Effect of hydrogen on diffusion bonding of commercially pure titanium and hydrogenated Ti6Al4V alloys

The diffusion bonding of commercially pure titanium and hydrogenated Ti6Al4V alloys was carried out, and the effect of hydrogen was investigated by SEM, XRD, TEM and TG/DSC. The β_H phase increased with the hydrogen content increasing in hydrogenated alloys, and the δ titanium hydride and α′ martensite were found in high hydrogen content. The TG curves of hydrogenated alloys descended between 600 °C and 950 °C, and the DSC curves represented a large endothermic peak correspondingly. Moreover, some voids were observed at the diffusion bonding interface. The amount of voids decreased and diffusion bonding quality improved gradually with the increase of hydrogen content, which was attributed to the increase of soft β_H phase and release speed of hydrogen as well as the occurrence of more dehydrogenation reaction.     

Microstructural investigation and electrochemical property of Mg63Ni27Nd10 amorphous alloy

Amorphous Mg-based alloy Mg₆₃Ni₂₇Nd₁₀ was prepared by melt-spinning. The phase structures and the electrochemical properties of the ribbons before and after charge/discharge cycling were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) analysis and galvanotactic charge–discharge cycle test, respectively. It was found that the amorphous structure begins to crystallize after four cycles of hydrogenation/dehydrogenation and a new nano-crystallized phase NdMg₂Ni₉ was detected with average grain size in the range 5–10 nm. A good cycle life and high discharge capacity were also observed in the Mg₆₃Ni₂₇Nd₁₀ alloy electrode. The highest discharge capacity reached 580.5 mAh g⁻¹ at the discharge current densities of 50 mA g⁻¹. It is suggested that homogeneous microstructure contributes to the enhancement in electrochemical characteristics, the presence of NdMg₂Ni₉ being beneficial for the improvement in cycle life of Mg₆₃Ni₂₇Nd₁₀ alloy electrode.     

A discussion on decay of discharge capacity for amorphous Mg–Ni–Nd hydrogen storage alloy

Amorphous Mg–Ni–Nd alloys were prepared by melt-spun technique and their discharge capacity was measured. Microstructure of amorphous ribbons after different charge–discharge cycles was observed using high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and selected area electron diffraction (SAED). The structure evolution of amorphous (Mg₆₀Ni₂₅)₉₀Nd₁₀ alloy during the charge–discharge cycles was investigated in detail. Experimental results showed that the structure evolution of amorphous ribbon was responsible for the decay of discharge capacity. During the charge–discharge cycles of amorphous (Mg₆₀Ni₂₅)₉₀Nd₁₀ alloy, NdMg₂Ni₉ phase appeared after four cycles and Mg₂Ni phase appeared after six cycles. After 20 cycles the stable phases Mg₂Ni, α-Mg and Nd₂H₅ were present and the former NdMg₂Ni₉ phase disappeared. With crystallization of amorphous ribbon, the discharge capacity of amorphous (Mg₆₀Ni₂₅)₉₀Nd₁₀ alloy dropped off. After 10 cycles the curve of discharge capacity tended to be smooth.     

Dependence and mechanism of hydrogenation behavior on absorption conditions in hypo-eutectic Mg–Ni–Cu alloy

In this article, the isothermal hydrogenation curves of hypo-eutectic Mg–6Ni–3Cu (at. %) alloy under various temperatures and applied hydrogen pressures are fitted by Johnson-Mehl-Avrami equation. The hydrogenation behavior and its dependence on absorption conditions are investigated, as different from that of pure Mg. A three-stage hydrogenation behavior is illustrated and an extra absorption process (Stage III) resulted from H dissolution or the hydride formation in boundaries appears when the hydrogen pressure reaches a critical value. Due to the same role of increased ΔP and decreased ΔT on hydride nucleation driving force and their opposite impacts on H diffusion, temperature and hydrogen pressure are influencing the absorption process differently. As the great H permeability in Mg–Mg₂Ni–Mg₂Cu eutectic, more hydrogen is absorbed under larger hydrogenation driving force, rather than hydrogen uptake deficit. It is revealed that the reaction order η corresponding to H diffusing in thin MgH₂ layer is larger than that in thick MgH₂ shell, which can preliminarily identify the hydrogenation behavior after MgH₂ impinging around primary Mg (Stage II).     

Mechanical activation of air exposed TiFe + 4 wt% Zr alloy for hydrogenation by cold rolling and ball milling

In this paper we report the effect of air exposure on TiFe + 4% Zr alloy and how the hydrogen capacity could be recovered by processing the alloy by cold rolling and ball milling. It has been noted that the alloy could not absorb hydrogen after 7 days of air exposure. To restore the hydrogenation capability of the alloy, it was mechanically treated using cold rolling and ball milling. It was found that the air exposed alloy could be hydrogenated after 30 min of ball milling under argon or after cold rolling. Effect of cold rolling and ball milling on first hydrogenation for pure TiFe alloy was also investigated.     

Effect of hydrogen on the superplasticity of Ti600 alloy

The influence of hydrogen on microstructure evolution and superplastic behavior of a new near α high-temperature titanium alloy-Ti600 alloy were studied. The results show that hydrogen increases the amount of β phase and δ hydride with fcc structure exists in the specimens when the hydrogen content is over 0.3 wt.%. After hydrogenation, the deformation temperature of Ti600 alloy can be decreased about 80 °C and the strain rate can be increased by at least one order. Addition of proper hydrogen can reduce the flow stress of Ti600 alloy significantly. The flow stress of Ti600–0.5H alloy decreases about 78% of that unhydrogenated Ti600 alloy at 840 °C and 5 × 10⁻⁴ s⁻¹. Moreover, introducing hydrogen into Ti600 alloy decreases the dislocation density, promotes the dislocation motion and facilitates the β phase flow.     

Ti–Fe based alloys prepared by ball milling for electrochemical hydrogen storage

In this study, the Ti_1.04Fe_0.6Ni_0.1Zr_0.1Mn_0.2Sm_0.06 composite was prepared by using vacuum induction melting under inert atmosphere. Then, the specimen was milled with 5 wt% Ni powders for 10–40 h to realize the general improvements in hydrogenation performance. The phase component was determined and the morphology and microscopic structure were observed using XRD, SEM and HRTEM, respectively. The electrochemical properties of the alloys were studied. The results showed that the as-milled specimens got the maximal discharge capacity without any activation. It reached 305 mAh/g for the 30 h milling specimen, which was better than the other specimens. Besides, ball milling can enhance the electrochemical cyclic stability of the experimental alloys. The capacity retention rate (S₁₀₀) increased from 57.6 to 70.2% after 100 charging and discharging cycles with increasing milling duration from 10 to 40 h. The high rate discharge ability of the 30 h milling specimen had the maximal value of 92.8%.     

Study on hydrogenation behaviors of a Mg-13Y alloy

The Mg-13Y bulk alloy was prepared by conventional casting process and the Mg-13Y powder was processed by ball milling using the casting alloy under the protection of argon. The hydrogenation thermodynamics, hydrogenation process and phase transitions were carefully investigated in the Mg-13Y powder alloy. It is shown that the Mg-13Y casting alloy consists of Mg₂₄Y₅ phase and α-Mg containing yttrium which have different hydrogenation enthalpies, −195 kJ/mol H₂ (by calculation) and −42 kJ/mol H₂ (by Pressure–Composition–Temperature experiment), respectively. The structure evolution and phase transition in the Mg-13Y bulk alloy treated at 673 K and at 4 MPa for 40 h were observed by an optical microscopy (OM), a scanning electron microscopy (SEM), a transmission electron microscopy (TEM) and X-ray diffraction (XRD). The large Mg₂₄Y₅ phase in the bulk Mg-13Y alloy could be destroyed into fine cuboid-shaped YH₂ phases during the hydrogenation process, which is probably responsible for the improvement of mechanical properties of Mg-13Y alloy.     

First-principles study of hydrogen behavior in vanadium-based binary alloy membranes for hydrogen separation

Vanadium-based alloys are considered to be one of the most promising hydrogen separation membranes due to their high hydrogen permeability. In this study, we investigate the dissolution and diffusion behaviors of hydrogen in vanadium-based binary alloys, V₁₅M (where M = Al, Ti, Cr, Fe, Ni and Nb) alloys, using first-principles method based on density functional theory. The dissolution of hydrogen in V₁₅M alloys is affected by both the elastic and electronic properties, but the elastic effect is the main factor. The H solution energies in the alloys follow the sequence: VTi < VNb < VAl < VCr < VNi < VFe, and a smaller atom size increase the H solution energy. Therefore, the addition of alloying elements with smaller atomic sizes can reduce the solubility of hydrogen in vanadium and inhibit hydrogen embrittlement. For hydrogen diffusion, alloying elements Al, Ti and Nb can be good candidates because they have a higher diffusion coefficient. The VTi alloy has the highest hydrogen permeability, but will have serious hydrogen embrittlement due to the increased H solubility.     

Hydrogen desorption/absorption properties of the extensively cold rolled β Ti–40Nb alloy

β Ti–Nb BCC alloys are potential materials for hydrogen storage in the solid state. Since these alloys present exceptional formability, they can be processed by extensive cold rolling (ECR), which can improve hydrogen sorption properties. This work investigated the effects of ECR accomplished under an inert atmosphere on H₂ sorption properties of the arc melted and rapidly solidified β Ti40Nb alloy. Samples were crushed in a rolling mill producing slightly deformed pieces within the millimeter range size, which were processed by ECR with 40 or 80 passes. Part of undeformed fragments was used for comparison purposes. All samples were characterized by scanning electron microscopy, x-ray diffractometry, energy-dispersive spectroscopy, hydrogen volumetry, and differential scanning calorimetry. After ECR, samples deformed with 40 passes were formed by thick sheets, while several thin layers composed the specimens after 80 passages. Furthermore, deformation of β Ti–40Nb alloys synthesized samples containing a high density of crystalline defects, cracks, and stored strain energy that increased with the deformation amount and proportionally helped to overcome the diffusion's control mechanisms, thus improving kinetic behaviors at low temperature. Such an improvement was also correlated to the synergetic effect of resulting features after deformation and thickness of stacked layers in the different deformation conditions. At the room temperature, samples deformed with 80 passes absorbed ∼2.0 wt% of H₂ after 15 min, while samples deformed with 40 passes absorbed ∼1.8 wt% during 2 h, excellent results if compared with undeformed samples hydrogenated at 300 °C that acquired a capacity of ∼1.7 wt% after 2 h. The hydrogen desorption evolved in the same way as for absorption regarding the deformation amount, which also influenced desorption temperatures that were reduced from ∼270 °C, observed for the undeformed and samples deformed with 40 passes, to ∼220 °C, for specimens rolled with 80 passes. No significant loss in hydrogen capacity was observed in the cold rolled samples.     

Structure and hydrogenation performances of as-cast Ti1.1-xRExFe0.8Mn0.2 (RE = Pr,Sm and Nd; x = 0, 0.01) alloys

The AB-type Ti_1.1-xRE_xFe_0.8Mn_0.2 (RE = Pr, Sm and Nd; x = 0, 0.01) alloys in cast are fabricated. The relationships of phase composition, surface morphology and crystal structure to the hydrogenation performances of the alloys are studied. X-ray diffraction (XRD) results show that the as-cast Ti_1.1Fe_0.8Mn_0.2 (TFM for short) alloy includes the base phase of TiFe phase and the second phases of TiFe₂ phase and TiO phase. Backscatter electron (BES) images show that there is Ti segregation phase besides TiFe phase and TiFe₂ phase in the TFM alloy. Substituting Ti with RE partly causes Ti phase to increase and RE phases to generate. The lowest activation temperature decreases from 573 K for the TiFe alloy to 423 K for the Ti_1.1-xRE_xFe_0.8Mn_0.2 (RE = Pr, Sm and Nd; x = 0, 0.01) alloys. Substituting Ti with RE partly makes the incubation time reduce from 5900 s for the TFM alloy to 3850, 3900 and 4050 s for the Pr_0.01, Sm_0.01 and Nd_0.01 alloys, respectively. And it also is able to weaken the stability of the hydrides, while causes variable degrees of the hydrogenation capacity decrease.     

Hydriding behavior in Zr-based AB2 alloy by gas atomization process

The hydriding characteristics of Zr-based AB₂ alloy produced by gas atomization have been investigated during its absorption–desorption reaction with hydrogen gas. Its gas-phase hydrogenation properties are different from those of specimens prepared by conventional methods. For the particle morphology of the as-cast and gas-atomized powders, it can be seen that the mechanically crushed powders are irregular, while the atomized powder particles are spherical. In PCT (Pressure–Composition–Temperature) measurements, for the gas-atomized particles smaller than 50 μm, the hydrogen storage capacity is dramatically decreased and the hysteresis loop becomes larger than that of the gas-atomized particles larger than 50 μm. In addition, the increase of jet pressure of gas atomization results in the decrease of hydrogen storage capacity and the slope of plateau pressure significantly increases. TEM and EDS studies showed the increase of jet pressure in the atomization process accelerated the phase separation within grain of the gas-atomized alloy, which brought about a poor hydrogenation property. In the measurements of hydrogen absorption–desorption kinetics, the improvement of desorption kinetics of gas-atomized AB₂ alloys was mainly caused by the higher plateau pressure, which is attributed to the smaller grain size and higher site energy for hydrogen in the gas-atomized alloys.     

A study of stability of MgH2 in Mg–8at%Al alloy powder

To investigate the effect of Al addition on the stability of magnesium hydride, the hydrogenation characteristics of a Mg–8at%Al alloy powder synthesized using the electrodeposition technique were evaluated. The characterization of the hydrogenation behavior within the 180 °C–280 °C temperature range and the subsequent microstructural analysis elucidated that the amount of Al present in the hydride decreased with increasing temperature. This observation suggests that Al has very low solubility in magnesium hydride but Al can be accommodated in MgH₂ by processing under non-equilibrium conditions. Pressure–composition isotherms were developed at different temperatures for the Mg–Al powder as well as pure Mg powder. The results indicate that the enthalpy of formation was slightly lower for the Mg–8at%Al powder while the enthalpy of dissociation did not change. The absence of noticeable influence of Al addition on the stability of magnesium hydride is attributed to its lack of solubility.