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.
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•Several Mg–Ni alloys with increasing Ni content were prepared by induction melting.•Reaction mechanisms and rate-limiting steps were investigated by solid-gas models.•H2 desorption enthalpy/activation energy were estimated by pressurized DSC measurements.•After 100 cycles Ni16.3 displayed a low storage capacity decay (4.5%).
The influence of hydrogen on the mechanical behavior of the CoCrFeMnNi high-entropy alloy (HEA) was examined through tensile and nanoindentation experiments on specimens hydrogenated via gaseous and ...electrochemical methods. Results show that the HEA's resistance to gaseous hydrogen embrittlement is better than that of two representative austenitic stainless steels, in spite of the fact that it absorbs a larger amount of hydrogen than the two steels. Reasons for this were discussed in terms of hydrogen-enhanced localized plasticity mechanism and the critical amount of hydrogen required for it. These were further substantiated by additional experiments on electrochemically charged specimens.
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Time-dependent plastic deformation behavior of nanocrystalline (nc) and coarse-grained (cg) CoCrFeMnNi high-entropy alloys (HEAs) was systematically explored through a series of spherical ...nanoindentation creep experiments. High-pressure torsion (HPT) processing was performed for achieving nc microstructure in the HEA, leading to a reduction in grain size from ∼46 μm for the as-cast state to ∼ 33 nm at the edge of the HPT disk after 2 turns. Indentation creep tests revealed that creep deformation indeed occurs in both cg and nc HEAs even at room temperature and it is more pronounced with an increase in strain. The creep stress exponent, n, was estimated as ∼3 for cg HEA and ∼1 for nc HEA and the predominant creep mechanisms were investigated in terms of the values of n and the activation volumes. Through theoretical calculations and comparison of the creep strain rates for nc HEA and a conventional face-centered-cubic nc metal (Ni), the influence of sluggish diffusion on the creep resistance of nc HEA was analyzed. In addition, sharp indentation creep tests were performed for comparison purposes and the results confirmed that the use of a spherical indenter is clearly more appropriate for investigating the creep behavior of this HEA.
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High temperature tensile and creep properties of face-centered cubic (FCC) high-entropy alloys (HEAs), CrMnFeCoNi and CrFeCoNi, were evaluated at 500–700 and 650–725 °C, respectively, to evaluate ...high temperature structural integrity of the most well-known FCC HEAs with special attention on (i) sigma phase formation in CrMnFeCoNi and (ii) difference in solid solution strengthening between the two alloys in the temperature range where commercial heat-resistant wrought alloys including austenitic heat-resistant steels and Ni-based superalloys are used. No remarkable difference was detected in tensile behavior measured both at room and high temperatures between the two alloys. However, creep rupture life turned out to be significantly longer for the CrFeCoNi quaternary alloy. On top of the longer creep rupture life, CrFeCoNi alloy is characterized by lower minimum creep rate and larger creep activation barrier energy which could be attributed to higher level of lattice distortion of CrFeCoNi than CrMnFeCoNi resulting in higher solid solution strengthening. Also, grain boundary strength of CrMnFeCoNi was compromised by the formation of sigma phase during creep deformation which led to lower elongation especially for long-term creep conditions. Therefore, the longer creep life of CrFeCoNi is mainly attributed to the combination of enhanced solid solution strengthening and stronger grain boundary which is free from the deleterious sigma phase due to the reduced thermodynamic driving force for formation of intermetallic phase.
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•Creep behavior of CrMnFeCoNi and CrFeCoNi high-entropy alloys were investigated.•CrFeCoNi had significantly longer creep rupture life than CrMnFeCoNi.•CrFeCoNi turned out to have higher creep activation energy than CrMnFeCoNi.•Sigma phase precipitation during creep in CrMnFeCoNi degraded the creep strain.
In the present study, equiatomic CoCrFeMnNi high entropy alloy (HEA) was subjected to thickness reductions of 20, 40, and 60% during cold rolling in order to thoroughly investigate the evolutions of ...both the microstructure and the deformation texture. Important aspects of deformed microstructures such as the activation of multiple twin variants and the formation of shear bands in the matrix were captured using electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) techniques. Twin trace analysis (TTA) was performed in conjunction with resolved shear stress (RSS) analysis for the identification of active twin variants. The RSS ratio, which is a ratio of the maximum RSS values for corresponding twin and slip systems, was used to reveal the orientation dependence of deformation twinning. Visco-plastic self-consistent (VPSC) simulations were carried out to predict the evolution of the crystallographic texture, the transition routes of ideal orientations subjected to multiple deformation twinning, and the role that deformation modes play in the rotation of orientation. Experimental and simulation results substantiated the key finding of the deformation twinning of a Brass orientation, which established new perspectives concerning the evolution of microstructure and texture. One twin variant of the Copper orientation was moved to a Goss orientation by dislocation slip while the other two variants were rotated towards Brass and S orientations. Meanwhile, twin variants of the S and Brass orientations primarily transitioned to a Brass orientation. The Goss orientation showed great resistance to the twinning mode. Furthermore, dislocation slip and the formation of shear bands contributed to the evolution of a strong texture while deformation twinning had the opposite effect.
•The evolutions of both the microstructure and the deformation texture in HEA cold rolled up to 60% reduction in thickness were investigated.•RSS analysis was used with experimentally observed twin traces for the identification of active twin variants.•A preliminary analysis using the RSS ratio (max(RSSTwin)/max(RSSSlip)) predicted both the favorable (Copper and S) and unfavorable orientations (Brass and Goss) for twinning.•Twinning of the Brass orientation was found to delay the shifting of Copper-type texture to Brass-type texture leading to a stronger Goss component at 60% rolling reduction.•VPSC simulations predicted the twinning of the Brass orientation and role of multiple twin variants in the texture transitions.
The selection and design of modern high-performance structural engineering materials is driven by optimizing combinations of mechanical properties such as strength, ductility, toughness, elasticity ...and requirements for predictable and graceful (non-catastrophic) failure in service. Highly processable bulk metallic glasses (BMGs) are a new class of engineering materials and have attracted significant technological interest. Although many BMGs exhibit high strength and show substantial fracture toughness, they lack ductility and fail in an apparently brittle manner in unconstrained loading geometries. For instance, some BMGs exhibit significant plastic deformation in compression or bending tests, but all exhibit negligible plasticity (<0.5% strain) in uniaxial tension. To overcome brittle failure in tension, BMG-matrix composites have been introduced. The inhomogeneous microstructure with isolated dendrites in a BMG matrix stabilizes the glass against the catastrophic failure associated with unlimited extension of a shear band and results in enhanced global plasticity and more graceful failure. Tensile strengths of ∼1 GPa, tensile ductility of ∼2-3 per cent, and an enhanced mode I fracture toughness of K1C 40 MPa m1/2 were reported. Building on this approach, we have developed 'designed composites' by matching fundamental mechanical and microstructural length scales. Here, we report titanium-zirconium-based BMG composites with room-temperature tensile ductility exceeding 10 per cent, yield strengths of 1.2-1.5 GPa, K1C up to ∼170 MPa m1/2, and fracture energies for crack propagation as high as G1C 340 kJ m-2. The K1C and G1C values equal or surpass those achievable in the toughest titanium or steel alloys, placing BMG composites among the toughest known materials.
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.
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Cardiac magnetic resonance (CMR) imaging is widely used in various medical fields related to cardiovascular diseases. Rapid technological innovations in magnetic resonance imaging in recent times ...have resulted in the development of new techniques for CMR imaging. T1 and T2 image mapping sequences enable the direct quantification of T1, T2, and extracellular volume fraction (ECV) values of the myocardium, leading to the progressive integration of these sequences into routine CMR settings. Currently, T1, T2, and ECV values are being recognized as not only robust biomarkers for diagnosis of cardiomyopathies, but also predictive factors for treatment monitoring and prognosis. In this study, we have reviewed various T1 and T2 mapping sequence techniques and their clinical applications.
Herein, real‐time observations of dehydrogenation of a Mg2FeH6–MgH2 composite by means of in situ transmission electron microscopy (TEM) with advanced spatial (≈0.8 Å) and temporal (25 frames s−1) ...resolution are reported. Careful control and systematic variations of the reaction temperature and electron dose rate enable detailed and direct visualization of the characteristic decomposition of Mg2FeH6 into Mg and Fe, which occurs on the nanometer scale under optimal experimental conditions defined to minimize the electron‐beam‐driven Mg oxidation and dehydrogenation that take place in TEM. First, the formation of nanostructured fine Fe clusters in Mg metal and their growth via coalescence during dehydrogenation are verified. Additionally, fine monitoring of the in situ diffraction patterns acquired during decomposition of the composite allows separate evaluations of the desorption kinetics of the two coexisting phases, which confirm the synergetic dehydrogenation of this dual‐phase system. It is envisioned that these findings will provide useful guidelines for reducing the gaps between nanoscale and bulk‐scale research and designing hydrogen sorption conditions to enable efficient operation of a solid‐state hydrogen storage system.
Dehydrogenation of a Mg2FeH6–MgH2 composite by in situ transmission electron microscopy is investigated. By setting up the optimal conditions to minimize electron‐beam‐induced Mg oxidation and dehydrogenation, hydrogen release process is imaged and thoroughly characterized, and nanostructuring of Fe clusters in Mg metal is observed. Furthermore, enhanced desorption kinetics of MgH2 induced by Fe catalytic effect is confirmed.
We present that the equilibrium hydrogen pressure of titanium iron (TiFe) alloy, a room-temperature hydrogen storage material, can be tailored via vanadium alloying. While many 3d transition metal ...alloying elements (e.g., Mn, Cr, Co, and Ni) typically replace the Fe sublattice in TiFe, vanadium can replace both the Ti and Fe sublattices. Density functional theory calculation predicts that the substitution of Ti with V yields a unique effect: the equilibrium pressures of TiFe/TiFeH (P1) and TiFeH/TiFeH2 (P2) are closer, resulting in a decreased P2/P1 ratio. Experimental pressure-composition isotherms confirm this theoretical prediction. The lower P2/P1 is beneficial because the two-step TiFe hydrogenation reactions can be contained within a narrow pressure range. In contrast, the substitution of V for Fe lowers both P1 and P2, but lowers P1 more, resulting in a higher P2/P1 ratio. The contrasting effects contingent on the substitution site is a crucial factor in alloy design. It highlights the significance of vanadium as a versatile alloying element that modifies the hydrogen storage property of TiFe.
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•Alloying effect on the hydrogenation of TiFe is predicted by DFT calculation.•Vanadium is a versatile element tailoring the hydrogenation thermodynamics of TiFe.•Significance of the sublattice of substitution in TiFe-alloy design is demonstrated.
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