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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]

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]

Developing strong-yet-ductile alloys with wide temperature capabilities is crucial for modern industrial and scientific applications; however, this is often difficult to realize using conventional wisdom. In this work, we innovatively designed a nanoparticle-strengthened high-entropy alloy (HEA) through elaborate compositional design and thermomechanical regulation. The designed HEA exhibits extraordinary mechanical properties across a wide temperature range (−196∼1000 °C), resulted from the unique microstructures consisting of high-density hierarchical L1 2 nanoparticles combined with serrated grain boundaries. A high ultimate tensile strength of 1355 MPa combined with a ductility of 35.0 % were achieved when tested at 25 °C. The tensile strength and ductility were enhanced simultaneously with the decrease of temperature from 25 to −196 °C. More prominently, the intermediate-temperature intergranular embrittlement, a severe ductility loss at 600∼800 °C for most conventional superalloys, is effectively suppressed even under high tensile stress, which is attributed to the high resistance to crack propagation of serrated grain boundaries. An apparent anomalous yielding behavior occurred at 700 °C, leading to an ultrahigh yield strength of ∼1045 MPa while maintaining a large ductility of ∼22.1 %. Meanwhile, an excellent strength of 325 MPa can still be maintained even at 1000 °C combined with a good ductility over 25 %. Systematic characterizations revealed that such a temperature-dependent mechanical behavior is highly related to various deformation substructures activated at different temperatures. This work would open a pathway to developing a series of advanced alloys with superior strength and ductility over a wide temperature range. [Display omitted] [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]

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]