Strengthening of Ti–6Al–4V alloys by thermohydrogen processing

The relationships among the hydrogen-induced phase transformation, grain refinement and improvement of mechanical properties of Ti–6Al–4V alloys are investigated. For this purpose, the decomposition of the α″ martensite and metastable β phase of hydrogenated Ti–6Al–4V alloys was investigated first. As a result, it is found that the key role leading to grain refinement is the decomposition of metastable phase. On the other hand, the precipitation of the TiH₂ hydride under lower temperature aging shows no significant refining effect after dehydrogenation. Mechanical properties of the materials after dehydrogenation show that the decomposition of the metastable phase strengthens the materials. In particular, specimens containing 0.45 wt% H in their fully martensites structures show a marked increase in strength. However, a fine grain and comprehensive mechanical properties are obtained only in specimens containing 0.8 wt% H.     

Effect of hydrogen on fracture behavior of Ti–6Al–4V alloy by in-situ tensile test

The fracture behaviors of non-hydrogenated, hydrogenated and dehydrogenated Ti–6Al–4V alloys were investigated by in-situ tensile test at room temperature. The distributions of stress and strain near the notch of in-situ tensile specimen were calculated using finite element method. Results indicate that hydrogen has an important effect on the fracture behavior of Ti–6Al–4V alloy. Crack initiation sites differ in the specimens treated by various procedures, and they are related to the stress intensity in specimens under different loads. The fracture mode of non-hydrogenated specimen is a ductile fracture by the initiation and coalescence of microvoids. The hydrogenated specimen shows a mixture of intergranular and transgranular brittle fractures. The dehydrogenated specimen is characterized by a mixture of intergranular and transgranular ductile fractures. The transition of fracture mode is attributed to the hydrogen atoms in solid solution and hydrides.     

Evolution of Ti2Ni precipitations during annealing and their effects on properties of Ti–Nb–Ni foil as PEMFC bipolar plates substrate

In this work, Ti–Nb–Ni foil was cold rolled and different annealing temperatures were conducted. For foils annealed at 600 °C, 650 °C and 700 °C, Ti₂Ni particles were evenly distributed in the matrix. Grain growth was obviously confined during annealing at these temperatures. While more (α+Ti₂Ni) eutectoids along grain boundaries (GBs) formed in the cases of higher temperatures. Plasticity of as-cold rolled foil was obviously improved by annealing and elongations gradually decreased with annealing temperature. Elongations of foil at 600 °C possessed the highest value, viz, 31.6% and 39.8% for RD and TD. For corrosion resistance, 850 °C annealed foil had the lowest i_corr compared with other counterparts. ICR of as-cold rolled and annealed specimens at 1.4 MPa were all lower than 10 mΩ cm⁻², which satisfied the DOE 2025 target.     

Influence of surface roughness on consecutively hydrogen absorption cycles in Ti–6Al–4V alloy

In the present work the influence of roughness of the material surface with hydrogen absorption in Ti–6Al–4V alloy during four hydrogenated cycles is studied. The Ti–6Al–4V alloy samples were hydrogenated during several cycles at 650 °C for two hours, in 50% hydrogen and 50% argon atmospheres, 1 atm pressure and a flux of 50 cm³/min each one. The hydrogen concentrations are measured using Elastic Recoil Detection Analysis technique; meanwhile the roughness is measured using an Atomic Force Microscope. X-ray Diffraction analysis shows changes in crystal orientation due to hydrogen absorption. The hydrogen capacity of the Ti–6Al–4V alloy is observed to be directly correlated to the surface quality of the sample during the first hydrogenation cycles, but in the fourth cycle, the hydrogen absorption is almost equal for all the samples independently of their surface roughness.     

Microstructural evolution and hydride precipitation mechanism in hydrogenated Ti–6Al–4V alloy

Use of hydrogen as a temporary alloying element in titanium alloys is an attractive approach for enhancing processability, and also for controlling the microstructure and improving final mechanical properties. In this study, the α + β titanium alloy, Ti–6Al–4V, was hydrogenated with hydrogen levels of 0.1, 0.3 and 0.5 wt%. The microstructure, phases and phase transformations were investigated by optical microscopy, X-ray diffraction and transmission electron microscopy. The results showed that the hydrogen addition had a noticeable influence on the microstructure of Ti–6Al–4V alloy. Hydrogen stabled the β-phase and leaded to the formation of hexagonal close packed α′ martensite as well as face-centered cubic δ hydride. Microstructural evolution and hydride precipitation mechanism in hydrogenated Ti–6Al–4V alloy was revealed.     

Effect of cell compression on the performance of a non–hot-pressed MEA for PEMFC

In this study, the effect of cell compression on the performance of a non–hot-pressed membrane electrode assembly (MEA) for a polymer electrolyte membrane fuel cell (PEMFC) is presented. The MEA is made without hot pressing, by carefully placing the gas diffusion electrodes (GDEs) and a membrane in a fuel cell fixture. Cell performance is assessed at five different compression ratios between 3.6% and 47.8%. It has been shown that ohmic resistance of the cell, mass transport resistance of reactants, charge transfer resistance at electrode, and overall cell performance are strongly dependent on the cell compression. On increasing the cell compression gradually, cell performance improves initially, reaches the best, and then deteriorates. The cell performance is assessed at fully humidified condition and at dry condition. Optimum cell performances are obtained at compression ratios of 14.2% and 25.7% for 100% relative humidity (RH) and 50% RH, respectively. It is also found that the cell with proper compression and at fully humidified conditions can deliver similar performance to a conventional hot-pressed MEA. Finally, it is shown that after the tests, GDEs can be peeled out, and the membrane inspection can be done as a postexperimental analysis.     

Effect of 0.3 wt% hydrogen addition on the friction stir welding characteristics of Ti–6Al–4V alloy and mechanism of hydrogen-induced effect

The α + β titanium alloy, Ti–6Al–4V, was thermohydrogen processed with 0.3 wt% hydrogen. The friction stir welding characteristics of Ti–6Al–4V alloy and the hydrogenated alloy with 0.3 wt% hydrogen were investigated in contrast. The results showed that welding parameters range for the unhydrogenated alloy was narrow and flash was prone to occur in the welding process. Hydrogenation could help to widen welding parameters range, improve weld appearance, and increase service life of pin tool. The reason was attributed to the hydrogen-enhanced thermoplasticity of titanium alloys, while the fundamental cause was the hydrogen-induced microstructural evolution in titanium alloys. Hydrogen almost did not escape from the hydrogenated alloy during the friction stir welding and could be successfully removed through post-weld vacuum annealing. The mechanism of hydrogen-induced effect during the friction stir welding and post-weld dehydrogenation was discussed in detail.     

Catalytic performance of Ni–Al nanoparticles fabricated by arc plasma evaporation for methanol decomposition

The catalytic properties of Ni-25 at% Al (Ni25Al) nanoparticles fabricated by arc plasma evaporation toward methanol decomposition were studied at temperatures ranging from 513 to 753 K. The Ni25Al nanoparticles showed much higher activity than gas atomized Ni25Al powders. They showed a high degree of selectivity for methanol decomposition into H₂ and CO. Side reactions such as methanation and water-gas shift reaction were suppressed to a high temperature of 673 K, which is hardly achieved for common Ni catalysts. Detailed characterization of the Ni25Al nanoparticles showed that they were composed of Ni, Ni₃Al, and Al₂O₃ phases with Ni and Al oxides on the surface of the Ni and Ni₃Al phases. The Ni oxides were reduced to Ni phase by a hydrogen reduction prior to methanol decomposition, while the Al oxides remained unchanged. It is supposed that the Ni phase provided the active sites for methanol decomposition, and the Ni₃Al and Al₂O₃ phases acted as supports for the Ni phase. Probably the Ni₃Al and Al₂O₃ phases provided good resistance to agglomeration of the Ni phase during the reaction, which might contribute to maintain the high catalytic performance of the nanoparticles for methanol decomposition.     

Corrosion resistant quaternary Al–Cr–Mo–N coating on type 316L stainless steel bipolar plates for proton exchange membrane fuel cells

Insufficient corrosion resistance, electrical conductivity and wettability of bipolar plates are some of the important issues affecting the performance of hydrogen fuel cells. To address these issues, an amorphous Al–Cr–Mo–N coating is deposited on type 316L stainless steel using direct current (DC) magnetron sputtering. The electrochemical corrosion behaviour is investigated under simulated fuel cell anode (H₂-purging) and cathode (air-purging) environment consisting of 0.5 M H₂SO₄ + 2 ppm NaF at 70 ± 2 °C. The corrosion current density is reduced to 0.02 μA cm⁻² comparable to the commercially used Ta/TaN coatings. The polarization resistance increases by two orders of magnitude and the interfacial contact resistance (ICR) reduces significantly due to the application of the coating. Further, the coating shows better water management due to high hydrophobicity than the bare stainless steel.     

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.     

Electrochemical impedance spectroscopy analysis of V–I characteristics and a fast prediction model for PEM-based electrolytic air dehumidification

PEM-based electrolytic air dehumidification is innovative due to its high efficiency, compact size and cleanness. However, high polarization loss and severe performance degradation have been observed, especially at high applied voltages (>2.5 V). Understanding the V–I characteristics is critical to performance optimization. This study experimentally investigated the V–I characteristics and internal response of materials under various operating conditions, with in-situ Electrochemical Impedance Spectroscopy (EIS) methods. Real-time mass transfer, electrochemical polarization and reaction dynamics of PEM components during dehumidification were derived by EIS. Then, a fast prediction model was built to directly predict the dehumidification rate and attenuation without any iteration, suitable for online monitoring and adjustment. Compared to other models, this model can take a quick understanding of the impact of operating conditions on the material characteristics inside the PEM element. The deviations of current density, PEM proton conductivity and moisture removal were 3%, 11.2% and 15.3%, respectively, compared to experiment data. Results showed that when the applied voltage changed from 1.5 to 3.5 V, the high-frequency resistance of the PEM element increased from 1.69 to 2.69 Ω, and the PEM proton conductivity decreased by about 38 times. The sharp drop in PEM proton conductivity resulted in a current attenuation. With this model, requirements for key components of PEM dehumidification were also obtained. Analysis of the overpotential distribution showed that increasing the water retention and reducing the dependence of proton conductivity on water molecules of the PEM can effectively improve the performance. This research provides guidance for the performance optimization and material selection of PEM-based dehumidification.     

Deposition of gold–titanium and gold–nickel coatings on electropolished 316L stainless steel bipolar plates for proton exchange membrane fuel cells

The effects of electropolishing and coating deposition on electrical resistance and chemical stability were studied for the stainless steel bipolar plates in proton exchange membrane fuel cell (PEMFC). A series of 316L stainless steel plates, selected as the substrate for a proton exchange membrane fuel cell (PEMFC) bipolar plate, were electropolished with a solution of H₂SO₄ and H₃PO₄ at temperatures ranging from 70 to 110 °C. The surface regions of the two electropolished stainless steel plates were coated with gold and either a titanium or nickel layer using electron beam evaporation. The electropolished stainless steel plates coated in 2-μm thick gold with a 0.1-μm titanium or nickel interlayer showed remarkably smooth and uniform surface morphologies in AFM and FE-SEM images compared to the surfaces of the plates that were coated after mechanical polishing only. The electrical resistance and water contact angle of the deposited stainless steel bipolar plates are strongly dependent on the surface modification treatments (i.e., mechanical polishing versus electropolishing). ICP-MS and XPS results indicate that after electropolishing, the coating layers show excellent chemical stability after exposure to an H₂SO₄ solution of pH 3. Finally, it was concluded that before coating deposition, the surface modification using electropolishing was very suitable for enhancing the electrical property and chemical stability of the stainless steel bipolar plate.     

The effects of plasma treatment on electrochemical activity of Co–W–B catalyst for hydrogen production by hydrolysis of NaBH4

A plasma treatment of Co–W–B catalyst increases the rate of hydrogen generation from the hydrolysis of NaBH₄. The catalytic properties of Co–W–B prepared in the presence of plasma have been investigated as a function of NaBH₄ concentration, NaOH concentration, temperature, plasma applying time, catalyst amount and plasma gases. The Co–W–B catalyst prepared with cold plasma effect hydrolysis in only 12 min, where as the Co–W–B catalyst prepared in known method with no plasma treatment in 23 min. The activation energy for first-order reaction is found to be 29.12 kJ mol⁻¹.     

Mechanical behavior of electrochemically hydrogenated electron beam melting (EBM) and wrought Ti–6Al–4V using small punch test

The influence of electrochemical charging of hydrogen at j = ?5 mA/cm² for 6, 12, 48 and 96 h on the structural and the mechanical behavior of wrought and electron beam melting (EBM) Ti–6Al–4V alloys containing 6 wt% β and similar impurities level was investigated. The length of the α/β interphase boundaries in the EBM alloy was larger by 34% compared to that in the wrought alloy. The small punch test (SPT) technique was used to characterize the mechanical behavior of the non-hydrogenated and hydrogenated specimens. It was found that the maximum load and the displacement at maximum load of the wrought alloy remained nearly stable after 6 h of charging, showing a maximum decrease of ～32% and 11%, respectively. Similarly, hydrogenation of the EBM alloy resulted in a gradual degradation in mechanical properties with charging time, up to ～81% and 86% in pop-in load and displacement at the “pop-in” load, respectively. The mode of fracture of the wrought alloy changed from ductile to semi-brittle with mud-cracking in all hydrogenated specimens. In contrast, the mode of fracture of the EBM alloy changed from a mixed mode ductile-brittle fracture to brittle fracture with star-like morphology. The degraded mechanical properties of the EBM alloy are attributed to its α/β lamellar microstructure which acted as a short-circuit path and enhanced hydrogen diffusion into the bulk as well as δ_a and δ_b hydride formation on the surface. In contrast, a surface layer with higher concentration of δ_a and δ_b hydrides in the wrought alloy served as a barrier to hydrogen uptake into the bulk and increased the alloy resistivity to hydrogen embrittlement (HE). This study shows that EBM Ti–6Al–4V alloy is more susceptible to mechanical degradation due to HE than wrought Ti–6Al–4V alloy.     

An improvement on cycling stability of Ti–V–Fe-based hydrogen storage alloys with Co substitution for Ni

In this work, the effects of Co substitution for Ni on the microstructures and electrochemical properties of Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.6Ni_1.25−xCo_xFe_0.2 (x = 0.00–0.25) alloys were investigated systematically by XRD, SEM and electrochemical measurements. The structural investigations revealed that the main phases of all of the alloys were the C14 Laves phase in a three-dimensional network and the V-based solid solution phase with a dendritic structure. The lattice parameters and unit cell volumes of the two phases gradually increased with the increase of Co concentration. The relative abundance of the C14 Laves phase slightly increased from 47.3% to 49.6%, accordingly that of the V-based solid solution phase decreased, with the increase of x from 0.00 to 0.25. The crystal grain of the V-based solid solution phase was obviously refined after Co substitution. The electrochemical investigations showed that the proper substitution of Co for Ni improved the cycling durability of the alloy electrodes mainly due to the suppression of both the pulverization of the alloy particles and the dissolution of the main hydrogen absorbing elements (V and Ti) into the KOH solution. The cycling stability of the alloy electrode with x = 0.1 was 79.8% after 200 cycles. However, the maximum discharge capacity (C_max) was decreased from 340.5 to 305.6 mAh g⁻¹, and the high rate dischargeability (HRD) gradually decreased from 66.8% to 55.0% with increasing x from 0.00 to 0.25.     

Study of the influence of the amount of PBI–H3PO4 in the catalytic layer of a high temperature PEMFC

The influence of the amount of polybenzimidazole (PBI)-H₃PO₄ (normalized with respect to the PBI loading, which expressed as C/PBI weight ratio) content in both the anode and cathode has been studied for a PBI-based high temperature proton exchange membrane (PEM) fuel cell. The electrodes prepared with different amounts of PBI have been characterized physically, by measuring the pore size distribution, and visualizing the surface microstructure. Afterwards, the electrochemical behaviour of the electrodes has been evaluated. The catalytic electrochemical activity has been measured by voltamperometry for each electrode prepared with a different PBI content, and the cell performance results have been studied, supported by the impedance spectra, in order to determine the influence of the PBI loading in each electrode. The best results have been achieved with a C/PBI weight ratio of 20, for both the anode and the cathode. A lower C/PBI weight ratio (larger amount of PBI in the catalytic layer) reduced the electrocatalytic activity, and impaired the mass transport processes, due to the large amount of polymer covering the catalyst particle, lowering the cell performance. A higher C/PBI weight ratio (lower amount of PBI in the catalytic layer) reduced the electrocatalytic activity, and slightly increased the ohmic resistance. The low amount of the polymeric ionic carrier PBI–H₃PO₄ limited the proton mobility, despite of the presence of large amounts of “free” H₃PO₄ in the catalytic layer.     

Coke resistance of Ni-based catalysts enhanced by cold plasma treatment for CH4–CO2 reforming: Review

Carbon dioxide reforming of methane can reduce emissions of greenhouse gases, and has been extensively studied. The conventional Ni-based catalysts easily coke, sinter, and deactivate in the CRM reaction. The studies suggested that the cold plasma treatment can improve the structure of Ni-based catalysts, and so enhance coke resistance of the catalysts. The review summarized the effect of cold plasma treatment on the coke resistance of Ni-based catalysts for the CRM reaction. The main goal of the paper was to illuminate: the structure change of catalysts treated by cold plasma, such as crystal planes of Ni particles, the particles size of Ni, the Ni dispersion, the metal-support interaction, and CO₂ absorption capacity; the correlation between plasma treatment conditions (treatment way and parameters) and coke resistance of the catalyst treated by cold plasma; and the mechanism of plasma treatment to improve the coke resistance of the catalysts.