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An equiatomic VCoNi medium-entropy alloy possesses high sensitivity to grain-boundary strengthening, achieved by severe lattice distortions. Its ultrafine-grain structure enables 1.5 Gigapascal yield strength even for the fully recrystallized alloy with a single face-centered cubic structure. The high density of grain boundaries also generates high back stresses via piling up of massive dislocations, and the low cross-slip probabilities produce not only robust dislocation-mediated plasticity but also high back stress contribution to flow stress, which affords high strain-hardening capability to ultrafine-grain alloys, with 1.7 Gigapascal ultimate tensile strength with remarkable ductility. Our approach provides a new method for developing ultrastrong metallic materials.

This article reports the hydrogenation properties of several Mg–Mg2Ni composite specimens with increasing Ni content: 4.4, 11.3 (eutectic), and 16.3 (in at%). Mg–Mg2Ni composites were prepared by means of induction melting, followed by simple mechanical chipping of the casts. The hydrogenation and dehydrogenation reaction kinetics were studied, and reaction mechanisms were described by means of solid-gas reaction modeling. Absorption and desorption properties were evidenced to be diffusion controlled, with hydrogen diffusion through hydrided/dehydrided phases being the rate limiting step in most cases (the migration of metal/hydride interface at a constant velocity being rate-limiting in only few of them). The kinetics study was supported by a thorough thermal analysis to provide in-depth insights of the decomposition reaction. Hence, thermogravimetry (TG) and pressurized differential scanning calorimeter (PDSC) were combined to investigate the dehydrogenation properties such as hydrogen gravimetric density, reaction onset temperature, enthalpy and activation energy as a function of Ni content. Structural analysis included X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM) to discuss the structural stability and microstructural evolution as a function of cycles, notably during the activation procedure. Finally, cyclic performance was evaluated for 100 cycles, using a custom-made large-scale reactor to demonstrate scale-up feasibility.

We investigated the effects of hydrogen and temperature on hydrogen embrittlement (HE) of cold-rolled equimolar CoCrFeMnNi high-entropy alloy (HEA). The HE exhibited intergranular fracture in this HEA at 298 and 177 K. At 177 K, more twins formed than at 298 K, and this acted as a hydrogen-diffusion path. During deformation, local stress was concentrated at the triple junction consisting of grain and twin boundaries. Hydrogen diffused predominantly along the boundary and encountered stress-concentration regions. Cracks initiated and propagated predominantly through the grain/twin boundaries by hydrogen diffusion at 298 and 177 K. Therefore, HE occurred at 298 and 177 K. At 77 K, hydrogen was distributed throughout the specimen as twin formation was more active. The cryogenic temperature of 77 K caused the hydrogen to become trapped and thus not diffuse into the stress-concentration region. Thus, there was no significant HE at 77 K.

In the present study, the self-healing behavior of Inconel 617B superalloy (617B), used as a high-temperature material, was investigated through both preliminary test consisting of room-temperature tension and annealing and creep rupture test. Micro-cracks in the solution-treated specimen, which had been fractured at room temperature, were filled in by the precipitation of M23C6 and the segregation of B, C, and S after annealing at 800 °C for 16 h. Due to this healing effect and dislocation recovery, a 33% pre-strained and annealed specimen exhibited the longer total elongation, compared to the specimen strained until fracture without annealing after 33% pre-strain. Finally, creep rupture tests revealed that the creep strength of 617B is higher than Inconel 617 superalloy (617) due to the self-healing effect by the high-temperature precipitation of M23C6 and the segregation of B, C, and S as well as grain boundary strengthening of B and C.

The high entropy alloy CrMnFeCoNi has been shown to have promising structural properties. For a new alloy to be used in a structural application it should be weldable. In the present study, friction stir welding (FSW) and laser welding (LW) techniques were used to butt weld thin plates of CrMnFeCoNi. The microstructure, chemical homogeneity and mechanical behavior of the welds were characterized and compared with the base metal. The tensile stress-strain behavior of the welded specimens were reasonable when compared with that of the base metal. FSW refined the grain size in the weld region by a factor of ∼14 when compared with the base metal. High-angle annular dark field transmission electron microscopy in combination with energy dispersive X-ray spectroscopy showed chemical inhomogeneity between dendritic and interdendritic regions in the fusion zone of LW. Large fluctuations in composition (up to 15 at%) did not change the crystal structure in the fusion zone. Hardness measurements were carried out in the weld cross section and discussed in view of the grain size, low angle grain boundaries and twin boundaries in FSW specimens and the dendritic microstructure in LW specimens.

Both self-healable conductors and stretchable conductors have been previously reported. However, it is still difficult to simultaneously achieve high stretchability, high conductivity, and self-healability. Here, we observed an intriguing phenomenon, termed “electrical self-boosting”, which enables reconstructing of electrically percolative pathways in an ultrastretchable and self-healable nanocomposite conductor (over 1700% strain). The autonomously reconstructed percolative pathways were directly verified by using microcomputed tomography and in situ scanning electron microscopy. The encapsulated nanocomposite conductor shows exceptional conductivity (average value: 2578 S cm–1; highest value: 3086 S cm–1) at 3500% tensile strain by virtue of efficient strain energy dissipation of the self-healing polymer and self-alignment and rearrangement of silver flakes surrounded by spontaneously formed silver nanoparticles and their self-assembly in the strained self-healing polymer matrix. In addition, the condu...

We report our findings of a detailed investigation into the frequent failures of ethylene furnace radiant tubes at a petrochemical plant. One of the failed samples having gas tungsten arc welded joint between Kanthal-APMT(Fe-based) and KHR45A(Ni-based) which had served for 2 years at 950 °C as an actual ethylene furnace tube was analyzed and turned out to have fracture occurred at the welding interface between the different metals. The detailed microstructural study revealed that the failure was induced by the massive formation of NiAl intermetallic phase and Cr23C6 carbide along the weld interface. To further discuss the failure mechanism, three additional specimens were made in as-welded and heat-treated (at 950 °C for 170 and 1000 h) conditions. Microstructural observation on the additional specimens revealed the enhanced diffusion of Al from APMT weld metal into KHR45A and, as a result, the early-stage precipitation of NiAl intermetallic phase in Al-enriched KHR45A side almost covering the weld interface was successfully detected, which plausibly explains the premature failures in the petrochemical plant.

Cr–Mn–N stainless steels have a cost and strength advantage over conventional Cr–Ni stainless steels. In this study microstructure and mechanical property of hydrogen-charged 18Cr10Mn-0.4N was comp...

Alloying elements are known to enhance the first hydrogenation of TiFe alloys, although the mechanism has not yet been fully understood. The effects of Zr on the bond character of a TiFe alloy was investigated using scanning Kelvin probe force microscopy. Adding Zr led to increased formation of the TiFe2 and Ti2Fe phases. As for TiFe2 phase, where the added Zr substituted Fe, the substitution caused the bond character of the phase to become more covalent and less ionic due to the lower electronegativity of Zr compared to Fe. Subsequently, the work functions increased as a function of Zr content.

The hydrogen-embrittlement susceptibility of austenitic high-manganese steels according to different hydrogen-charging test methods was discussed in terms of hydrogen-embrittlement process and fracture mechanism. Ex-situ electrochemical and high-pressure thermal hydrogen-charging methods exhibited a difference in hydrogen-embrittlement susceptibility because they affect the permeability and diffusivity of hydrogen. Moreover, the in-situ high-pressure gaseous hydrogen-charging method showed the most apparent hydrogen-embrittlement susceptibility because higher triaxial stress in the necked region caused by plastic instability accelerates hydrogen charging in the specimen during tensile testing in hydrogen environment, thus leading to intergranular fracture in all regions.

The tensile behavior of hydrogen-charged 304-type austenitic stainless steel, with and without prestrain, is investigated. The specimens are thermally charged with hydrogen in 15 MPa hydrogen gas at 300 °C for 72 h. Tensile behavior of the specimen is compared with the specimen aged in vacuum at 300 °C. The effect of the charging condition on the stability of microstructure is determined by characterizing prestrained specimens before and after charging. The hydrogen content in the specimens is determined using thermal desorption spectroscopy (TDS). Analysis of X-ray diffraction (XRD) data and electron backscattered diffraction (EBSD) shows that the fraction of martensite increases after charging in hydrogen by 5–10%. The fracture surfaces of the uncharged and charged specimens are examined for characteristic features. Flow stress and ductility of the charged and prestrained and charged specimens are discussed in terms of the martensite fraction and hydrogen content.