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.     

Thermal decomposition of titanium hydrides in electrochemically hydrogenated electron beam melting (EBM) and wrought Ti–6Al–4V alloys using in situ high-temperature X-Ray diffraction

Thermal decomposition of titanium hydrides in electrochemically hydrogenated electron beam melting (EBM) and wrought Ti–6Al–4V alloys containing 6 wt% β is compared. Differential scanning calorimetry (DSC) is used to identify phase transitions. High-temperature X-ray diffraction (HTXRD) is used to identify phases and determine their contents and crystallographic parameters. Both alloys are found to contain α_H (hcp) and β_H (bcc) solid solutions, as well as δ_a (fcc) and δ_b (fcc) hydrides after hydrogenation. δ_a is found to decompose between room temperature and 350 °C to α_H (in both alloys) plus either β_H and δ_b (wrought alloy) or δ_b only (EBM alloy). δ_b fully decomposes at either 450 °C (wrought alloy) or 600 °C (EBM alloy) to α_H plus H₂ desorption (which starts at 300 and 350 °C in the wrought and EBM alloys, respectively). In the case of the wrought alloy, β_H is also formed in this decomposition reaction due to faster diffusion of hydrogen. The non-continuous, finer needle-like morphology of the β-phase in the as-printed EBM alloy combined with its smaller lattice constants seem to inhibit hydrogen diffusion into the bulk alloy through the β-phase, thus triggering δ_a dissociation into δ_b (rather than to β_H+δ_b) and δ_b decomposition into α_H (rather than to α_H + β_H). Hydrogen incorporation in the α_H phase results in its expansion in the c direction in both alloys. HTXRD allows to conclude that both δ_a and δ_b hydrides decompose up to 600 °C. Hydrogen peaks measured at higher temperatures are due to hydrogen desorption from the hydride that is decomposed from the sample's bulk and/or hydrogen desorption from β_H and/or α_H during heating. These findings indicate that the EBM Ti–6Al–4V alloy might be more prone to hydrogen damage at elevated temperatures than its wrought counterpart when both have a similar β-phase content.     

Mechanism of hydrogen-induced defects and cracking in Ti and Ti–Mo alloy

Mechanism of Hydrogen-induced defects and cracking in Ti and Ti–Mo alloy was systematically analyzed from the perspective of microstructure. Results show that more hydrogen atoms can be dissolved in Ti–Mo alloy due to the existence of β phase, and the precipitation amount of hydride is reduced under the same hydrogen charging conditions. The analysis of positron annihilation results show that the solid solution of Mo element reduces the hydrogen induced defect concentration in the alloy and inhibit the combination of hydrogen and defects. At the same time, Mo atom can affect the electronic structure of Titanium hydrides, and then reduce the bonding between titanium atoms and hydrogen atoms, thereby reducing the formation of brittle titanium hydride. The reduction of hydride and hydrogen induced defect concentration significantly decrease the hardening rate of Ti–Mo alloy, thereby effectively reducing the hydrogen embrittlement sensitivity of titanium and inhibiting its hydrogen absorption cracking tendency.     

High throughput optical characterization of alloy hydrogenation

Transition from the metallic to the hydride phase is of fundamental importance to achieving hydrogen storage in the solid state. Multi-component metal hydrides belong to one of the promising categories of materials that can potentially offer high hydrogen storage capacity. Despite extensive research on metal hydrides over the past decades, the progress remains limited partly due to the inability of screening a nearly infinite number of possible alloy compositions. High throughput materials fabrication and characterization techniques therefore offer an advantage in studying multi-component alloys and their phase transition to metal hydrides. We fabricated an Mg–Ni–Al and Ca–B–Ti ternary alloy libraries using a continuous combinatorial material synthesis technique, and measured the optical reflectance to examine the formation of metal hydride phase when the alloy library was exposed to hydrogen. The results indicate that mapping the change in reflectance is a viable method to study the kinetics of hydride formation. Monitoring the optical properties provides evidence for the “black state” formed during the transition from α-phase to β-phase. In addition, we found that the fastest reflectance change occurred when the alloy has an Mg to Ni ratio of approximately 2:1, and with low concentration of Al.     

Enhanced hydrogen storage properties of Ti–Cr–Nb alloys by melt-spin and Mo-doping

Ti–Cr–Nb hydrogen storage alloys with a body centered cubic (BCC) structure have been successfully prepared by melt-spin and Mo-doping. The crystalline structure, solidification microstructural evolution, and hydrogen storage properties of the corresponding alloys were characterized in details. The results showed that the hydrogen storage capacity of Ti–Cr–Nb ingot alloys increased from 2.2 wt% up to around 3.5 wt% under the treatment of melt-spin and Mo-doping. It is ascribed that the single BCC phase of Ti–Cr–Nb alloys was stabilized after melt-spin and Mo-doping, which has a higher theoretical hydrogen storage site than the Laves phase. Furthermore, the melt-spin alloy after Mo doping can further effectively increase the de-/absorption plateau pressure. The hydrogen desorption enthalpy change ΔH of the melt-spin alloy decreased from 48.94 kJ/mol to 43.93 kJ/mol after Mo-doping. The short terms cycling test also manifests that Mo-doping was effective in improving the cycle durability of the Ti–Cr–Nb alloys. And the BCC phase of the Ti–Cr–Nb alloys could form body centered tetragonal (BCT) or face center cubic (FCC) hydride phase after hydrogen absorption and transform to the original BCC phase after desorption process. This study might provide reference for developing reversible metal hydrides with favorable cost and acceptable hydrogen storage characteristics.     

Formation Ti6Al4V alloy by hydride cycle mode and its (Ti6Al4V)H1.606 hydride in self-propagating high-temperature synthesis mode

The wide use of titanium alloys is limited by high costs of both their producing technology and processing. The expansion of area of their application depends on development of more efficient, resource-saving, economical methods providing reduction in the cost of titanium alloys and titanium made products. The hydride cycle (HC) method for synthesis of binary and multicomponent refractory alloys and hydrides can be suitable alternative to traditional metallurgy for production of the alloy in small quantities (small-sized products for technical/medical purposes). This work presents the results of study on the mechanism of Ti–6Al–4V alloy formation in HC. The synthesis parameters are evaluated and optimized. The physicochemical, structural and hydrogen-sorption characteristics of the alloy samples are described. Actually, a resource-saving, efficient technology for synthesis of Ti–6Al–4V alloy by HC is developed and proposed. The synthesized alloy interaction with hydrogen in Self-Propagating High-Temperature Synthesis mode was investigated.     

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.     

Structural characterization and hydrogen storage properties of the Ti31V26Nb26Zr12M5 (M = Fe,Co, or Ni) multi-phase multicomponent alloys

Structure and hydrogen storage properties of three Ti₃₁V₂₆Nb₂₆Zr₁₂M₅ multicomponent alloys with M = Fe, Co and Ni are investigated. The alloys synthesized by arc melting are characterized via X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The as-cast ingots present multi-phase dendritic structures composed mainly of BCC phases and small amounts of C14 Laves phases. Upon hydrogenation, each alloy absorbs around 1.9 H/M (number of hydrogen atoms per metal atoms) at room temperature. XRD of fully hydrogenated samples shows the formation of multi-phase structures composed of FCC and C14 hydrides. Thermo Desorption Spectroscopy (TDS) shows that the hydrogenated alloys present multi-step desorption processes with wide temperature ranges and low onset temperatures. XRD of partially hydrogenated samples indicate the presence of intermediate BCC hydrides. XRD of desorbed samples suggest reversible reactions of absorption/desorption: BCC + C14 alloy ? intermediate BCC hydride + C14 hydride ? FCC + C14 hydrides.     

Synthesis and hydrogen storage behavior of Mg–V–Al–Cr–Ni high entropy alloys

The ternary MgVAl, MgVCr, MgVNi, quaternary MgVAlCr, MgVAlNi, MgVCrNi and quinary MgVAlCrNi alloys were produced by high energy ball milling (HEBM) under hydrogen pressure (3.0 MPa) as a strategy to find lightweight alloys for hydrogen storage applications. Most of the ternary and quaternary alloys presented multiphase structure, composed mainly of body-centered cubic (BCC) solid solutions and Mg-based hydrides. Only the quinary MgVAlCrNi high entropy alloy (HEA) formed a single-phase structure (BCC solid solution), which is a novel lightweight (ρ = 5.48 g/cm³) single-phase HEA. The hydrogen storage capacity of this alloy was found to be very low (approximately 0.3 wt% of H). Two non-equiatomic alloys with higher fraction of Mg and V (strong hydride former elements), namely Mg₂₈V₂₈Al₁₉Cr₁₉Ni₆ and Mg₂₆V₃₁Al₃₁Cr₆Ni₆, were then designed, aiming at higher storage capacity. Both alloys were produced by HEBM. The results show that the non-stoichiometric alloys also presented low hydrogen storage capacity. The low affinity of these alloys with hydrogen was discussed in terms of enthalpy of hydrogen solution and enthalpy of hydride formation of the single components. This study brought to light the importance of considering both enthalpy of hydrogen solution and enthalpy of hydride formation of the alloying elements for designing Mg-containing HEA for hydrogen storage. Once Mg has a positive enthalpy of hydrogen solution, the alloys composition must be balanced with alloying elements with higher hydrogen affinity, i.e., negative values of enthalpy of solution and hydride formation.     

Effect of Zr7Ni10 additive on hydrogen mobility in (TiCr1.8)1-xVx (x = 0.2, 0.4, 0.6, 0.8): An 1H NMR SFG study

In order to optimize hydrogen storage properties of bcc Ti–V–Cr alloys it was found that alloying with a few 4 at% of Zr₇Ni₁₀ results in acceleration the hydrogen sorption kinetics in the composite material. The novel intergranular phase plays a role of gate for hydrogen, leading parallel to its easy decrepitate, thus enhancing fast formation of Ti–V–Cr hydrides. Nevertheless, the question on how such a composite microstructure affects hydrogen mobility in the material is still open. Here we report on the results of the studies of hydrogen self-diffusion in hydrogenated (TiCr_1.8)_1-xV_x based alloys (x = 0.2, 0.4, 0.6 and 0.8) carried out using proton nuclear magnetic resonance diffusiometry in a static field gradient_. For all compounds the method has proved itself as a powerful tool to probe the microstructure of the multicomponent alloys with inhomogeneous element distribution in a few micrometer scale. It has been found that addition of Zr₇Ni₁₀ lowers the activation energy of hydrogen motion in (TiCr_1.8)_1-xV_x alloys and leads appear two different diffusion areas. They can be associated with a redistribution of elements within the intra-granular phase due to opposite substitution of Ti and Ni atoms during synthesis and blurring the boundaries between the intra-granular and inter-granular phases.     

Effect of yttrium addition on microstructure and orientation of hydride precipitation in Zr-1Nb alloy

The purpose is to investigate the role of yttrium addition in improving hydride embrittlement resistance in Zr-1Nb alloy, because yttrium addition has been widely used to improve oxidation resistance of alloys. The results suggest that 0.2 wt.% yttrium addition can effectively lower hydrogen absorption in Zr-1Nb alloy. The microstructural characterization shows that the precipitated hydrides are interlinked to have a width of 200–600 nm and a shape of lamellar or “dendritic”. The electron backscattered diffraction (EBSD) results show that the crystallographic orientation relationship between precipitated hydrides and zirconium matrix is (0001)α-Zr//{111}δ-H_0.62Zr_0.38 or (0001)α-Zr//{100}δ-H_0.62Zr_0.38, depending on the position and morphology of hydrides. The depression of hydrogen absorption is attributed to the reduction of transgranular hydride precipitation by yttrium addition. This study provides an experimental basis for designing new zirconium alloys with enhanced hydrogen absorption resistance.     

Effects of alloying elements (Al,Mn, Ru) on desorption plateau pressures of vanadium hydrides: An experimental and first-principles study

Vanadium-based alloys are promising for the reversible and compact storage of renewable hydrogen. To improve the hydrogen desorption plateau pressure of vanadium hydrides, V–3A binary alloys of V₉₇Al₃, V₉₇Mn₃, and V₉₇Ru₃ were prepared by vacuum arc melting. Hydrogen absorption and desorption properties of the newly prepared V–3A alloys were studied, and compared to vanadium hydrides, by pressure-composition-temperature measurements and first-principles calculations. Among the studied materials, V–3Ru showed the highest hydrogen desorption plateau pressures between T = 373 and 433 K. During hydrogen loading, V–3Ru also reached the highest hydrogen to metal ratio between the alloys. Meanwhile, V–3Ru was also the alloy with the highest hardness. Findings of hydrogen absorption and desorption measurements were supported with DFT calculations of hydride formation energies. The calculated DOS, ELF and atomic charges were used to study the effects of alloying on the electronic properties of hydrides. The bonding interactions in vanadium dihydrides were influenced the most through Al alloying, whereas V–3Mn and V–3Ru hydrides showed comparable electronic properties.     

Thermodynamics,kinetics and microstructural evolution of Ti0.43Zr0.07Cr0.25V0.25 alloy upon hydrogenation

Zr substituted Ti₂CrV alloy with Ti_0.43Zr_0.07Cr_0.25V_0.25 composition was synthesized by arc melting method and its crystal structure, microstructure and hydrogen storage performance were investigated. XRD and microstructural analyses confirmed that the alloy forms Laves phase related BCC solid solution. The enthalpy of hydride formation as derived from pressure composition absorption isotherms is ?56.33 kJ/mol H₂. The desorption temperature of the hydride is significantly lower (by ～50 K) than that of Ti₂CrV hydride indicating lower thermal stability of the hydride compared to its unsubstituted analogue. The alloy shows better cyclic stability over the unsubstituted one. This work also offers mechanistic insight into hydrogen absorption reaction of Ti_0.43Zr_0.07Cr_0.25V_0.25 alloy by analyzing the hydriding kinetics data with standard kinetic models. The rate-determining steps of hydrogen absorption reaction were identified as random nucleation and growth of hydride followed by 1D and 3D diffusion of hydrogen atoms through the hydride layer. The present study is expected to provide valuable information for the better development of Ti–Cr–V based hydrogen storage alloys.     

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.     

Tailoring the activation behaviour and oxide resistant properties of TiFe alloys by doping with Mn

Ti–Fe alloy is the most investigated material for H₂ storage, however, the poor activation kinetics and surface oxide formation limits its practical application. Herein, Mn substituted Ti–Fe alloys are investigated for hydrogen storage application and the effect of air exposure on their performance is evaluated. The alloys were synthesized using arc melting method and characterized for structure, composition and morphology analysis. The XRD analysis confirmed the partial substitution of Fe by Mn in the TiFe_1-xMn_x alloys. The activation kinetics of the alloys are improved by Mn substitution, and the rate of reaction increased with Mn concentration. The desorption PCIs showed a distinct but dual plateau for the low Mn content and the slope of plateau increased with Mn content. The surface oxide layer formation upon air exposure was analysed by XPS technique. The combined XRD and XPS results illustrated a thin surface oxide layer formation. It was also observed that Mn acts as a sacrificial element to prevent the bulk oxidation of alloys. The overall study depicts synergetic effect of Mn addition on hydrogen absorption kinetics of TiFe_1-xMn_x alloys.     

Metal hydride storage for mobile and stationary applications

Metal hydrides offer the possibility of a convenient and safe method for the storage of hydrogen. These compounds provide for compact storage in a form that is equal to or better than cryogenic liquid hydrogen on a volume basis. Considerable research has gone into the study of hydrides derived from rare earth, iron-titanium and magnesium alloys. The formation of these compounds is reversible and the chemistry of relevant hydrides has been discussed. Heat must be provided to decompose these compounds and release the hydrogen, while heat is liberated when the compounds are formed and must be removed to allow the hydriding reactions to proceed to completion.The iron-titanium and magnesium alloys are especially promising hydride storage media, the former in stationary applications, or where weight is not a limiting consideration, and the latter for mobile applications, Each of these materials has unique pressure-temperature characteristics and reaction kinetics which must be considered in the design of a hydrogen storage system. These special characteristics are discussed for particular applications. The results of recent work on hydrogen storage development and the engineering design of storage systems are reviewed.     

First-principle modeling of hydrogen site solubility and diffusion in disordered Ti–V–Cr alloys

Hydrogen diffusion and solubility in disordered alloys are of paramount importance to a variety of practical applications from hydrogen storage materials to separation membranes and protection against hydrogen embrittlement. By employing density functional theory calculations we unveil the atomic-level understanding of hydrogen diffusion in disordered Ti–V–Cr alloys used for hydrogen storage. Hydrogen distribution over interstitial sites of the bcc and fcc lattices of TiV_0.8Cr_1.2 has been simulated using a supercell approach. Taking into account both structural and energy factors we identify tetrahedral sites coordinated by three different metal atoms as the most favorable for hydrogen. The calculations carried out within the nudged elastic band method show that hydrogen diffusion between two tetrahedral site in fcc TiV_0.8.Cr_1.2H_5.25 occurs nearby an intermediate octahedral site with the activation barrier of 0.158 eV for the most probable diffusion pathway. An estimation of the hydrogen diffusion coefficient in fcc TiV_0.8.Cr_1.2H_5.25 at 294 K provides the value of 2.6 × 10^?11 m²/s that is in fair agreement with experiment data. Despite the modeling was done for a hydride of a definite composition we anticipate that the present results could be extended to Ti–V–Cr hydrides with various compositions.     

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.     

Hydrogen storage properties of new A3B2-type TiZrNbCrFe high-entropy alloy

Crystal structure and hydrogen storage properties of a novel equiatomic TiZrNbCrFe high-entropy alloy (HEA) were studied. The selected alloy, which had a A₃B₂-type configuration (A: elements forming hydride, B: elements with low chemical affinity with hydrogen) was designed to produce a hydride with a hydrogen-to-metal atomic ratio (H/M) higher than those for the AB₂- and AB-type alloys. The phase stability of alloy was investigated through thermodynamic calculations by the CALPHAD method. The alloy after arc melting showed the dominant presence of a solid solution C14 Laves phase (98.4%) with a minor proportion of a disordered BCC phase (1.6%). Hydrogen storage properties investigated at different temperatures revealed that the alloy was able to reversibly absorb and fully desorb 1.9 wt% of hydrogen at 473 K. During the hydrogenation, the initial C14 and BCC crystal structures were fully converted into the C14 and FCC hydrides, respectively. The H/M value was 1.32 which is higher than the value of 1 reported for the AB₂- and AB-type HEAs. The present results show that good hydrogen storage capacity and reversibility at moderate temperatures can be attained in HEAs with new configurations such as A₃B₂/A₃B₂H₇.     

Structure and hydrogen storage properties of mechanically alloyed Ti-V alloys

Ti-V alloys are potential candidates for hydrogen storage materials. In this study, mechanical alloying under an argon atmosphere was used to produce Ti_2?xV_x nanocrystalline alloys (x = 0.5, 0.75, 1, 1.25, 1.5). Shaker type ball mill was used. An objective of the present study was to investigate an influence of chemical composition and method of production on hydrogenation and dehydrogenation properties of Ti-V alloys. X-ray diffraction analyses revealed formation of BCC solid solution after 14 h of milling. It is the first time of obtaining this phase directly from mechanical alloying method. HRTEM images confirmed formation of nanocrystalline materials. Synthesized materials were studied by a conventional Sievert's type apparatus at 303 K. It was observed that the maximum hydrogen storage capacity is increased with increased Ti content in the alloy. Ti_1.5V_0.5 alloy showed high hydrogen storage capacity at room temperature, which reached about 3.67 wt.%. Simultaneously, it was noticed that Ti-rich alloys form more stable hydride phases than V-rich alloys. Observed properties resulted mainly from structure of studied materials.