher than that for conventional alloys and suggests superior ballistic properties. The slower diffusion rates resulting from the multi-element environment contribute to the excellent intermediate-temperature performance. We review the principal mechanical properties of these alloys with emphasis on the CrCoNi-based systems and their nano-/micro-structural features. We also discuss high-entropy ceramics and more complex systems having more than one phase. Due to the very favorable mechanical properties of some of these HEAs, and to the fact that most can be processed by conventional means, we anticipate that they will find extensive utilization in many future structural applications.

- by Robert Ritchie and +2
- •
- High Entropy Alloys

In human-made malleable materials, microdamage such as cracking usually limits material lifetime. Some biological composites, such as bone, have hierarchical microstructures that tolerate cracks but cannot withstand high elongation. We demonstrate a directionally solidified eutectic high-entropy alloy (EHEA) that successfully reconciles crack tolerance and high elongation. The solidified alloy has a hierarchically organized herringbone structure that enables bionic-inspired hierarchical crack buffering. This effect guides stable, persistent crystallographic nucleation and growth of multiple microcracks in abundant poor-deformability microstructures. Hierarchical buffering by adjacent dynamic strainhardened features helps the cracks to avoid catastrophic growth and percolation. Our self-buffering herringbone material yields an ultrahigh uniform tensile elongation (~50%), three times that of conventional nonbuffering EHEAs, without sacrificing strength.

- by Dierk Raabe
- •
- Microstructure, Composites, Metallurgy, Bio-Inspired Systems

High-entropy alloys (HEAs) have recently become a vibrant field of study in the metallic materials area. In the early years, the design of HEAs was more of an exploratory nature. The selection of compositions was somewhat arbitrary, and there was typically no specific goal to be achieved in the design. Very recently, however, the development of HEAs has gradually entered a different stage. Unlike the early alloys, HEAs developed nowadays are usually designed to meet clear goals, and have carefully chosen components, deliberately introduced multiple phases, and tailored microstructures. These alloys are referred to as advanced HEAs. In this paper, the progress in advanced HEAs is briefly reviewed. The design strategies for these materials are examined and are classified into three categories. Representative works in each category are presented. Finally, important issues and future directions in the development of advanced HEAs are pointed out and discussed.

A B S T R A C T A novel (TiZrNbTaHf)N/MoN nanocomposite coatings, which consist of the nitride of the high-entropy alloy and the binary nitride, were synthesized by vacuum-arc deposition at various substrate biases. The elemental composition, chemical bonding state, phase structure, microstructure and mechanical properties of the coatings were studied by high-resolution experimental methods: SIMS, GDMS, XPS, XRD, HR-TEM and nano-indentation. It was found that the chemical state of the (TiZrNbTaHf)N/MoN coatings has a complex nature, which consist of a mixture of nitrides of constituting elements. It was also shown that the coatings are based on B1 NaCl-structured γ-Mo 2 N-phase with a mixture of crystallographic orientations (111), (200), (220) and (311) together with the B1 NaCl-structured (TiZrNbTaHf)N solid-solution phase. First-principles calculations demonstrated that the metal sub-lattice of the (TiZrNbTaHf)N solid solution can be based on Ti 1-x Hf y Ta 1-x-y , Zr 1-x Hf y Ta 1-x-y , Zr 0.25 Ti 0.25 Ta 0.5 ternary alloys, which have the lowest mixing energy. The HR-TEM results showed that the nanocomposite nitride coatings have nano-scale multilayer structure with modulation periods ranged from 20 nm to 25 nm. The maximum hardness of approximately 29 GPa demonstrated the coating deposited at a higher energy condition (−200 V) with the thinnest modulation period of bilayer of 20 nm (15 nm of (TiZrNbTaHf)N and 5 nm of Mo 2 N).

- by Artem Bagdasaryan and +3
- •
- Protective Coatings, High Entropy Alloys, Multilayered Structure

The recently developed interstitial high-entropy alloys (iHEAs) exhibit an enhanced combination of strength and ductility. These properties are attributed to dislocation hardening, deformation-driven athermal phase transformation from the face-centered cubic (FCC) g matrix into the hexagonal close-packed (HCP) ε phase, stacking fault formation, mechanical twinning and precipitation hardening. For gaining a better understanding of these mechanisms as well as their interactions direct observation of the deformation process is required. For this purpose, an iHEA with nominal composition of Fe-30Mn-10Co-10Cr-0.5C (at. %) was produced and investigated via in-situ and interrupted in-situ tensile testing in a scanning electron microscope (SEM) combining electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques. The results reveal that the iHEA is deformed by formation and multiplication of stacking faults along {111} microbands. Sufficient overlap of stacking faults within microbands leads to intrinsic nucleation of HCP ε phase and incoherent annealing twin boundaries act as preferential extrinsic nucleation sites for HCP ε formation. With further straining HCP ε nuclei grow into the adjacent deformed FCC g matrix. g regions with smaller grain size have higher mechanical stability against phase transformation. Twinning in FCC g grains with a size of ~10 mm can be activated at room temperature at a stress below ~736 MPa. With increasing deformation, new twin lamellae continuously nucleate. The twin lamellae grow in preferred directions driven by the motion of the mobile partial dislocations. Owing to the individual grain size dependence of the activation of the dislocation-mediated plasticity, of the athermal phase transformation and of mechanical twinning at the different deformation stages, desired strain hardening profiles can be tuned and adjusted over the entire deformation regime by adequate microstructure design, providing excellent combinations of strength and ductility.

A B S T R A C T In this study, (Zr-Ti-Nb)N, (Zr-Ti-Cr-Nb)N and (Zr-Ti-Cr-Nb-Si)N nitride coatings were obtained using a well-developed vacuum arc deposition. The systematical investigations demonstrate that the chemical composition, microstructure, and properties of the coatings intimately rely on the deposition parameters (pressure of working gas and substrate bias). Effects of Cr and Si additions on microstructure and mechanical properties of the (Zr-Ti-Nb)N coatings have been investigated using scanning electron microscopy (SEM) equipped with energy dispersive spectrum (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscope (TEM), hardness measurements and adhesion testing. First-principles band-structure calculations and Gibbs-Rosenbaum triangle representation have been used to investigate the elemental and phase compositions in nitride coatings. The multi-component (Zr-Ti-Cr-Nb-Si)N and (Zr-Ti-Nb)N coatings are found to be a simple face-centered cubic (FCC) solid solution. For the coatings without Si, the structure is mainly composed of TiN fcc phase and Cr 2 N trigonal modification. The hardness values were in the ranges (24– 42 GPa). The (Zr-Ti-Nb)N, (Zr-Ti-Cr-Nb)N coatings provided the best adhesive strength in different conditions. The (Zr-Ti-Cr-Nb-Si)N coatings exhibited the worst adhesive strength, which may be attributed to the relative low hardness.

- by Artem Bagdasaryan and +1
- •
- Materials Science, Thin Films and Coatings, High Entropy Alloys

Recently, high-entropy alloys (HEAs) have attracted much interest in the materials community, as they offer massive opportunities to observe new phenomena, explore new structure, and develop new materials. Particularly, it is attractive to prepare high-performance HEA coatings by laser-induced rapid solidification, which can be formed on the surface of components and parts in a variety of sizes and shapes with a lower cost in comparison with those bulk material fabrication methods. From the technical point of view, laser-induced rapid solidification could hamper the compositional segregation, improve the solubility in solid-solution phases, and lead to the strengthening effect by the grain refinement. This article reviews the recent work on the typical microstructural features and the mechanical and chemical properties in laser-induced rapidly solidified HEAs, and these data are compared with conventional Co- and Ni-based alloy coatings. The article concludes with suggestions for future research and development in HEAs, from considerations of their characteristic properties.

- by Sheng Guo
- •
- Surface Coatings, High Entropy Alloys

High entropy alloys (HEA) are multicomponent (5 or more) massive solid solutions with an equiatomic or a near equiatomic composition. The original ideal of investigating multicomponent alloys in equal or near-equal proportions represents a new alloy exploration strategy. Instead of starting from a corner of a phase diagram with one prevalent base element, it has been suggested that new materials could be identified by directly producing equiatomic compositions with multiple components. The term ‘‘high entropy alloys’’ was introduced by Yeh et al., based on the hypothesis that the high configurational entropy would stabilize the solid solution phase over competing intermetallic and elemental phases. A well-studied HEA is the Cantor alloy i.e. Co20Cr20Fe20Mn20Ni20 (at.%) which develops a single phase fcc solid solution. Recently, it has been shown that a non-equiatomic composition of this alloy system also exhibits a single phase fcc solid solution irrespective of its slightly lower mixing entropy. The objective of this study is two-fold. One focus is the prediction and analysis of the phase stability of this alloy system i.e. FexMn62ÿxNi30Co6Cr2 (at.%, x = 22, 27, 32, 37, and 42), while varying the Fe and Mn contents, and maintaining the compositions of Cr, Co and Ni constant. The configurational entropy of these alloys ranges from 1.295 to 1.334 kB/atom (kB is the Boltzmann constant) which yields 80–83% of that in equiatomic composition (1.6094 kB/atom) as shown in Fig. 1. Another focus is to explore the feasibility of using the CALPHAD (CALculation of PHAse Diagrams) method for future knowledge based approaches to the design of HEAs. Compared with other approaches for designing HEAs (e.g. empirical rules, or ab initio based methods), the CALPHAD method provides an optimal balance between efficiency and accuracy. On the other hand, most multicomponent systems are not fully covered by the available CALPHAD databases. Instead, current CALPHAD simulations of multicomponent systems are based on the extrapolation from binary, ternary, and, (perhaps) quaternary systems. Hence, the accuracy of the corresponding predictions yielded by
using a CALPHAD approach needs to be critically evaluated.
The objective of this study is to experimentally and theoretically investigate the phase stability of non-equiatomic FexMn62ÿxNi30Co6Cr2 based high entropy alloys, where x ranges from 22 to 42 at.%. Another aim is to systematically and critically assess the predictive capability of the CALPHAD approach for such high entropy alloy systems. We find that the CALPHAD simulations provide a very consistent assessment of phase stability yielding good agreement with experimental observations. These include the equilibrium phase formation at high temperatures, the constituent phases after non-equilibrium solidification processes, unfavorable segregation profiles inherited from solidification together with the associated nucleation and growth of low temperature phases, and undesired martensitic transformation effects. Encouraged by these consistent theoretical and experimental results, we extend our simulations to other alloy systems with equiatomic compositions reported in the literature. Using these other equiatomic model systems we demonstrate how systematic CALPHAD simulations can improve and accelerate the design of multicomponent alloy systems.

- by M. Duancheng and +1
- •
- Thermodynamics, Computational Materials Science, Segregation, Electron Backscatter Diffraction (EBSD)
- by Ming-Hung Tsai
- •
- High Entropy Alloys, Electrical Properties

We present a brief overview on recent developments in the field of strong and ductile non-equiatomic high-entropy alloys (HEAs). The materials reviewed are mainly based on massive transition-metal solute solutions and exhibit a broad spectrum of microstructures and mechanical properties. Three relevant aspects of such non-equiatomic HEAs with excellent strength–ductility combination are addressed in detail, namely phase stability-guided design, controlled and inexpensive bulk metallurgical processing routes for appropriate microstructure and compositional homogeneity, and the resultant microstructure–property relations. In addition to the multiple principal sub-stitutional elements used in these alloys, minor interstitial alloying elements are also considered. We show that various groups of strong and ductile HEAs can be obtained by shifting the alloy design strategy from single-phase equiatomic to dual-or multiphase non-equiatomic compositional configurations with carefully designed phase instability. This design direction provides ample possibilities for joint activation of a number of strengthening and toughening mechanisms. Some potential research efforts which can be conducted in the future are also proposed.

- by Dierk Raabe
- •
- Thermodynamics, Phase Transformations, Corrosion Science, Metallurgy

High-entropy alloy and nitride coatings (TiHfZrVNb)N were prepared by the cathodic-arc-vapor-deposition method under various deposition conditions. The composition, crystal structure, strain-stress state, profiles of defects and atoms in-depth and at surfaces of the (TiHfZrVNb)N coatings were characterized by EDS and SEM analysis, X-ray diffraction with “α-sin2ψ” method of measurements and slow positron beam. The oxidation behavior of nitride films after annealing at 600 °C temperature was studied. The results indicate that nitride coatings show the face-centered cubic crystal structure. The redistributions of elements and defects, their arrangement (segregation) due to the thermally stimulated diffusion and termination of the spinodal segregation near the interfaces, around the grains and subgrains were found. The peak hardness and modulus of the nitride films were 44.3 and 384 GPa, respectively. The tribological properties of the (TiHfZrVNb)N coatings against AISI 1045 were evaluated by a ball-on-disc tribometer with a 3.0 N applied load.

- by Artem Bagdasaryan and +3
- •
- Thin Films and Coatings, Surface Coatings, High Entropy Alloys

Soft magnetic materials (SMMs) serve in electrical applications and sustainable energy supply, allowing magnetic flux variation in response to changes in applied magnetic field, at low energy loss1. The electrification of transport, households and manufacturing leads to an increase in energy consumption due to hysteresis losses2. Therefore, minimizing coercivity, which scales these losses, is crucial3. Yet, meeting this target alone is not enough: SMMs in electrical engines must withstand severe mechanical loads, i.e., the alloys need high strength and ductility4. This is a fundamental design challenge, as most methods that enhance strength introduce stress fields that can pin magnetic domains, thus increasing coercivity and hysteretic losses5. Here, we introduce an approach to overcome this dilemma. We have designed a Fe-Co-Ni-Ta-Al multicomponent alloy with ferromagnetic matrix and paramagnetic coherent nanoparticles (~91 nm size, ~55% volume fraction). They impede dislocation motion, enhancing strength and ductility. Their small size, low coherency and small magnetostatic energy create an interaction volume below the magnetic domain wall width, leading to minimal domain wall pinning, thus maintaining the soft magnetic properties. The alloy has a tensile strength of 1336 MPa at 54% tensile elongation, extremely low coercivity of 78 A/m (<1 Oe), moderate saturation magnetization of 100 Am2/kg, and high electrical resistivity of 103 μΩ·cm.

In this study we present and discuss the influence of compositional inhomogeneity on the mechanical behavior of an interstitially alloyed dual-phase non-equiatomic high-entropy alloy (Fe49.5Mn30-Co10Cr10C0.5). Various processing routes including hot-rolling, homogenization, cold-rolling and recrystallization annealing were performed on the cast alloys to obtain samples in different compositional homogeneity states. Grain sizes of the alloys were also considered. Tensile testing and microstructural
investigations reveal that the deformation behavior of the interstitial dual-phase high-entropy alloy samples varied significantly depending on the compositional homogeneity of the specimens probed. In the case of coarse-grains (~300 mm) obtained for cast alloys without homogenization treatment, ductility and strain-hardening of the material was significantly reduced due to its compositional inhomogeneity. This detrimental effect was attributed to preferred deformation-driven phase transformation occurring in the Fe enriched regions with lower stacking fault energy, promoting early stress-strain localization. The grain-refined alloy (~4 mm) with compositional heterogeneity which was obtained for recrystallization annealed alloys without homogenization treatment was characterized by almost total loss in work-hardening. This effect was attributed to large local shear strains due to the inhomogeneous planar slip. These insights demonstrate the essential role of compositional homogeneity through applying corresponding processing steps for the development of advanced high-entropy alloys.

e corrosion behavior of high-entropy alloys (HEAs) CoCrFeNi 2 and CoCrFeNi 2 Mo 0.25 was investigated in 3.5 wt. percent sodium chloride (NaCl) at 25 ° C by electrochemical methods. eir corrosion parameters were compared to those of HASTELLOY ® C-276 (UNS N10276) and stainless steel 316L (UNS 31600) to assess the suitability of HEAs for potential industrial applications in NaCl simulating seawater type environments. e corrosion rates were calculated using corrosion current determined from electrochemical experiments for each of the alloys. In addition, potentiodynamic polarization measurements can indicate active, passive, and transpassive behavior of the metal as well as potential susceptibility to pitting corrosion. Cyclic voltammetry (CV) can confirm the alloy susceptibility to pitting corrosion. Electrochemical impedance spectroscopy (EIS) elucidates the corrosion mechanism under studied conditions. e results of the electrochemical experiments and scanning electron microscopy (SEM) analyses of the corroded surfaces revealed general corrosion on alloy CoCrFeNi 2 Mo 0.25 and HASTELLOY C-276 and pitting corrosion on alloy CoCrFeNi 2 and stainless steel 316L.

The high entropy alloy (HEA) concept has triggered a renewed interest in alloy design, even though some aspects of the underlying thermodynamic concepts are still under debate. This study addresses the shortcomings of this alloy design strategy with the aim to open up new directions of HEA research targeting specifically non-equiatomic yet massively alloyed compositions. We propose that a wide range of massive single phase solid solutions could be designed by including non-equiatomic variants. It is demonstrated by introducing a set of novel non-equiatomic multi-component CoCrFeMnNi alloys produced by metallurgical rapid alloy prototyping. Despite the reduced configurational entropy, detailed characterization of these materials reveals a strong resemblance to the well-studied equiatomic single phase HEA: The microstructure of these novel alloys exhibits a random distribution of alloying elements (confirmed by Energy-Dispersive Spectroscopy and Atom Probe Tomography) in a single face-centered-cubic phase (confirmed by X-ray Diffraction and Electron Backscatter Diffraction), which deforms through planar slip (confirmed by Electron-Channeling Contrast Imaging) and leads to excellent ductility (confirmed by uniaxial tensile tests). This approach widens the field of HEAs to non-equiatomic multi-component alloys since the concept enables to tailor the stacking fault energy and associated transformation phenomena which act as main mechanisms to design useful strain hardening behavior.

- by Dierk Raabe
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- Thermodynamics, Crystallography, Phase Transformations, Microstructure

Although refractory high-entropy alloys have exceptional strength at high temperatures, they are often brittle at room temperature. One exception is the HfNbTaTiZr alloy, which has a plasticity of over 50% at room temperature. However, the strength of HfNbTaTiZr at high temperature is insufficient. In this study, the composition of HfNbTaTiZr is modified with an aim to improve its strength at high temperature, while retaining reasonable toughness at room temperature. Two new alloys with simple BCC structure, HfMoTaTiZr and HfMoNbTaTiZr, were designed and synthesized. The results show that the yield strengths of the new alloys are apparently higher than that of HfNbTaTiZr. Moreover, a fracture strain of
12% is successfully retained in the HfMoNbTaTiZr alloy at room temperature.

High-entropy alloys (HEAs) have been investigated considerably in the last decade. The phase selection in HEAs has attracted much attention recently, especially on forming of the solid solutions. Up to now, phase diagrams of most HEAs are still not well developed, and the empirical phase selection rules play an important role in HEAs area. In this brief review, the physical factors controlling the phase stability in HEAs are discussed, and the phase selection rules are identified. Different from previous results, the rules on equilibrium phase selection within a certain temperature range are carefully reviewed and presented in this article.

- by Sheng Guo
- •
- High Entropy Alloys, Phase Stability
- by Erwin Huang and +2
- •
- High Entropy Alloys, High Entropy Alloy

Evolution of microstructure and texture after heavy cold rolling and subsequent annealing in a wide temperature range was first studied in an FCC equiatomic CoCrFeMnNi high-entropy alloy (HEA). Development of a submicron-cell structure and a strong brass-type texture was observed after 90% cold rolling. An ultra-fine microstructure having average recrystallized grain size $1 lm with profuse annealing twins was observed after annealing at 650 °C. Remarkable resistance against grain coarsening was observed at least up to 800 °C. The mechanisms for these features were closely related with the distinct whole-solute matrix in HEAs. The recrystallization texture was characterized by the retention of deformation texture components similar to those of TWIP and 316 stainless steels. But notable differences exist. The S ({1 2 3}h6 3 4i) component is stronger than brass ({1 1 0}h1 1 2i) and Goss ({1 1 0}h0 0 1i), and strengthened with increasing annealing temperatures. Strong a-fiber (h1 1 0i//ND) components other than the deformation components B S and G, and higher fraction of random components also develop. It could be attributed to profuse annealing twin formation due to the low stacking fault energy of the alloy.

- by Jagga R Gatti and +1
- •
- Microstructure, Thermomechanical Processing, Crystallographic Texture, High Entropy Alloys

In this work, we study the inﬂuence of hydrogen on the deformation behavior and microstructure evolution in an equiatomic CoCrNi medium entropy alloy (MEA) with an ultimate tensile strength of ∼1 GPa. Upon deforma- tion, hydrogen-charged... more

- by Dierk Raabe
- •
- Hydrogen, Corrosion Science, Corrosion, Crack

The microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure was performed. The phase state and chemical homogeneity of the solid solution were analysed with respect to crystal structure, phase stability, and oxide formation. It was found that Y-rich precipitates form at grain boundaries and that the alloy is prone to oxidation, leading to a homogeneous distribution of ~10 nm-sized oxides in the grain interiors. The plastic response at the sub-grain level was studied in terms of the activated slip systems, critical resolved shear stresses (CRSS), and strain hardening using micropillar compression tests. We observe plastic slip on the basal system, with a CRSS of 196 ± 14.7 MPa. Particle strengthening and strength dependence on sample size are discussed on the basis of dislocation particle interaction and mechanical size effects.

Metals have been mankind's most essential materials for thousands of years; however, their use is affected by ecological and economical concerns. Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength– ductility trade-off 1,2. Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization 3–6. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection 7–11 , the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase 12); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase 13). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels 14,15 and massive solid-solution strengthening of high-entropy alloys 3. In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials 16,17. This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys. To realize the TRIP-DP-HEA concept, we switch from the equi-atomic Fe 20 Mn 20 Ni 20 Co 20 Cr 20 (atomic per cent, at%) 6 system to the non-equiatomic Fe 80 − x Mn x Co 10 Cr 10 (at%) system, which exhibits partial martensitic transformation of the face-centred cubic (f.c.c.) to the hexagonal close-packed (h.c.p.) phase upon cooling from the high-temperature single-phase region. This change enables development of a dual-phase microstructure in which both phases obtain the maximum benefit of the solid-solution strengthening effect and one phase, owing to the decreased stacking fault energy 18 , undergoes deformation-induced displacive transformation. The partial martensitic transformation during quenching is the only possible approach that can lead to the formation of a DP-HEA with phases of identical chemical composition (that is, high-entropy phases). The alloys were synthesized with varying Mn contents in a vacuum induction furnace using pure metals, hot-rolled to 50% thickness at 900 °C, homogenized at 1,200 °C for 2 h in an Ar atmosphere, and water-quenched. Further grain refinement was achieved by cold-rolling (to 60% thickness) and 3-min annealing at 900 °C in an Ar atmosphere. The chemical composition of the HEAs measured by wet-chemical analysis is given in Extended Data Table 1. Microstructure characterization down to 30-nm resolution reveals that the Fe 80 − x Mn x Co 10 Cr 10 (at%) system indeed demonstrates the targeted change in phase stability (see the X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) data in Fig. 1). A single f.c.c. phase structure was obtained when the Mn content was 45 at% and 40 at% (Fe 35 Mn 45 Co 10 Cr 10 and Fe 40 Mn 40 Co 10 Cr 10 , respectively). Figure 1 | XRD patterns and EBSD phase maps of Fe 80 − x Mn x Co 10 Cr 10 (x = 45 at%, 40 at%, 35 at% and 30 at%) HEAs. θ is the Bragg angle. The Mn content plays an important part in phase constitution, tuning phase stability for the activation of specific displacing transformation mechanisms, for example, enabling TWIP or TRIP effects. We note that the 35 at% Mn alloy has only trace amounts of the h.c.p. phase, and hence is not referred to as a DP-HEA.

High-entropy alloys (HEAs) consisting of multiple principle elements provide an avenue for realizing exceptional mechanical, physical and chemical properties. We report a novel strategy for designing a new class of HEAs incorporating the additional interstitial element carbon. This results in joint activation of twinning-and transformation-induced plasticity (TWIP and TRIP) by tuning the matrix phase's instability in a metastable TRIP-assisted dual-phase HEA. Besides TWIP and TRIP, such alloys benefit from massive substitutional and interstitial solid solution strengthening as well as from the composite effect associated with its dual-phase structure. Nanosize particle formation and grain size reduction are also utilized. The new interstitial TWIP-TRIP-HEA thus unifies all metallic strengthening mechanisms in one material, leading to twice the tensile strength compared to a single-phase HEA with similar composition, yet, at identical ductility. Exploring strong and yet ductile materials is paramount for reducing the weight and hence the energy consumption in all fields where mobile structures are used 1,2. However, strength and ductility of current engineering materials are generally conflicting 3 , limiting traditional alloy design strategies. Over the past years, high-entropy alloys (HEAs) have drawn great attention as it opens an entirely new realm of compositional opportunities for designing novel materials with exceptional properties 4–11. HEAs were originally proposed to contain multiple principal elements in near-equimolar ratios to stabilize single-phase solid solutions through maximizing con-figurational entropy 4,5. Recently, motivated by the fact that maximized configurational entropy is not the sole factor determining phase stability of HEAs 12–16 , a novel metastable transformation-induced plasticity dual-phase (TRIP-DP) HEA with exceptional strength and ductility has been developed 6,17. Based on this approach, we propose a new class of HEAs which is interstitially alloyed and unifies all known metallic strengthening mechanisms in one material. We use carbon as interstitial element in line with two main trends which can be deduced from previous studies on advanced steels: (i) First, the addition of interstitial carbon into a recently developed TRIP-DP-HEA 6 leads to an increase in stacking fault energy and hence phase stability 18. Tuning the stability of the face-centered cubic (f.c.c.) matrix phase in the dual-phase structure to a critical point triggers the twinning-induced plasticity (TWIP) effect while maintaining the TRIP effect, thereby further improving the alloy's strain-hardening ability 19,20. (ii) Second, HEAs benefit profoundly from interstitial solid solution strengthening instead of only the established massive solid solution strengthening provided by its multiple principle elements 4,5. This is due to the circumstance that carbon, nitrogen and other interstitials lead to much higher lattice distortions than substitutional elements which strongly affects their interaction with dislocations 21,22. We produced the interstitial HEA (iHEA) by melting and casting in a vacuum induction furnace using pure metals and carbon with nominal composition Fe 49.5 Mn 30 Co 10 Cr 10 C 0.5 (at%). The cast alloy was hot-rolled, homogenized and water-quenched. Further grain refinement was achieved by cold-rolling and annealing in Ar

The effect of thermo-mechanical processing on the evolution of microstructure and mechanical properties was investigated in an AlCoCrFeNi 2.1 high entropy alloy. For this purpose, the alloy was cold-rolled to 90% reduction in thickness and annealed at temperatures ranging from 800 °C to 1200 °C. The as-cast alloy revealed eutectic lamellar mixture of (Ni, Al) rich but Cr depleted B2 phase and Al-depleted L1 2 phases, having volume fractions of $ 35% and 65%, respectively. Nanosized precipitates enriched in Cr and having disordered BCC structure were found dispersed inside the B2 phase. Cold-rolling resulted in progressive disordering of the L1 2 phase but the B2 phase maintained the ordered structure. The disordering of the L1 2 phase was accompanied by the evolution of ultrafine lamellar structure and profuse shear band formation. Annealing of the 90% cold-rolled material at 800 °C resulted in the formation of a duplex microstructure composed of two different phases with equiaxed morphologies, having significant resistance to grain growth up to 1200 °C. The annealed materials showed disordered FCC and precipitate-free B2 phases. This indicated that quenching of the annealed specimens to room temperature was sufficient to prevent the ordering of the L1 2 phase and the formation of the Cr-rich nano-precipitates which were dissolved in the B2 phase during annealing. Significant improvement in tensile properties compared to the as-cast alloy could be achieved by thermo-mechanical processing. All the specimens annealed at 800 °C to 1200 °C were having good tensile ductility over 10% as well as high tensile strength greater than 1000 MPa. These indicated that the properties of the EHEA could be successfully tailored using thermo-mechanical processing for a wide range of engineering applications.

- by Sheng Guo
- •
- Thermomechanical Processing, High Entropy Alloys, Eutectic Alloys, High Entropy Alloy

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.

A B S T R A C T Refractory high-entropy alloys (RHEAs) are promising candidates for new-generation high temperature materials , but they generally suffer from room temperature brittleness and unsatisfactory high-temperature oxidation resistance. There currently lack efforts to address to these two critical issues for RHEAs at the same time. In this work, the high temperature oxidation resistance of a previously identified ductile Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr RHEA is studied. An accelerated oxidation or more specifically, pesting, in the temperature range of 600–1000 °C is observed for the target RHEA, where the oxidation leads the material to catastrophically disintegrate into powders. The pesting mechanism is studied here, and is attributed to the failure in forming protective oxide scales accompanied by the accelerated internal oxidation. The simultaneous removal of zirconium and hafnium can eliminate the pesting phenomenon in the alloy. It is believed that pesting can also occur to other equiatomic and non-equiatomic quinary Hf-Nb-Ta-Ti-Zr or quaternary Hf-Nb-Ti-Zr and Hf-Ta-Ti-Zr RHEAs, where all currently available ductile RHEAs are identified. Therefore, the results from this work will provide crucial perspectives to the further development of RHEAs as novel high-temperature materials, with balanced room-temperature ductility and high-temperature oxidation resistance.

The recently developed dual-phase high-entropy alloys are characterized by pronounced strain hardening and high ductility under monotonic loading owing to the associated transformation induced plasticity effect. Fatigue properties of high-entropy alloys have not been studied in depth so far. The current study focuses on the low-cycle fatigue regime. Cyclic tests were conducted and the microstructure evolution was studied post-mortem. Despite deformation-induced martensitic transformation during cycling at given plastic strain amplitudes, intense strain hardening in the cyclic stress-strain response is not observed. This behavior is attributed to the planar nature of slip and partial reversibility of deformation.

- by Dierk Raabe
- •
- Fatigue, Crystal Plasticity, Mechanical Properties of Materials, High Entropy Alloys

Phase stability is an important topic for high entropy alloys (HEAs), but the understanding to it is very limited. The capability to predict phase stability from fundamental properties of constituent elements would benefit the alloy design greatly. The relationship between phase stability and physicochemical/thermodynamic properties of alloying components in HEAs was studied systematically. The mixing enthalpy is found to be the key factor controlling the formation of solid solutions or compounds. The stability of fcc and bcc solid solutions is well delineated by the valance electron concentration (VEC). The revealing of the effect of the VEC on the phase stability is vitally important for alloy design and for controlling the mechanical behavior of HEAs.

Strategies involving metastable phases have been the basis of the design of numerous alloys, yet research on metastable high-entropy alloys is still in its infancy. In dual-phase high-entropy alloys, the combination of local chemical environments and loading-induced crystal structure changes suggests a relationship between deformation mechanisms and chemical atomic distribution, which we examine in here in a Cantor-like Cr 20 Mn 6 Fe 34 Co 34 Ni 6 alloy, comprising both face-centered cubic (fcc) and hexagonal closed packed (hcp) phases. We observe that partial dislocation activities result in stable three-dimensional stacking-fault networks. Additionally, the fraction of the stronger hcp phase progressively increases during plastic deformation by forming at the stacking-fault network boundaries in the fcc phase, serving as the major source of strain hardening. In this context, variations in local chemical composition promote a high density of Lomer-Cottrell locks, which facilitate the construction of the stacking-fault networks to provide nucleation sites for the hcp phase transformation.