Mechanical high-energy treatment of TiNi powder and phase changes after electrochemical hydrogenation

The paper presents the results obtained for the effect of ball milling of Ti–Ni powder, which is close in composition to the equiatomic one, on electrochemical hydrogenation. It is shown that the average size of the powder particles measured by BET and laser diffraction methods is found to reduce during milling, while the average size of the powder particles measured by SEM changes to attain its minimum within 30-s milling due to destruction and subsequent aggregation of particles. The powder in its initial state consists of a mixture of TiNi (austenite, martensite), Ti₂Ni, and TiNi₃ phases, and after ball milling, an X-ray amorphous phase is formed. The CDD size of the TiNi phase (austenite) reduces from 25 to 4 nm. It is found that the lattice parameters of the TiNi (austenite) and Ni₃Ti phases do not change during electrochemical hydrogenation, whereas the crystal lattice parameter of the Ti₂Ni phase increases, which indicates the predominant interaction of hydrogen with the Ti₂Ni phase. The lattice parameter of the Ti₂Ni based phase corresponds to Ti₂NiH_0.5 and Ti₂NiH_0.8 hydrides depending on the milling time and hydrogenation time. It is found that there is an “incubation period” of hydrogenation of the Ti₂Ni phase, which attains 90 min.     

Relationship between hydrogen permeation and microstructure in Nb–TiNi two-phase alloys

The relationship between the microstructure and hydrogen permeability of the as-cast two-phase Nb–TiNi alloys is investigated and discussed on the basis of the mixing rule. The alloy compositions consisted of the bcc-(Nb, Ti) and B2–TiNi phases are expressed as (Nb₄Ti₄₆Ni₅₀)_1−x(Nb₈₅Ti₁₃Ni₂)_x. The alloy for x = 0.19 has a fully eutectic structure of the (Nb, Ti) and TiNi phases in the as-cast state. The hydrogen permeability of this alloy corresponds to that of a model alloy in which these two phases are distributed randomly. The primary TiNi and (Nb, Ti) phases are formed in the alloys for x < 0.19 and x > 0.19, respectively. Their volume fractions decrease and increase with x, respectively. According to the mixing rule, the hydrogen permeability of alloys having a primary (Nb, Ti) phase can be expressed as the model alloy in which the primary phase is isolated in the eutectic structure. However, the hydrogen permeability of alloys having a primary TiNi phase is higher than that expected for the above model alloy.     

Hydrogenation method based on electrodeposited layers controlling the hydrogen desorption rate

An innovative hydrogenation method to investigate the hydrogen embrittlement of metals and alloys is hereby presented. The benefits of electroplating samples with copper and nickel prior to gaseous hydrogenation at mid-range temperatures are quantified. It is showed that these electrodeposited layers allow to control the hydrogen desorption rate occurring after hydrogenation, during the cooling of the hydrogenated specimen. The present study demonstrates the capability of the method to control the introduced total hydrogen concentration within a margin of 0.2 wt.ppm. The applicability of the described method to further investigations into hydrogen concentrations effects on hydrogen embrittlement of ferritic alloys by the means of mechanical tests is evaluated.     

Structural and hydrogen storage properties of melt-spun Mg–Ni–Y alloys

Nanocrystalline magnesium-rich Mg–Ni–Y alloys were produced by melt-spinning. They were characterized regarding their microstructure, crystallization behaviour, and cyclic hydrogenation/dehydrogenation properties in view of their application as reversible hydrogen storage materials. Transmission electron microscopy reveals that these alloys consist in the as-spun state of mixtures of nanocrystalline Mg(Ni;Y) grains that are embedded in an amorphous matrix. Differential scanning calorimetry and X-ray diffraction analysis show that these alloys undergo several crystallization steps in the temperature range between 180 and 370 °C. It was found that only a few thermal activation cycles of the as-quenched ribbons are required in order to reach excellent hydrogenation/dehydrogenation properties of these alloys. In thermogravimetric analyses using a magnetic suspension balance it could be shown that these alloys can reach reversible gravimetric hydrogen storage densities of up to 5.3 wt.%-H with hydrogenation and dehydrogenation rates of up to 1 wt.%-H/min even at temperatures of 250 °C. The structure of the alloys remains nanocrystalline even after several hydrogenation/dehydrogenation cycles.     

Composition dependent hydrogen storage performance and desorption factors of Mg–Ce based alloys

The widespread application of Mg as a hydrogen storage material has been limited by its slow absorption and desorption kinetics at moderate temperatures. Aiming at improving the de-/absorption kinetics of Mg-based alloys by in situ formed catalysts and understanding the desorption factors, Mg–Ce and Mg–Ce–Ni alloys with different Ce contents are prepared. The phase components, microstructure and hydrogen storage properties have been carefully investigated. It is shown that an 18R-type long-period stacking ordered (LPSO) phase is formed in as-melt Mg–Ce–Ni ternary alloy together with random stacking faults. Abundant in situ formed CeH_2.73 particles with particle size less than 100 nm are observed on the matrix after hydrogenation. It is found in isothermal hydrogenation and dehydrogenation kinetic curves that Ni significantly favors desorption process, while Ce is more conducive to absorption. After partial dehydrogenation of Mg–Ce binary alloy, the initial desorption temperature decreases significantly when desorbing again. The primary-formed Mg phase on the surface of MgH₂ accounts for the improved desorption performance.     

Hydrogen activation and storage properties of laves phase Ti1-xScxMn1.6V0.4 alloys

Hydrogen activation, storage properties and associated crystal structures of Ti_1-xSc_xMn_1.6V_0.4 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) alloys are investigated by hydrogenation and XRD. The unit-cells of alloys and hydrides expand with Sc content and hydrogen concentration. Minor addition of Sc significantly improves hydrogen activation and storage properties. The plateau pressure decreases, whereas the sloping factor and relative partial molar enthalpy for hydrogenation increase with Sc content. The activation properties strongly depend on the particle sizes. The bulk samples can easily be activated by implementing a hydrogen-induced cracking mechanism, which avoids removal of protective oxide films and compensates the lack of metallic B-metal in catalysis of hydrogen dissociation. Samples with smaller particle sizes are difficult to activate. The unusual particle size effect is interpreted by activation kinetics, and attributed to the high oxygen binding ability of B-metals and their contribution to the surface oxide films.     

Characteristic and kinetics of hydrogen absorption during the heat preservation stage and the cooling stage of TC21 alloy

TC21 alloy is hydrogenated under different initial hydrogen pressures at hydrogenation temperatures in the range of 450 °C–850 °C. Hydrogen absorption characteristic and kinetics during the heat preservation stage and cooling stage, hydrogen content and activation energy are investigated. The hydrogen absorption reaches equilibrium first at higher hydrogenation temperature and initial hydrogen pressure during the heat preservation stage. The hydrogen absorption reaches equilibrium first at lower hydrogenation temperature and initial hydrogen pressure during the cooling stage. Mechanisms of hydrogen absorption are analyzed during the heat preservation stage and the cooling stage. Phase compositions of the hydrogenated TC21 alloys are analyzed by XRD. Hydrogen content increases first and then decreases, then increases slightly, and finally decreases with the increase of hydrogenation temperature. Hydrogen content increases gradually with the increase of initial hydrogen pressure. The activation energy of hydrogen absorption in TC21 alloy is about 18.304 kJ/mol.     

Effect of cold rolling on the structure and hydrogen properties of AZ91 and AM60D magnesium alloys processed by ECAP

This investigation was conducted to evaluate the effect of cold rolling on the structure and hydrogen properties of two magnesium alloys, AZ91 and AM60D, after processing by equal-channel angular pressing (ECAP). The results show that the use of cold rolling after ECAP significantly increases the preferential texture for hydrogenation and increases the potential for the use of these alloys as hydrogen storage materials. The ECAP was performed through two different numbers of passes in order to give different grain sizes and both materials were subsequently cold-rolled through the same numbers of passes for a comparison of the hydrogenation absorption. It is shown that the hydriding properties are enhanced by an (0001) texture which improves the kinetics primarily in the initial stages of hydrogenation. The results demonstrate that optimum sorption properties may be acquired through a combination of fine grains and appropriate texture.     

Transformation-assisted hydrogen desorption during deformation in steels: Examples of α′- and ε-Martensite

Hydrogen desorption behavior associated with γ-α′ and γ-ε martensitic transformation during tensile tests is investigated in four kinds of alloys with different austenite stabilities. Remarkable deformation-induced hydrogen desorption is detected not only as a result of the γ-α′ martensitic transformation, but also because of the γ-ε martensitic transformation, and the amount of desorbed hydrogen from transformed α′ martensite is more than that of transformed ε martensite. This suggests that the solubility of hydrogen in the ε phase is higher than that in the α′ martensite but lower than that in austenite.     

Hydrogen-induced softening of Ti–44Al–6Nb–1Cr–2V alloy during hot deformation

The effect of hydrogen on the hot deformation behavior and microstructural evolution of Ti–44Al–6Nb–1Cr–2V (at.%) alloys were investigated at temperature range of 1373–1523 K under strain rate of 0.01 s^?1. The true stress–strain curves show that the peak stress decreases from 323 MPa to 97 MPa when deformation temperature increases from 1373 K to 1523 K. The peak stress is decreased by 30% after hydrogenation with 2% H, which corresponds to the decrease of deformation temperature by about 50 K, it denotes that hydrogen can promote a solution softening effect in TiAl alloys. This is attributed to hydrogen-promoted the dynamic recrystallization, hydrogen-induced dislocation movement and hydrogen-stabilized the B2 phase. For dynamic recrystallization, the calculated results show that hydrogen accelerates the onset of dynamic recrystallization, which means that hydrogen promotes the dynamic recrystallization kinetics. For dislocation movement, EBSD results show that the fraction of low-density dislocation region increases from 59.6% to 79.7% after hydrogenation with 2% H, which indicates that hydrogen reduces the dislocation tangles and dislocation density. For B2 phase, more softening B2 phases are observed in hydrogenated alloy compared with that in unhydrogenated alloy, which results from hydrogen-promoted the transition of L (α₂/γ) → γ + B2. The positive effect of hydrogen on TiAl alloys provides an effective method to improve the hot workability of TiAl alloys.     

Hydrogen permeation in anisotropic Nb–TiNi two-phase alloys formed by forging and rolling

The formation of an anisotropic microstructure by forging and rolling of a Nb–TiNi two-phase alloy and the effects of direction and annealing on hydrogen permeability were investigated. After forging and rolling, the granular (Nb, Ti) phase was strongly elongated along the rolling direction (RD) and compressed along the normal direction (ND). Hydrogen permeability along the RD (ND) increased (decreased) dramatically. The hydrogen permeability of this anisotropic microstructure can be explained by the law of mixtures using the hydrogen permeabilities of (Nb, Ti) and TiNi single-phase alloys. The hydrogen permeabilities along RD and ND correspond to parallel- and series-type hydrogen permeability, respectively. The 94-μm-thick RD sample shows a large hydrogen flux of 0.57 mol H₂ m^?2 s^?1 (77 ccH₂ cm^?2 min^?1) without hydrogen embrittlement. Phase boundary between (Nb, Ti) and TiNi phases, aligned parallel to the hydrogen flux, is one of the hydrogen permeation path.     

Hydrogenation-induced microstructure evolution in as cast and severely deformed Mg-10 wt.% Ni alloy

We determined the kinetics of hydrogen absorption of the hypoeutectic Mg-10 wt.% Ni alloy in the as-cast state and after processing by four passes of equal channel angular pressing (ECAP). While during the first hydrogenation cycle the ECAP-modified alloy exhibited faster absorption than its as-cast counterpart, this advantage was lost after the second hydrogenation cycle; parity was regained after six cycles. We attributed these differences in the hydrogen absorption kinetics to the formation of large (tens of micrometers) faceted Mg crystals observed during the first hydrogenation cycle. These crystals were significantly larger in the ECAP-modified alloy than in its as-cast counterpart. We discussed the growth of large Mg crystals during hydrogenation in terms of self-diffusion of Mg atoms driven by the metal-hydride transformation stress. The larger size of these crystals in the ECAP-processed alloy was attributed to the acceleration of diffusion by ECAP. Our metallographic studies revealed a number of microstructural changes in the alloys upon hydrogenation, such as cracking, accumulation of plastic strain in large Mg crystals, and re-distribution of the dispersed particles of Mg₂Ni phase in the partly hydrogenated alloys.     

Mg-based metastable nano alloys for hydrogen storage

Mg-based materials have been widely researched for hydrogen storage development due to the low price of Mg, abundant resources of Mg element in the earth's crust and the high hydrogen capacity (ca. 7.7 mass% for MgH₂). However, the challenges of poor kinetics, unsuitable thermodynamic properties, large volume change during hydrogen sorption cycles have greatly hindered the practical applications. Here in this review, our recent achievements of a new research direction on Mg-based metastable nano alloys with a Body-Centered Cubic (BCC) lattice structure are summarized. Different with other metals/alloys/complex hydrides etc. which involve significant lattice structure and volume change from hydrogen introduction and release, one unique nature of this kind of metastable nano alloys is that the lattice structure does not change obviously with hydrogen absorption and desorption, which brings interesting phenomenon in microstructure properties and hydrogen storage performances (outstanding kinetics at low temperature and super high hydrogen capacity potential). The synthesis results, morphology and microstructure characterization, formation evolution mechanisms, hydrogen storage performances and geometrical effect of these metastable nano alloys are discussed. The nanostructure, fresh surface from ball milling process and fast hydrogen diffusion rate in BCC lattice structure, as well as the unique nature of maintaining original BCC metal lattice during hydrogenation result in outstanding hydrogen storage performances for Mg-based metastable nano alloys. This work may open a new sight to develop new generation hydrogen storage materials.     

Effect of Ni on electrochemical and hydrogen storage properties of V-rich body-centered-cubic solid solution alloys

V-rich solid solution alloys are potential candidates for Ni-MH_x negative electrodes and hydrogen sorbing materials. Mechanical alloying (MA) is used in this paper to produce Ti_0.5V_1.5?xNi_x nanocrystalline alloys (x = 0, 0.1, 0.2, 0.3). A SPEX 8000 M mill is used. The aim of this work is to study the effect of chemical modification by Ni on hydrogen sorption/desorption and electrochemical properties of V-rich body-centered-cubic (BCC) alloys. Presented measurements results show formation of BCC phase after 14 h of MA. The nanocrystallinety of obtained materials is confirmed by high resolution transmission electron microscopy images. MA alloys are tested by a Sievert's device at near room temperature. Partial substitution of V by Ni causes improved hydrogenation kinetics, reduced hysteresis and increased hydrogenation/dehydrogenation reversibility. Observed properties are mainly due to differences in structures of studied materials. Electrochemical studies on chemically modified V-rich alloys show that capacity retaining rate and discharge capacity increase with higher Ni content in the material.     

Effect of rapid solidification on hydrogen solubility in Mg-rich Mg-Ni alloys

The hydrogenation properties of Mg_100−xNi_x alloys (x = 0.5, 1, 2, 5) produced by melt spinning and subsequent high-energy ball milling were studied. The alloys were crystalline and, in addition to Mg matrix, contained finely dispersed particles of Mg₂Ni and metastable Mg₆Ni intermetallic phases. The alloys exhibited excellent hydrogenation kinetics at 300 °C and reversibly absorbed about 6.5 mass fraction (%) of hydrogen. At the same temperature, the as prepared Mg_99.5Ni_0.5 and Mg₉₅Ni₅ powders dissolved about 0.6 mass fraction (%) of hydrogen at the pressures lower than the hydrogen pressure corresponding to the bulk Mg-MgH₂ two-phase equilibrium, exhibiting an extended apparent solubility of hydrogen in Mg-based matrix. The hydrogen solubility returned to its equilibrium value after prolonged hydrogenation testing at 300 °C. We discuss this unusually high solubility of hydrogen in Mg-based matrix in terms of ultrafine dispersion of nanometric MgH₂ precipitates of different size and morphology formed on vacancy clusters and dislocation loops quenched-in during rapid solidification.     

Nanoscale microstructures and hydrogenation properties of an as-cast vanadium-based medium-entropy alloy

Vanadium-based hydrogen storage alloys have been widely investigated; however, alloys in the cast state are typically coarse-grained. In this study, an as-cast V₄₅Fe₁₅Ti₂₀Cr₂₀ medium-entropy alloy was prepared by arc melting, and microstructural analysis revealed that the alloy was composed of nanocrystals. The initial pretreatment temperature of the alloy was approximately 100 K lower than that of the as-cast coarse-grained alloy. At room temperature, the time required for the alloy to reach 90% saturation was only 140 s, indicating excellent hydrogen absorption kinetics. The alloy is fully activated after two hydrogen absorption/desorption cycles. The phase transformation of the alloy in the early hydrogenation stage was investigated using X-ray diffraction, and the results showed that the BCC phase was completely transformed into the BCT phase when hydrogen uptake was performed for 6 s. Furthermore, the apparent activation energy of dehydrogenation in the present alloy calculated using the Kissinger method was 69.8 ± 0.8 kJ/mol. The pressure-composition-isotherms tests showed that the hydrogen absorption capacity of the alloy at 295 K was 2.12 wt%. The hydrogenation/dehydrogenation enthalpy change of the alloy was calculated by the Van't Hoff equation, which was 30.90 ± 1.47 and 33.95 ± 0.41 kJ/mol, respectively. The present work demonstrates that nanostructured vanadium-based hydrogen storage alloys can be fabricated using traditional casting techniques. Our study also enriches the understanding of the microstructures of medium-entropy alloys, which may provide positive guidance for the design of novel vanadium-based hydrogen storage alloys.     

Theoretical model on the electronic properties of multi-element AB5-type metal hydride

Nowadays, multi-element alloys are preferred over binary alloys for application point of view. The hydrogenation properties strongly depend on the thermodynamic, structural and electronic properties of the alloys. At present, no model is available which can predict the hydrogen storage properties of the multi-element alloy, before actual synthesis of the alloy. In the present investigation, efforts are made to develop a theoretical mathematical model to predict the hydrogenation properties of multi-element AB₅-type metal hydride. The present investigation deals with the various electronic parameters which may affect the hydrogenation characteristics of the metal hydride. Based on all such parameters, an electronic factor has been proposed for AB₅-type alloys. Electronic factor has been combined with the structural and thermodynamical factor to propose a new combined factor, which was further correlated with the hydrogen storage capacity of the alloy. Atomic radius and electronic configuration of substituted elements in the multi-element AB₅-type hydrogen storage alloy have been found as key players to predict the hydrogenation properties of the alloys before synthesis. It has been shown that in the case of alloy series with multiple substitutions, the combined factor is more relevant in deciding the hydrogen storage capacity in comparison to electronic factor alone. Combined factor is directly proportional to the hydrogen storage capacity. All the three factors thermodynamic, structural and electronic together may lead to the prediction of pressure-composition isotherm of the multi-element AB₅-type hydrogen storage alloy.     

Comparative investigations on the hydrogenation characteristics and hydrogen storage kinetics of melt-spun Mg10NiR (R = La,Nd and Sm) alloys

The hydrogenation characteristics and hydrogen storage kinetics of the melt-spun Mg₁₀NiR (R = La, Nd and Sm) alloys have been studied comparatively. It is found that the Mg₁₀NiNd and Mg₁₀NiSm alloys are in amorphous state but the Mg₁₀NiLa alloy is composed of an amorphous phase and minor crystalline La₂Mg₁₇ after melt-spinning. The alloys can be hydrogenated into MgH₂, Mg₂NiH₄ and a rare earth metal hydride RH_x. The rare earth metal hydride and Mg₂NiH₄ synergistically provide a catalytic effect on the hydrogen absorption–desorption reactions in the Mg−H₂ system. The hydrogen storage kinetics is not influenced by different rare earth metal hydrides but by the particle size of the rare earth metal hydrides.     

Effect of Cr addition on room temperature hydrogenation of TiFe alloys

This paper discusses the effect of AB₂ (Ti(Cr, Fe)₂) phase on the hydrogenation properties of a Ti–Fe–Cr alloy system. Five Ti–Fe–Cr based alloys were fabricated by varying the Cr content. The microstructural analysis results revealed that the fraction of the Ti(Cr, Fe)₂ phase increased with the increasing Cr content. The first hydrogenation test results indicated that all the alloys could absorb a significant amount of hydrogen at room temperature (30 °C) without a separate activation process. This behavior improved when the Ti(Cr, Fe)₂ phase existed in the AB phase; the kinetics of the first hydrogenation tended to increase with the fraction of Ti(Cr, Fe)₂ phase. The enhancement in the first hydrogenation kinetics of the Ti–Fe–Cr based alloys was attributed to the synergetic effect of the interface between the AB and Ti(Cr, Fe)₂ phases and the inherent fast hydrogenation of the Ti(Cr, Fe)₂ phase. However, the total hydrogen storage capacity decreased when the fraction of Ti(Cr, Fe)₂ phase increased.