Effect of hydrogen on the microstructure and mechanical properties of high temperature deformation of Ti6Al4V additive manufactured

The additive manufactured Ti6Al4V-xH titanium alloy was compressed at 600°C–750 °C on a Gleeble 3800 testing machine, and the compression rates were 1s⁻¹ and 0.01s⁻¹, respectively. The experimental results show that with the increase of hydrogen content, the flow stress of the alloy decreases firstly and then increases gradually. When the hydrogen content is 0.27 wt%, the flow stress of titanium alloy is the smallest. EBSD and TEM analysis were carried out and show that the α lamellar microstructure became larger at 0.27H, the corresponding flow stress also decreased, and slip bands appeared in the alloy. Dislocation slip was an important deformation mechanism of the alloy. When the hydrogen content continued to increase, the α phase in the alloy gradually decreased, and α″ appeared at 0.81H. Therefore, adding appropriate hydrogen can reduce the alloy flow stress and improve the performance of titanium alloy during hot deformation.     

Room-temperature compressive properties of TC21 alloy processed by thermohydrogen treatment at 1123 K

The influence of hydrogen level on compressive properties of TC21 alloy processed by thermohydrogen treatment (THT) at 1123 K was studied through room-temperature compression experiments. The microstructures of TC21 alloy processed by THT at 1123 K were investigated by OM, XRD, and TEM. Results showed that hydrogen level affected apparently the microstructures and compressive properties of TC21 alloy processed by THT at 1123 K. With increasing level of hydrogen, the yield strength of TC21 alloy decreased to a minimum initially and then increased; the deformation limit of TC21 alloy decreased slightly to a minimum first, then increased to a maximum, and ultimately decreased. At a hydrogen level of 0.78 wt%, the deformation limit of TC21 alloy was the highest among all THT-processed TC21 alloys and showed an increase of 244.33% compared with the original TC21 alloy.     

Gradient microstructure of TC21 alloy induced by hydrogen during hydrogenation

Through melt hydrogenation, a gradient microstructure (α″ + α′)/(α + β_H) has been observed in TC21 alloy. The addition of hydrogen induces martensite transformation and increases the volume fraction of β. It is found that the absorption process of hydrogen atoms can be divided into melting and cooling stages. During cooling, the continuous absorption of hydrogen and the corresponding decrease of freezing point of melt extend solidification time of melt and lead to hydrogen enrichment in the upper of the specimen, which induces the formation of the gradient structure. The hydrogenated TC21 alloy shows higher thermoplasticity compared with the unhydrogenated TC21 alloy. The flow stress of the upper part of the hydrogenated alloy is lower than that of the center part. A gradual variation has been observed in the microhardness along the gradient direction due to variation in the microstructure. The microhardness of the upper surface drops about 45% with 14.6 at.%H.     

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 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.     

Strengthening versus softening mechanisms by hydrogen addition in β-Ti40 alloy

The effects of hydrogenation (≈0–0.9 wt.%) on the flow stress behavior and microstructural evolution of Ti40 (Ti–25V–15Cr–0.2Si) alloys during hot deformation are investigated. Isothermal hot compression tests are performed in the temperature range of 1023–1223 K with a strain rate of 0.01 s⁻¹. The stress–strain curves of the Ti40-xH alloys are recorded and the relationship between steady-state stress and hydrogen content at different temperatures is determined. With an increase of hydrogen content, the steady-state flow stress initially increases, and then decreases before increasing again at higher hydrogen contents. This behavior implies that hydrogen exerts a strengthening effect at low and high concentrations and a softening effect at intermediate concentrations. The strengthening mechanism is explained in terms of solid solution strengthening of hydrogen, fine grain strengthening caused by hydrogen induced DRX (dynamic recrystallization) and precipitation strengthening of silicide. The softening mechanism is explained in terms of DRX, grain strength softening caused by grain growth and a continuous reticular precipitation of silicide.     

Hot-deformation behaviour and hot-processing map of melt-hydrogenated Ti6Al4V/(TiB+TiC)

The hydrogen was straight-forward added to the Ti6Al4V/(TiC + TiB) composites (TiCTiB=5 vol.%) by melt hydrogenation. The results of hot compression show that the peak resistance of titanium matrix composites (TMCs) decreased by 17.2% when hydrogen content was 0.035 wt% compared with the TMCs without hydrogen. Therefore, the TMCs with a hydrogen content of 0.035 wt% was performed to a thermal compression experiment. Thermal-deformation characteristics and hot processing map of TMCs with a hydrogen content of 0.035 wt% were analyzed in the light of the flow stress curve, constitutive relations, and the dynamic-material model. The computed apparent activation energy was 284.54 kJ/mol, and the corresponding strain-rate sensitivity, power dissipation, and instability parameter were calculated. The hot-processing map exhibited maximum efficiencies of power dissipation at 780–840 °C/0.005–0.06 s⁻¹ and there was only one instable region. The microstructures corresponding to the stable and instable region were verified, confirming the optimum processing parameters of hot-working that can be used as a reference for hot deformation of hydrogenated composites.     

The influence of melt hydrogenation on Ti600 alloy

In this paper, melt hydrogenation, which is a new hydrogen treatment method, was used to hydrogenate Ti600 alloy, the relationship between the hydrogen partial pressure and the hydrogen content was built by analyzing experimental data, and microstructure was observed and mechanical properties was tested. It was found that hydrogen addition made the thickness of the solidified shell thinner and a big temperature gradient existed from the top to bottom surface of the alloy melt. With increasing hydrogen partial pressure, the directional solidification structure can be formed in the Ti600 alloy ingots. Microstructure of Ti600 alloy was modified significantly and the amount of α′ and β phases after melt hydrogenation. When increasing the content of hydrogen to 7.2 at.%, γ hydride was obtained in Ti600 alloy. The flow stress and yield stress decrease with increasing the hydrogen content.     

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.     

Hydrogen storage in activated carbons produced from coals of different ranks: Effect of oxygen content

20 activated carbons (ACs) were prepared by activation of four coals of different ranks (bituminous, low-ash bituminous and sub-bituminous coals, and one anthracite) with potassium hydroxide, in order to evaluate their hydrogen storage capacities at −196 °C. The effect of surface area and oxygen content on hydrogen storage was examined. Oxygen content was determined by temperature-programmed desorption. The significance of oxygen content on hydrogen storage capacity was evaluated by Analysis of Variance (ANOVA). Apparent surface areas higher than 3000 m² g⁻¹ and hydrogen adsorption as high as 6.8 wt.% were obtained. The best results were obtained with ACs from bituminous coals. No significant effect of oxygen content on hydrogen adsorption was observed. We concluded that surface area controls hydrogen storage capacity at −196 °C.     

Modeling and analysis of microchannel autothermal methane steam reformer focusing on thermal characteristic and thermo-mechanically induced stress behavior

The application of fuel cells boosts the hydrogen demand particularly for distributed hydrogen production facility. As a potential candidate of hydrogen supply, microchannel autothermal methane steam reactor operates at high temperature and results in high thermal impact, which would decrease its stability and lifespan. A three-dimension numerical model based on finite element method was developed to evaluate the thermal characteristic and thermo-mechanically induced stress behavior of the reactor. Three different potential manufacturing materials, Fe–Cr–Al alloy, ceramic and quartz, were chosen. The results indicate that the cold-spot temperature appears near reactor inlet while the hotspot temperature appears near reactor outlet for reactor manufactured by different materials. Corresponding to the hot spot temperature, the maximum Von Mises stress appears near reactor outlet. The difference is the maximum Von Mises stress appears in catalyst layer for quartz reactor while it appears in interconnect rib for both Fe–Cr–Al alloy reactor and ceramic reactor. Meanwhile the maximum Von Mises stress reaches 1830 MPa for ceramic reactor. While the maximum Von Mises stress is 1197 MPa for quartz reactor and 1760 MPa for Fe–Cr–Al alloy reactor respectively. It implies the outlet catalyst layer region is vulnerable for quartz reactor. While the outlet interconnect rib is most vulnerable for both Fe–Cr–Al alloy reactor and ceramic reactor.     

Low-temperature direct diffusion bonding of hydrogenated TC4 alloy and GH3128 superalloy

Bonding at high temperatures can cause many problems, such as an induction of high stress, grain coarsening and strict requirements for bonding equipments, etc.. In this paper, a hydrogenation approach was utilized for the TC4 alloy before the dissimilar materials bonding process. Effects of hydrogen contents on the diffusion behavior of the TC4/GH3128 joints were investigated. Particularly, the mechanism that the hydrogenation affected the low-temperature bonding process of the TC4/GH3128 joints was discussed. By regulating the bonding temperatures, holding durations and hydrogen contents, a maximum of 92 MPa was achieved. The formation mechanism of the diffusion bonded TC4/GH3128 joint was proposed. This novel metal hydrogenation idea can offer new insights on the development of the low-temperature joining particularly suitable for dissimilar materials joining.     

Effect of pore orientation on the catalytic performance of porous NiMo electrode for hydrogen evolution in alkaline solutions

Porous NiMo alloys with Mo content of 5 at.% were fabricated by freezing casting method. The pores are elongated, and the media pore size is 8.1 μm. The electrocatalytic activity of the synthesized NiMo alloy foam as cathodes for hydrogen evolution reaction (HER) in 6.0 M potassium hydroxide solution was investigated. Results show that the electrodes which pore orientation is parallel to the hydrogen overflow direction present higher electrocatalytic activity than the electrodes with pore orientation perpendicular to the hydrogen overflow direction. The Tafel slope is 94 and 117 mV dec⁻¹, respectively at a current density of 10 mA/cm² at room temperature.     

The effect of hydrogen on phase transformation and mechanical properties of a β containing γ–TiAl based alloy

The effect of hydrogen on phase transformation and mechanical properties of the unhydrogenated Ti–45Al–5Nb–0.8Mo–0.3Y (in at.%) alloy and Ti–45Al–5Nb–0.8Mo–0.3Y alloy with 1.5 at.% hydrogen addition is investigated in the temperature range 1323–1473 K with a strain rate of 0.01 s⁻¹. The results show that the flow stresses of the hydrogenated alloy are lower than those of the unhydrogenated alloy. H as an interstitial atom occupies B2 phase tetrahedral interstice. The clearance radius of tetrahedron of B2 phase is decreased by approximately 4.6% after B2→ω_o transformation. So, H can impede such phase transformation by means of impeding the decrease of clearance radius of tetrahedron. The volume fractions of B2 phase of the hydrogenated alloy are more than those of the unhydrogenated alloy under all deformation conditions due to hydrogen-stabilized β phase. The lower flow stress of the hydrogenated alloy compared with the unhydrogenated alloy is mainly attributed to hydrogen-increased B2 phase, hydrogen-induced twinning and stacking faults of γ phase, and hydrogen-impeded formation of ω_o and α₂ phases.     

Effect of thermal deformation on leakage clearance of claw hydrogen circulating pump for fuel cell system

The leakage clearance have a strong impact on the performance and reliability of hydrogen circulating pump in a fuel cell system, and the thermal deformantion is the most significant factor because of its high working temperature, small size. In this paper, the distribution of leakage clearance with different temperatures and rotors materials is obtained. The radial leakage clearance decreases with the increase of temperature in the working chamber. The axial clearance is increased by about 100  μm at rated working condition. When the material of rotors is aluminum alloy, the minimum radial clearance under rated working condition is about 8.5 μm, and the risk of interference is large. When the material of rotors is structural steel and titanium alloy, the radial clearance is maintained above 30 μm, and the risk of interference is small. The variation of leakage clearance leads to the variation of leakage area. The variation of total leakage clearance first increases and then decreases with the increase of angle. The minimum variation of total leakage area is 6.25  mm² at 0° and the maximum is 7.578  mm² at 84°. The total leakage area increased by 7.144 mm² on average. The results can be used as guidelines for the structural optimization of hydrogen circulating pump.     

Differences between internal and external hydrogen effects on slow strain rate tensile test of iron-based superalloy A286

To investigate the evaluation method of hydrogen compatibility of A286 superalloy in high pressure hydrogen gas, SSRT tests of hydrogen-charged specimens were conducted at ambient temperature at various strain rates. The relative reduction in area (RRA), one of the ductility parameters, was determined. The hydrogen content in the hydrogen-charged specimen was the same as the equilibrium hydrogen content on the specimen surface at 150 °C in 70 MPa hydrogen gas. The strain rate dependence of RRA was smaller than that of RRA obtained in 70 MPa hydrogen gas at 150 °C. All the hydrogen-charged specimens showed slip-plane fractures in the grains in their cores. However, the specimens in 70 MPa hydrogen gas at 150 °C showed fracture surfaces morphology ranging from dimples to quasi-cleavages and intergranular fractures with decreasing strain rate. These dissimilarities are expected to arise from differences in the hydrogen concentration behaviors of the specimens during the deformation process.     

Hydrogen storage thermodynamic and dynamic properties of as-milled CeMgNi-based CeMg12-type alloys

Ni was chosen to partially substitute the Mg of alloys to investigate the effect on hydrogen storage dynamics of NdMg₁₂-type alloy. The amorphous and nanocrystalline alloys were synthesized by mechanical milling technology based on CeMg₁₁Ni + x wt% Ni (x = 100, 200) alloys. This paper systematically narrates and investigates the influences of Ni content and milling duration on hydrogen storage performance. Sievert apparatus and differential scanning calorimetry (DSC) were utilized for investigating the de-/hydriding performances of samples. Both Arrhenius and Kissinger methods were utilized in this paper for estimating the dehydrogenation activation energy of hydrides, and found that enhancing Ni content can decrease the thermodynamic parameters (ΔH and ΔS) of alloys slightly and improve the dehydriding dynamics significantly. Furthermore, the hydrogen storage property can be affected significantly by adjusting milling time. With varying milling time, the hydrogen storage capacities can reach the maximum values of 5.691 and 5.904 wt% for x = 100 and 200 alloys separately. The hydrogen absorption saturation ratio (R^a(10)) at 573 K and 3 MPa also obtains maximum values with the variation of milling time, namely 90.17% and 99.32% for x = 100 and 200 alloys separately. The hydrogen desorption ratio (R^d(20)) always increases with milling time increasing. To be specific, prolonging milling time from 5 to 60 h results in the increase of R^d(20) at 593 K from 37.55% to 47.21% for x = 100 alloy and 47.29%–61.70% for x = 200 alloy.     

Effects of ball milling time on the microstructure and hydrogen storage performances of Ti21.7Y0.3Fe16Mn3Cr alloy

TiFe alloy can store hydrogen at room temperature and low hydrogen pressure, and its theoretical hydrogen storage capacity is up to 1.8 wt%. However, TiFe alloy needs to be activated at high pressure (5 MPa hydrogen) and high temperature (673–723 K), which limits the practical application of TiFe alloy. The as-cast Ti_21.7Y_0.3Fe₁₆Mn₃Cr alloy was milled for 0, 0.5, 0.75, 1, and 3 h to study the effects of ball milling on phase structures and hydrogen storage performances. Emphasis was focused on the activation process of as-milled alloys at different temperatures, including the activation process at 483, 443, and 403 K. The results show that the alloys were consisted of TiFe phase, and [Fe, Cr] solid solution. The nanocrystalline boundary produced by milling and the phase boundary provided by the second phase provide a large number of channels for hydrogen diffusion and promote the improvement of hydrogen storage performances. The time required for activation process of as-milled alloys was significantly reduced, and the activation time of as-milled (0.75 h) was only 4 min, and its enthalpy variation for hydrogen absorption and desorption was 22.943 and 26.215 kJ mol⁻¹ H₂, respectively.     

Novel nanocomposite materials for oxygen and hydrogen separation membranes

Design of oxygen and hydrogen separation membranes is the point of current interest in producing syngas from biofuels. Nanocomposites with a high mixed ionic-electronic conductivity are known to be promising materials for these applications. This work aims at studying performance of oxygen and hydrogen separation membranes based on nanocomposites PrNi_0.5Co_0.5O_3-δ + Ce_0.9Y_0.1O_2-δ and Nd_5.5WO_11.25-δ + NiCu alloy, respectively. A high and stable performance promising for the practical application was demonstrated for these membranes. For oxygen separation membrane CH₄ conversion is up to 50% with H₂ content in the outlet feed being up to 25% at 900 °C. For reactor with hydrogen separation membrane complete EtOH conversion was achieved at T ∼ 700 °C even at the highest flow rate, and a high hydrogen permeation (≥1 ml H₂ cm⁻² min⁻¹) was revealed.     

Hydrogen generation through rolling using Al–Sn alloy

Mechanically treated aluminum-tin (Al–Sn) alloy, a novel hydrogen-generating material, was fabricated and found to react directly and immediately with water at room temperature. The maximum yield of hydrogen per unit volume of alloy was 2259 mL/cm³ (0.202 g/cm³). The mass ratio of the generated hydrogen and the Al–Sn alloy material was 4.86%. This percentage is much higher than that of traditional hydrogen storage alloys and can compete with metal hydrides. The combination of Al–Sn alloy powder and carbon nanotubes (CNTs) produced a new kind of Al–Sn/CNT composite that also reacts with water at room temperature. Al–Sn/CNT composites were synthesized using a high temperature and high-pressure method. When CNT content was held constant, composites with single-walled CNTs had higher reaction rates than those with multi-walled CNTs. The effects of mechanical treatment and CNT addition on enhancing the reaction between Al–Sn alloys or Al–Sn/CNTs and water were also analysed.