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[Display omitted] Intermetallic materials are bestowed by diverse ordered superlattice structures together with many unusual properties. In particular, the advent of chemically complex intermetallic alloys (CCIMAs) has received considerable attention in recent years and offers a new paradigm to develop novel metallic materials for advanced structural applications. These newly emerged CCIMAs exhibit synergistic modulations of structural and chemical features, such as self-assembled long-range close-packed ordering, complex sublattice occupancy, and interfacial disordered nanoscale layer, potentially allowing for superb physical and mechanical properties that are unmatched in conventional metallic materials. In this paper, we critically review the historical developments and recent advances in ordered intermetallic materials from the simple binary to chemically complex alloy systems. We are focused on the unique multicomponent superlattice microstructures, nanoscale grain-boundary segregation, and disordering, as well as the various extraordinary mechanical and functional properties of these newly developed CCIMAs. Finally, perspectives on the future research orientation, challenges, and opportunities of this new frontier are provided. [ABSTRACT FROM AUTHOR]

Dual-phase high-entropy alloys strengthened by multicomponent intermetallic nanoparticles represent a unique class of hetero-structured materials with potentially superior mechanical properties for structural applications. Due to the multicomponent chemistries of both nanoparticles and the matrix, as well as the coherent interfaces between them, these alloys can possess excellent strength-ductility combinations over a wide temperature range. This brief viewpoint article gives an overview of the major developments achieved recently. Advantages over other classes of metallic materials as well as limitations in this field are also carefully discussed. The results provide new insights into the development of high-performance structural materials via multicomponent-nanoparticle engineering. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Thermally stable high-entropy alloys (HEAs) consisting of a high density of coherent precipitates show a great potential for high-temperature applications. In this work, we systematically investigated the phase stability and coarsening kinetics of L1 2 -type coherent precipitates in a Ni-30Co-13Fe-15Cr-6Al-6Ti-0.1B (at.%) HEA isothermally aged at 800, 900 and 1000 °C. Aged microstructures in the grain interiors under this temperature range were essentially dominated by the uniform precipitation of multicomponent L1 2 (Ni, Co, Fe, Cr) 3 (Ti, Al)-type precipitates. The coarsening kinetics of these intragranular L1 2 precipitates were quantitatively determined, which were adequately characterized by the classical Lifshitz-Slyozov-Wagner model. The activation energy for coarsening was determined to be 378 kJ/mol, which is relatively higher than that of conventional Ni or Co-based superalloys, suggesting a slow elemental diffusion in the HEA matrix. More importantly, the heterogeneous precipitation and the associated metastable phase transformation mechanism along grain boundaries (GBs) were carefully analyzed. Localized chemical heterogeneity was identified within the discontinuous L1 2 phase at the GBs, which thermodynamically destabilizes the L1 2 structure and encourages the formation of brittle Heusler phase. Finally, we establish a unique duplex-aging strategy that can be efficiently utilized for GB stabilization, by which these detrimental intergranular heterostructures can be greatly eliminated, leading to an exceptional resistance to intermediate-temperature embrittlement, along with enhanced tensile strengths. These findings will not only shed light on the precipitation mechanisms in compositionally complex HEAs but also generate new opportunities to the interfacial design of HEAs for advanced high-temperature applications with superior properties. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We developed a novel high-performance L1 2 -strengthened high-entropy alloy (HEA) in the multicomponent Ni-Co-Fe-Cr-Al-Nb system. The phase transformation, mechanical properties and associated deformation behaviors were systematically investigated through combinational analyses involving the three-dimensional atom probe tomography (3D-APT), transmission electron microscopy (TEM) and first-principles calculations. In contrast to conventional alloys that generally strengthened by Ni 3 (Al, Ti)-type precipitates, a high density of coherent L1 2 nanoprecipitates with a new chemical constitution of (Ni, Co, Fe, Cr) 3 (Al, Nb) can be controllably introduced via elaboratively tuning the content of Al and Nb, resulting in a large lattice misfit of ~0.78% that rarely achieved in previous HEAs. The newly developed (Ni 2 Co 2 FeCr) 92 Al 4 Nb 4 HEA enables excellent tensile properties at a large temperature window from room temperature to 870 °C. More remarkably, an anomalous growth in yield strength can be observed at the temperatures above 600 °C, showing a peak yield stress over 720 MPa when deformed at 760 °C, which surpasses most of the previous L1 2 -strengthened HEAs, as well as the commercial superalloys. Detailed TEM analyses revealed that the multicomponent L1 2 precipitates are mainly sheared by the super-partial dislocations, forming superlattice intrinsic stacking fault (SISF) loops coupled with antiphase boundaries (APBs). Such an interesting deformation substructure enables sustained work hardening and produces high tensile strengths at the high temperatures. The underlying mechanisms of those SISF loops were carefully discussed, which could be possibly ascribed to the local elemental segregation on the planner faults. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• The corrosion of FeCoNiCrMo high-entropy alloy was studied in CO 2 /CO gas mixture at 700 and 950 °C. • This high-entropy alloy experienced an exclusive oxidation in various CO 2 /CO mixed gases. • The oxidation rates steadily increased with increasing oxygen pressure and temperature. • TEM analyses confirmed that (Mn,Fe) 3 O 4 and MnO formed in the outer layer while (Mn,Cr) 3 O 4 and MnO developed in the inner layer at 950 °C. The corrosion behavior of an equimolar FeCoNiCrMn high-entropy alloy was studied in various combination of CO 2 /COmixed gases at 700 and 950 °C, with the oxygen partial pressure range from 5.0 × 10−16 to 7.9 × 10−8 Pa. The alloy exclusively experienced oxidation, and its kinetics followed the parabolic rate law at both temperatures with the oxidation rates steadily increasing with oxygen pressure. This result demonstrated that a typical oxide scale exhibited a p-type semiconductivity. The scale formed on the alloy at 700 °C was thin, consisting of a mixture of MnO, (Mn,Fe) 3 O 4 , and (Mn,Cr) 3 O 4 , while the duplex scales formed at 950 °C consisted of an outer layer of (Mn,Fe) 3 O 4 and, MnO, and an inner layer of (Mn,Cr) 3 O 4 andMnO. [ABSTRACT FROM AUTHOR]

Abstract Eutectic high entropy alloys (EHEAs) have shown promising mechanical properties and good castability. However, designing EHEAs is still a challenge because of highly complex chemical compositions and limited multiple phase diagrams. Here, we propose a grouping strategy to design EHEAs efficiently based on the distributions of mixing enthalpies of atom-pairs in multi-principal-component alloys. Following this idea, a new CoCrFeNiTa 0.43 EHEA is designed successfully. Moreover, those reported EHEAs are reviewed and show high consistence with the strategy proposed here. Highlights • A new strategy for the design of eutectic high entropy alloys is proposed. • The EHEAs can be designed by mixing a solid solution group and an intermetallic group. • The CoCrFeNiTa 0.43 HEA with eutectic structure is successfully designed following this group strategy. [ABSTRACT FROM AUTHOR]

Abstract The face centered cubic (FCC) high entropy alloys (HEAs) showed highly attractive properties because of their simple crystal structures and low stacking fault energies. However, deep understandings of micro-alloying in FCC HEAs, including solid solubility, precipitates formation and stacking fault energy variation, remain unsatisfactory. In present study, we systematically investigated the solubility of Ti, Al, and Mo in the CoCrFeNi HEA and evaluated the effect of every single element on the precipitation behavior. The results showed that at 750 °C the CoCrFeNi matrix can separately dissolve up to 0.6 at.% Ti, 4 at.% Al, and 3 at.% Mo. The roles of both additional and base elements in the precipitation behaviors were identified. Specifically, Co can stabilize the γ′ phase in the Ti-doped HEAs, while Al leads to the formation of B2 phase in the CoCrFeNiAl x system. The annealing stacking faults and twins in the matrix changed with the addition of different elements, which was mainly due to stacking fault energy variations. These insights will greatly benefit the design of precipitate-hardening HEAs and provide a deeper understanding of metallurgical behaviors of HEAs. Highlights • The CoCrFeNi HEA dissolve 0.6 at.%, 4 at.% Al and 3 at.% Mo at 750 °C, respectively. • In the CoCrFeNi system, Co stabilizes the γ′ phase through decreasing the VEC. • The Al leads to the formation of B 2 phase at 750 °C, rather than the γ′ phase. • The Al and Ti decrease the stacking fault energy of CoCrFeNi while Mo increases it. [ABSTRACT FROM AUTHOR]

The oxidation of three quinary FeCoNiCrSi x high-entropy alloys (x = 0.25, 0.5 and 1.0) was investigated at 700–900 °C in dry air and at 900 °C in various O 2 -containing atmospheres (FeCoNiCrSi 0.5 only). The two lower Si-content alloys contained an FCC single-phase structure, while the equimolar alloy had a BCC plus FCC dual-phase structure. The oxidation kinetics of all the alloys followed the parabolic rate law, with their oxidation rate constants increased with increasing temperature and decreasing Si content. In addition, the oxidation rate constants of the FeCoNiCrSi 0.5 alloy steadily increased with increasing oxygen partial pressure, indicative of a typical oxide scale exhibiting a p-type semiconductivity. The scales formed on all the alloys consisted mostly of Cr 2 O 3 and minor amounts of SiO 2 (α-quartz). The oxidation mechanism of the HEAs is due primarily to outward diffusion of cations. [ABSTRACT FROM AUTHOR]

The oxidation of three quinary equimolar high-entropy alloys, containing FeCoNiCrAl, FeCoNiCrMn, and FeCoNiCrSi was studied in dry air at 700–900 °C. The Mn-containing alloy was single-phase, while both Al- and Si-containing alloys were dual-phase. The oxidation kinetics of all the alloys followed the parabolic rate law, regardless of temperature and alloy composition. The oxidation rates of both Al- and Si-containing alloys were lower than those of the quaternary FeCoNiCr alloy, while those of the Mn-containing alloy went to an opposite direction. Multiple scales formed on the alloys were strongly composition-dependent. [ABSTRACT FROM AUTHOR]

Developing microstructurally stable alloys with the outstanding strength-ductility combination at both room temperature and elevated temperatures is highly desirable for advanced structural applications, which, however, still remains challenging despite years of efforts. In this study, leveraging on the computation-aided thermodynamic modeling, novel high-strength Ni 39.9 Co 20 Fe 30-x Cr x Al 6 Ti 4 B 0.1 (x = 0, 10, 15, and 20 at.%) high-entropy alloys (HEAs) were rationally designed, which maintained a stable coherent precipitation microstructure when continually substitute Fe with Cr. The structural features, mechanical properties, and underlying deformation micro-mechanisms were systematically investigated. We found that Cr mostly partitions into the face-centered-cubic (FCC) matrix relative to the L1 2 -type nanoparticles. The increased Cr concentration substantially decreases the stacking fault energy (SFE) of the matrix and results in the stacking-fault-prevailed deformation behaviors, thereby producing simultaneously enhanced strength and ductility at room temperature. By contrast, when tested at 600 °C, the Cr-free HEA suffers from an extremely brittle fracture along grain boundaries caused by environmental oxidation damages. Interestingly, with Cr doping up to 20 at.%, such a serve intergranular embrittlement can be completely eliminated due to the increased oxidation resistance, leading to a distinct brittle-to-ductile transition (from 3.4 to 25.2%) at a high strength level of ∼ 1136 MPa. These findings will give new insights into the controllable design of novel high-performance HEAs for achieving target microstructures and superior property combinations. [Display omitted] [ABSTRACT FROM AUTHOR]

Precipitates are known to be effective obstacles for dislocation gliding, which can substantially increase the strength of metallic materials. In particular, dense nanoscale precipitates under a peak-aged condition can achieve a favorable precipitate size for pronounced strengthening, which, however, often causes progressive stress localization and premature failure of alloys with reduced ductility. Here, we showcase an "unusual ductilization effect from the peak-aged state" in a high-entropy alloy (HEA) system with a medium-to-high stacking fault energy (SFE). Compared with the under-aged HEA, the peak-aged HEA with relatively large precipitates exhibits a pronounced increment in both the strength and the tensile ductility, distinguishing from the conventional wisdom. Such an anomalous precipitate-size-dependence of ductility is achieved by trigging a unique multi-staged strain-hardening mechanism from the activation of microbands, which is enabled through a delicate modulation of precipitate size, volume fraction, and SFE of the precipitate-hardened HEA. Specifically, the dynamically active microbands upon loading can simultaneously serve as dislocation obstacles for strengthening and also the origins of stress delocalization, leading to a sustainable strain hardening for balanced mechanical properties. These discoveries shed new insights into the innovative design of ultrastrong and ductile metallic materials for advanced structural applications. [Display omitted] [ABSTRACT FROM AUTHOR]

The oxidation behaviour of four NiCoCrAl x medium-entropy alloys (x = 0, 0.1, 0.3, or 0.5 mol.) was investigated in dry air at 600–900 °C. The oxidation kinetics of all the alloys followed the parabolic rate law at T > 700 °C, and the oxidation rates increased with an increase in temperature and a decrease in Al content. Discontinuous oxide granules were formed on all the alloys at 600 °C, while complex scales were detected, strongly depending on the Al content and temperature. Internal attack of AlN precipitates was also observed for Al-containing alloys at T > 700 °C. ● Air-oxidation of NiCoCrAl x medium-entropy alloys was investigated at 600–900 °C. ● The oxidation kinetics of all the alloys followed the parabolic-rate law. ● The oxidation rates increased with increasing temperature and decreasing Al content. ● Complex scales formed on the alloys strongly depended on temperature and Al content. [ABSTRACT FROM AUTHOR]

• The oxidation of the NiCoCr meduim-entropy alloy was studied in various oxygen pressures at 800 °C. • The oxidation kinetics of the alloy obeyed the parabolic-rate law. • The oxidation mechanism of the alloy was due to outward cation diffusion. • The main diffusing species were doubly- and triply-ionized cation vacancies. The oxidation behavior of NiCoCr medium-entropy alloy was investigated at 800 °C in various oxygen pressures from 10 to 105 Pa. The results showed that the oxidation kinetics of NiCoCr followed the parabolic rate law, regardless of oxygen pressure. The oxidation rate constants of the alloy steadily increased with increasing oxygen pressure. The oxidation mechanism of the MEA was mainly controlled by outward cation diffusion to form an exclusive Cr 2 O 3 layer although minor amounts of Co- or Ni-Cr spinels were observed at PO 2 = 105 Pa under TEM analyses. The main diffusing species were doubly- and triply-ionized cation vacancies. [ABSTRACT FROM AUTHOR]

It is well established that chemically-disordered alloys exhibit good irradiation resistance due to their ability to self-healing. On the other hand, ordered phases are usually utilized as strengthening components in alloys. In this work, the effects of ordering degree and the order/disorder interface on primary damage production, including point defects, defect clusters, and dislocations, are investigated through atomistic simulations. Exemplified by the Ni–Al and Ni–Fe alloy systems, we first compare defect evolution in the ordered and disordered phases. Our results indicate that the degree of order has profound influences on defect evolution, especially defect clustering properties. Specifically, ordered Ni 3 Al and disordered NiFe can strongly suppress defect growth, ascribing to their particular melting temperature and defect diffusion properties. For mixture alloys containing order/disorder interfaces, our results suggest that the ordered phase gets disordered gradually under displacement damage. The disordering is facilitated by the disparity of defect diffusivities between the ordered and disordered phases. These results shed light on the atomistic mechanism underlying the irradiation response of ordered and disordered alloys as well as their mixture systems, which advances the understanding of the defect process in superalloys and recent ordered-precipitate-strengthened high-entropy alloys. • Primary damage production in ordered and disordered Ni-Al and Ni-Fe alloys is compared by molecular dynamics. • Ordered Ni 3 Al and disordered NiFe strongly suppress defect growth. • Defect evolution in the ordered/disordered mixture system is governed by the disparity of defect diffusivities. [ABSTRACT FROM AUTHOR]

High-entropy alloys (HEAs) strengthened by coherent nanoparticles show great potentials for elevated-temperature structural applications, which however, generally suffer from a severe intergranular embrittlement when tested at intermediate temperatures. In this study, we demonstrated a novel "heterogenous columnar-grained" (HCG) approach that can effectively overcome this thorny problem. Different from the equiaxed counterpart which shows extreme brittleness along grain boundaries at 800 °C, the newly developed HCG-HEA exhibits an exceptionally high resistance to intergranular fractures originating from the unique grain-boundary characters and distributions. The presence of heterogenous columnar grain structure drastically suppresses the crack nucleation and propagation along with boundaries, resulting in an unusually large tensile ductility of ~18.4 % combined with a high yield strength of ~652 MPa at 800 °C. This finding provides a new insight into the innovative design of high-temperature materials with extraordinary mechanical properties. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The oxidation behavior of three Ni 2 FeCoCrAl x high-entropy alloys (where x = 0, 0.5, and 1.0) was studied at 600–900 °C in dry air. The oxidation kinetics of all the alloys obeyed the parabolic rate law at T ≥ 700 °C, with their oxidation rates steadily increasing with either increasing temperature or decreasing Al content. Discontinuous oxide particles grew on the alloys at 600 °C, while multiple layered scales and an exclusive Al 2 O 3 layer formed on the Al-free and the Al-containing alloys, respectively at T ≥ 700 °C. In addition, an internal nitrogen attack to form AlN was also observed for the Al-containing alloys at 900 °C. • Air-oxidation behavior of three Ni 2 FeCoCrAl x high-entropy alloys was studied at 600–900 oC. • All the alloys oxidized parabolically at T ≥ 700 °C. • Discontinuous oxides formed on the alloys at 600 °C, while layered scales formed at T ≥ 700 °C depended on Al content. [ABSTRACT FROM AUTHOR]

Diffusionless martensitic transformation (MT) exerts one of the most significant influences on the mechanical properties of alloys. However, the application of martensitic transformation to improve mechanical performance was seldom involved in the manufacture of high-entropy alloys (HEAs) and medium-entropy (MEAs) alloys. In this work, an innovative non-equiatomic MEA, Fe 42 Co 42 Cr 16 , was proposed with incorporation of martensitic transformation during water quenching and plastic deformation. Water quenching for the alloy in the high-temperature single-phase region produced a partial MT; i.e., transformation of γ –FCC austenite phase into an ε -HCP martensite phase, responsible for the coexistence of γ and ε phases in the dual-phase (DP) alloy. Another triple-phase (TP) alloy, including γ –FCC austenite, ε -HCP martensite and B2-BCC precipitates, was obtained by quenching the alloy in γ + B 2 phase region. Owing to the low intrinsic stacking fault energy (γ I), both DP (γ I = 10.9 mJ/m2) and TP (γ I = 12.2 mJ/m2) alloys involved the complete polymorphic MT process during plastic deformation; i.e., the transformation of the γ –FCC austenite phase into the α -BCT martensite phase with an intermediate ε -HCP martensite phase. Due to the transformation-induced plasticity effect and precipitation strengthening, the produced TP alloy exhibited a yield strength above 1 GPa with a total elongation of as high as 25%. • Partial martensitic transformation, i.e., γ -FCC → ε -HCP, occurred in the Fe 42 Co 42 Cr 16 medium-entropy alloy. • Triple-phase alloy with γ -FCC austenite, ε -HCP martensite and B2-BCC precipitates, was obtained. • Complete martensitic transformation, i.e., γ -FCC → ε -HCP → α-BCT, were conformed in the Fe 42 Co 42 Cr 16. • The triple-phase medium-entropy alloy exhibited a yield strength above 1 GPa with a total elongation of as high as 25%. [ABSTRACT FROM AUTHOR]