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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]

Abstract We designed a novel nanoparticles-strengthened high-entropy alloy, Ni 30 Co 30 Fe 13 Cr 15 Al 6 Ti 6 , which exhibits excellent strength-ductility combinations at both ambient and cryogenic temperatures. Especially at 77 K, an exceptional strength-ductility synergy can be observed, showing an ultrahigh tensile strength of 1.7 GPa and a large ductility of 51%, accompanied by a distinctive three-stage strain-hardening response. The precipitation-hardening behaviors and deformation micro-mechanisms were carefully investigated by the atom probe tomography and transmission electron microscope. The dynamic formation of nano-spaced stacking faults contributed to improved strain-hardening capacities, resulting in simultaneous enhancements of plastic deformation stability and tensile ductility at such high-strength levels. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract A series of novel Co-free high-entropy alloys (HEAs) were developed by engineering nanoprecipitation in Fe-Ni-Cr-Mn-Al-Ti system, which exhibit a good combination of low cost, high strength and good ductility. The effects of Ti/Al ratio (from 0.6 to 1.7) on phase structures and mechanical properties were carefully investigated. Specifically, the alloy with a high Ti/Al ratio of 1.7 gives a high tensile strength of 1.25 GPa with a good ductility over 20 %, obviously outperforming other Co-free HEAs. The strain-hardening behaviors and deformation micro-mechanisms were analyzed by transmission electron microscope, which reveals the structural origins of their superior mechanical properties. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

High-strength ordered intermetallic alloys are always sought-after in sophisticated structural applications. However, their practical applications have long been hindered by the severe grain-boundary embrittlement. Here, we designed a Co-rich chemically complex intermetallic alloy (CCIMA) with the L1 2 -type ordered structure, which exhibits an ultrahigh tensile strength of ∼1,611 MPa and a substantial ductility of ∼37%. These exceptional strength-ductility combination outperforms the majority of common Co-rich intermetallic alloys. The multi-addition of Ta, Ti, and V elements is largely responsible for the significantly increased antiphase boundary energy of the L1 2 superlattice, which accounts for the high strength. This high degree of ductility is attributed to both the increased fracture resistance of grain boundaries brought about by co-segregation-induced order-to-disorder phase transformation, as well as the high work-hardening capability. [Display omitted] [ABSTRACT FROM AUTHOR]

This work investigated the effects of L1 2 nanoprecipitates on the electrochemical corrosion behavior and passive film characteristics of CoCrNiAlTi-type high-entropy alloys (HEAs) in chloride-containing solutions. Compared with the single-phase recrystallized alloys, the precipitation of the L1 2 phase destabilized surface passive films, thus deteriorating the localized corrosion resistance and increasing the metastable pitting activities of the L1 2 -strengthened HEA. Influenced by the L1 2 -induced chemical inhomogeneities, the increase of Cr(OH) 3 content in passive films together with the high galvanic corrosion tendency between the two phases was considered as the main reason for the increased susceptibility of the L1 2 -strengthened alloys to localized corrosion attacks. [Display omitted] • A heterogeneous precipitation behavior characterizing the spherical L1 2 nanoparticles and lamellar L1 2 nanorods was observed. • L1 2 -induced chemical inhomogeneities increased the localized corrosion susceptibility of the aged alloy. • The nanoscale passive film with an amorphous structure was directly visualized by STEM. • The increased Cr(OH) 3 content combined with the high galvanic corrosion weakened the aged alloy passive film stability. [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]

Corrosion-resistant alloys with high strength and large ductility are desirable for applications in harsh environments where high structural load-carrying capacity is required. In this work, we designed an L1 2 -strengthened high-entropy alloy (HEA) of composition (NiCo) 77 Cr 13 Al 5 Nb 5 (at.%) that combines excellent mechanical properties and good corrosion resistance. The formation of a thin passive film of non-stoichiometric compounds with an amorphous structure, characterized by an enrichment of Cr 2 O 3 and Nb 2 O 5 , contributes to the superior localized corrosion resistance. The precipitation strengthening brought about by coherent L1 2 particles and the multi-stage plastic deformation dominated by stacking faults (SFs) provides ultrahigh strength and appreciable ductility, respectively. [Display omitted] • A novel corrosion-resistant L1 2 -strengthened HEA with high strength and ductility has been designed. • Ni, Al, and Nb are assigned to the L1 2 particles, while Co and Cr are left in the FCC matrix. • The oxide film enriched in Cr 2 O 3 and Nb 2 O 5 endows the HEA with high resistance to localized corrosion. • Ordering strengthening by coherent L1 2 precipitates and deformation-induced stacking faults provide an exceptional strength-ductility synergy. [ABSTRACT FROM AUTHOR]

Metallic materials are mostly susceptible to hydrogen embrittlement (HE), which severely deteriorates their mechanical properties and causes catastrophic failures with poor ductility. In this study, we found that such a long-standing HE problem can be effectively eliminated in the Fe x (CrCoNi) 1-x face-centered-cubic (fcc) high-entropy alloys (HEAs) by triggering the localized segregation of Cr at grain boundaries (GBs). It was revealed that increasing the Fe concentration from 2.5 to 25 at. % leads to substantially improved HE resistance, i.e., the ductility loss decreases from 70% to 6%. Meanwhile, the fracture mode transformed from the intergranular to the transgranular mode. Multiscale microstructural analyses demonstrated that the Fe 2.5 Cr 32.5 Co 32.5 Ni 32.5 and Fe 25 Cr 25 Co 25 Ni 25 alloys show negligible differences in the phase structure, grain size, and grain-boundary (GB) character. However, interestingly, the near atomic-resolution elemental mapping revealed that an increased Fe concentration promotes the nanoscale Cr segregation at the GBs, which is primarily motivated by the strong repulsive force between Cr and Fe and the low self-binding energy of Cr. Such unusual interfacial segregation of Cr, which has not been reported before in the Fe 25 Cr 25 Co 25 Ni 25 alloy, helps enhance the GBs' cohesive strength and suppresses the local hydrogen segregation at GBs due to the deceased GB energy, leading to the outstanding HE resistance. These findings decipher the origins of the vastly-improved HE resistance in current FeCrCoNi-type HEAs, and meanwhile, provide new insight into the future development of novel high-performance structural alloys with extraordinary immunity to hydrogen-induced damages. [Display omitted] [ABSTRACT FROM AUTHOR]

Precipitation-strengthened high-entropy alloys (HEAs) with tailored phase structures are expected to possess excellent strength-ductility combinations for advanced structural applications. In this work, we systematically studied the microstructural evolutions and mechanical behaviors of (Ni 2 Co 2 FeCr) 92 Al 4 Nb 4 HEAs at different aging temperatures (700–900 ℃). It was revealed that only spherical L1 2 phase existed in the specimen peak-aged at 700 °C without other precipitates, while in the specimen peak-aged at 800 °C, irregular precipitation of ε phase appeared on the grain boundaries in addition to the intragranular L1 2 phase. In strong contrast, upon peak aging at 900 ℃. the fine interleaving Widmanstätten-type ε phase with an ordered hexagonal structure (D0 19) became the dominated precipitate. We further revealed that the coherent L1 2 phase provided a more effective strengthening effect than that of the ε phase, leading to the highest yield strength of ~950 MPa in the alloy peak-aged at 700 °C. The stacking faults shearing of L1 2 phase and the planar dislocations shearing of ε phase were identified as the main deformation mechanisms of the specimens peak-aged at 700 and 900 °C, respectively. Interestingly, the ε phase was determined to have certain plastic deformability, enabling the 900 °C peak-aged alloy to exhibit excellent tensile ductility. These findings would provide valuable guidelines for the design of precipitation-strengthened HEAs with optimized microstructures and superior mechanical properties. • Temperature-dependent microstructural transformation of precipitation strengthened high-entropy alloys was established. • The L1 2 phase dominated the precipitation behaviors at low temperature, while the ε phase dominated at higher temperatures. • The L1 2 phase sheared by the stacking faults was the main deformation mechanism of the specimen peak-aged at 700 ℃. • The precipitation strengthening efficiency provided by the ε phase was lower than that of the L1 2 phase. • The ε phase in this HEA was shearable and deformable in contrast to many other brittle phases. [ABSTRACT FROM AUTHOR]