Hydrogen-induced Phase Transformations Research Articles

Effect of different hydrogen Fugacities on the microstructure of additively manufactured Ti-6Al-4V

In this study we compared the microstructural changes in the presence of hydrogen of Ti-6Al-4V alloy prepared by two different additive manufacturing (AM) methods: electron beam melting (EBM) and selective laser melting (SLM). Cathodic hydrogenation of AM Ti-6Al-4V resulted in significant expansion of β Ti phases due to the solute hydrogen, increasing of micro-strains, and α → titanium hydride/βH transformation. It was shown that SLM Ti-6Al-4V with acicular martensitic structure is more prone to hydrogen-induced phase transformation and hydrogen-assisted damage, compared to EBM. Moreover, it was revealed that different hydrogen charging environments cause different microstructural changes in EBM Ti-6Al-4V. Unlike cathodic hydrogen charging, gaseous hydrogen charging at elevated temperature reduced the process-induced micro-strains and promoted Al redistribution within the EBM Ti-6Al-4V. This resulted in massive precipitation of brittle α2-Ti3Al intermetallic compound, which act as a strong hydrogen trapping site aside from the Ti hydride phase.

Overview of hydrogen-resistant alloys for high-pressure hydrogen environment: on the hydrogen energy structural materials

Abstract With the progressive expansion of hydrogen fuel demand, hydrogen pipelines, hydrogen storage cylinders and hydrogen refuelling stations (HRSs) are the primary components of hydrogen energy systems that face high-pressure hydrogen environments. Hydrogen embrittlement (HE) is a typical phenomenon in metallic materials, particularly in the high-pressure hydrogen environment, that causes loss of ductility and potentially catastrophic failure. HE is associated with materials, the service environment and stress. The primary mechanisms for explaining the HE of materials are hydrogen-enhanced decohesion, hydrogen-induced phase transformation, hydrogen-enhanced local plasticity, adsorption-induced dislocation emission and hydrogen-enhanced strain-induced vacancy. To reduce the risk of HE for metallic structural materials used in hydrogen energy systems, it is crucial to reasonably select hydrogen-resistant materials for high-pressure hydrogen environments. This paper summarizes HE phenomena, mechanisms and current problems for the metallic structural materials of hydrogen energy systems. A research perspective is also proposed, mainly focusing on metal structural materials for hydrogen pipelines, hydrogen storage cylinders and hydrogen compressors in HRSs from an application perspective.

Composition and temperature influence on hydrogenation performance of TiZrHfMoxNb2–x high entropy alloys

This study thoroughly investigates the influences of temperature and composition on the hydrogenation kinetic performance and discusses the hydrogen-induced phase transformation process at RT or high temperature of BCC TiZrHfMoxNb2–x (x = 0, 1, 2).

Role of rare-earth yttrium in the hydrogen embrittlement of AISI 321 austenitic stainless steel

The hydrogen embrittlement of AISI 321 austenitic stainless steel without and with 0.045 wt% rare-earth yttrium was studied with slow strain rate test, X-ray diffraction, scanning electron microscopy, electron back scattered diffraction, and transmission electron microscopy. The results indicate that rare-earth microalloyed AISI 321 austenitic stainless steels show better ductility in a hydrogen environment, where the plastic loss of a specimen with 0.045 wt% yttrium is 25.27 ± 3.74%, which is significantly lower than that of a specimen without yttrium (35.02 ± 1.95%). There are two important roles of rare earth yttrium in hydrogen embrittlement of AISI 321 austenitic stainless steels. The first is that rare-earth yttrium tends to interact with titanium and improves the stress concentration around Ti(C, N) by reducing its size and increasing its number. In addition, rare-earth yttrium contributes to reducing the inclination of stress and hydrogen-induced martensitic phase transformation in metastable AISI 321 stainless steel. This work is valuable for improving resistance of austenitic stainless steel to hydrogen embrittlement.

Hydrogen Diffusion on, into and in Magnesium Probed by DFT: A Review

Hydrogen is an energy carrier that can be a sustainable solution for alternative energy with zero greenhouse gas emissions. Hydrogen storage is a key point for hydrogen energy. Metals provide an access for safe, controlled and reversible hydrogen storage and release. Magnesium, due to its outstanding hydrogen storage capacity, high natural abundance, low cost and non-toxicity is one of the most attractive materials for hydrogen storage. The economic efficiency of Mg as a hydrogen accumulator is limited by its sluggish hydrogen sorption kinetics and high stability of its hydride MgH2. Many attempts have been made to overcome these shortcomings. On a microscopic level, hydrogen absorption by metal is a complex multistep process that is impossible to survey experimentally. Theoretical studies help to elucidate this process and focus experimental efforts on the design of new effective Mg-based materials for hydrogen storage. This review reports on the results obtained within a density functional theory approach to studying hydrogen interactions with magnesium surfaces, diffusion on Mg surfaces, into and in bulk Mg, as well as hydrogen induced phase transformations in MgHx and hydrogen desorption from MgH2 surfaces.

Reversible Hydrogen-Induced Phase Transformations in La0.7Sr0.3MnO3 Thin Films Characterized by In Situ Neutron Reflectometry.

We report on the mechanism for hydrogen-induced topotactic phase transitions in perovskite (PV) oxides using La0.7Sr0.3MnO3 as a prototypical example. Hydrogenation starts with lattice expansion confirmed by X-ray diffraction (XRD). The strain- and oxygen-vacancy-mediated electron-phonon coupling in turn produces electronic structure changes that manifest through the appearance of a metal insulator transition accompanied by a sharp increase in resistivity. The ordering of initially randomly distributed oxygen vacancies produces a PV to brownmillerite phase (La0.7Sr0.3MnO2.5) transition. This phase transformation proceeds by the intercalation of oxygen vacancy planes confirmed by in situ XRD and neutron reflectometry (NR) measurements. Despite the prevailing picture that hydrogenation occurs by reaction with lattice oxygen, NR results are not consistent with deuterium (hydrogen) presence in the La0.7Sr0.3MnO3 lattice at steady state. The film can reach a highly oxygen-deficient La0.7Sr0.3MnO2.1 metastable state that is reversible to the as-grown composition simply by annealing in air. Theoretical calculations confirm that hydrogenation-induced oxygen vacancy formation is energetically favorable in La0.7Sr0.3MnO3. The hydrogenation-driven changes of the oxygen sublattice periodicity and the electrical and magnetic properties similar to interface effects induced by oxygen-deficient cap layers persist despite hydrogen not being present in the lattice.

Kinetics of hydrogen-induced reverse phase transformation in Y2Fe17 magnetic alloy

ABSTRACTA model is proposed for the kinetics of a hydrogen-induced reverse phase transformation, namely the recombination of a decomposed YH2 hydride phase and an α-Fe phase into the initial phase of a Y2Fe17 magnetic alloy. On the basis of the model, a kinetic equation is obtained to describe the kinetic curves of the reverse phase transformation as a function of transformation temperature and time. The proposed equation can be used to calculate the optimal conditions (temperature, time) during the reversible phase transformations in order to obtain the best magnetic properties of hydrogen-treated Y2Fe17 powders for use as permanent magnets.

Influence of Defects on the Stability and Hydrogen-Sorption Behavior of Mg-Based Hydrides.

This review deals with the destabilization methods for improvement of storage properties of metal hydrides. Both theoretical and experimental approaches were used to point out the influence of various types of defects on structure and stability of hydrides. As a case study, Mg, and Ni based hydrides has been investigated. Theoretical studies, mainly carried out within various implementations of DFT, are a powerful tool to study mostly MgH2 based materials. By providing an insight on metal-hydrogen bonding that governs both thermodynamics and hydrogen kinetics, they allow us to describe phenomena to which experimental methods have a limited access or do not have it at all: to follow the hydrogen sorption reaction on a specific metal surface and hydrogen induced phase transformations, to describe structure of phase boundaries or to explain the impact of defects or various additives on MgH2 stability and hydrogen sorption kinetics. In several cases theoretical calculations reveal themselves as being able to predict new properties of materials, including the ways to modify Mg or MgH2 that would lead to better characteristics in terms of hydrogen storage. The influence of ion irradiation and mechanical milling with and without additives has been discussed. Ion irradiation is the way to introduce a well-defined concentration of defects (Frankel pairs) at the surface and sub-surface layers of a material. Defects at the surface play the main role in sorption reaction since they enhance the dissociation of hydrogen. On the other hand, ball-milling introduce defects through the entire sample volume, refine the structure and thus decrease the path for hydrogen diffusion. Two Severe Plastic Deformation techniques were used to better understand the hydrogenation/dehydrogenation kinetics of Mg- and Mg2 Ni-based alloys: Equal-Angular-Channel-Pressing and Fast-Forging. Successive ECAP passes leads to refinement of the microstructure of AZ31 ingots and to instalment therein of high densities of defects. Depending on mode, number and temperature of ECAP passes, the H-sorption kinetics have been improved satisfactorily without any additive for mass H-storage applications considering the relative speed of the shaping procedure. A qualitative understanding of the kinetic advanced principles has been built. Fast-Forging was used for a "quasi-instantaneous" synthesis of Mg/Mg2 Ni-based composites. Hydrogenation of the as-received almost bi-phased materials remains rather slow as generally observed elsewhere, whatever are multiple and different techniques used to deliver the composite alloys. However, our preliminary results suggest that a synergic hydrogenation / dehydrogenation process should assist hydrogen transfers from Mg/Mg2 Ni on one side to MgH2 /Mg2 NiH4 on the other side via the rather stable a-Mg2 NiH0.3 , acting as in-situ catalyser.

Universal Scaling and Design Rules of Hydrogen-Induced Optical Properties in Pd and Pd-Alloy Nanoparticles.

Hydride-forming metal nanoparticles sustaining localized surface plasmon resonance have emerged as prototypical material to study the fundamentals of hydrogen-induced phase transformations. They have also been proposed as signal transducers in next-generation hydrogen sensors. However, despite high current interest in hydrogen sorption by nanomaterials in general and such sensors in particular, the correlations between nanoparticle size, shape, and composition, the amount of hydrogen absorbed, and the obtained optical response have not been systematically experimentally studied. Focusing on hydrogenated Pd, PdAu- and PdCu-alloy nanoparticles, which are of particular interest in hysteresis-free plasmonic hydrogen sensing, we find that at practically important Au/Pd and Cu/Pd ratios the optical response to hydrogen concentration is linear and, more interestingly, can be described by a single universal linear trend if constructed as a function of the H/Pd ratio, independent of alloy composition. In addition to this correlation, we establish that the amplitude of optical signal change is defined solely by the spectral plasmon resonance position in the non-hydrogenated state for a specific nanoparticle composition. Thus, it can be maximized by red-shifting the LSPR into the NIR spectral range via tailoring the particle size and shape. These findings further establish plasmonic sensing as an effective tool for studying metal-hydrogen interactions in nanoparticles of complex chemical composition. They also represent universal design rules for metal-hydride-based plasmonic hydrogen sensors, and our theoretical analysis predicts that they are applicable not only to the H/Pd/Au or H/Pd/Cu system investigated here but also to other H/Pd/Metal combinations.

Hydrogen-Induced Phase Transformation and Microstructure Evolution for Ti-6Al-4V Parts Produced by Electron Beam Melting

In this paper, phase transitions and microstructure evolution in titanium Ti-6Al-4V alloy parts produced by electron beam melting (EBM) under hydrogenation was investigated. Hydrogenation was carried out at the temperature of 650 °C to the absolute hydrogen concentrations in the samples of 0.29, 0.58, and 0.90 wt. %. Comparative analysis of microstructure changes in Ti-6Al-4V alloy parts was performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Furthermore, in-situ XRD was used to investigate the phase transitions in the samples during hydrogenation. The structure of Ti-6Al-4V parts produced by EBM is represented by the α phase plates with the transverse length of 0.2 μm, the β phase both in the form of plates and globular grains, and metastable α″ and ω phases. Hydrogenation to the concentration of 0.29 wt. % leads to the formation of intermetallic Ti3Al phase. The dimensions of intermetallic Ti3Al plates and their volume fraction increase significantly with hydrogen concentration up to 0.58 wt. % along with precipitation of nano-sized crystals of titanium δ hydrides. Individual Ti3Al plates decay into nanocrystals with increasing hydrogen concentration up to 0.9 wt. % accompanied by the increase of proportion and size of hydride plates. Hardness of EBM Ti-6Al-4V alloy decreases with hydrogen content.

Mechanism of improved hydrogen embrittlement resistance of low-temperature plasma carburised stainless steel

ABSTRACTThe present paper focuses on the underlying principles of the effect of S-phase on reducing hydrogen embrittlement susceptibility. Hydrogen permeation test indicates that S-phase formed by low-temperature carburising has minor effect on hydrogen diffusion. Moreover, hydrogen-induced phase transformation caused by hydrogen charging is suppressed in the presence of S-phase. The kernel average misorientation result indicates that lattice distortion introduced by hydrogen charging is suppressed in the presence of S-phase. Cross-sectional EBSD investigations on the surface layer of fractured samples show that S-phase remains extremely stable during tensile deformation after hydrogen charging.

Kinetic study of hydrogen-induced direct phase transformation in Y2Fe17 magnetic alloy on the basis of Kolmogorov’s model

The kinetics of the hydrogen induced direct phase transformation in Y2Fe17 magnetic alloy has been analysed within the framework of Kolmogorov’s kinetic model. It is established that the transformation can be classified as a diffusion-controlled transformation, which occurs by a mechanism of nucleation and growth with a decreasing nucleation rate of new phases, namely α-Fe and YH2. A kinetic equation has been obtained that well describes the isothermal kinetic curves of the phase transformation in Y2Fe17 as a function of the transformation temperature.

Hydrogen-Induced Phase Transformations: a Base for a New Sphere of the Science of Metals (an Analytical Review)

Data on the phase transformations induced in metals by hydrogen are generalized and analyzed. It is suggested to classify hydrogen-induced phase transformations on the basis of allowance for the temperature dependence of the parameters of diffusion of substitutional and interstitial (hydrogen) atoms like in the classical science of metals. A pioneer classification of hydrogen-induced phase transformations by this method is developed.

Hydrogen-Induced Reversible Phase Transformations and Hydrogen Storage Properties of Mg–Ag–Al Ternary Alloys

Overly stable thermodynamics and sluggish kinetics hinder the practical applications of Mg-based hydrogen storage alloys. Compositional and structural modifications are important strategies in tuning these hydrogen storage properties. In this study, Mg-based Mg–Ag–Al ternary alloys were investigated to explore their performance as hydrogen storage alloys. Mg80Ag15Al5 exhibits a reaction pathway that differs from that in pure Mg, in which the intermediate phase, consisting of a new ternary solid solution MgAg(Al), reacts with MgH2 during dehydriding and contributes to an increase in the dehydriding equilibrium pressure (0.22 MPa at 300 °C) and to a reversible hydrogen storage capacity of 1.7 wt %. Adjusting the composition to Mg85Ag5Al10 results in a reversible hydrogen storage capacity of approximately 3.8 wt % and an elevated equilibrium pressure (0.26 MPa at 300 °C). These Mg–Ag–Al ternary alloys also show enhanced hydrogen sorption kinetics relative to that of Mg, and the apparent activation energies f...

Relationship between hydrogen-induced phase transformations and pitting nucleation sites in duplex stainless steel

Abstract This paper demonstrates the hydrogen-induced phase transformation and the associated pitting nucleation sites of 2507 duplex stainless steel using scanning Kelvin probe force microscopy and magnetic force microscopy. The low potential sites in Volta potential images, which are considered as the pitting nucleation sites, are strongly dependent on the hydrogen-induced phase transformation. They firstly initiate on the magnetic martensite laths in the austenite phase or at the ferrite/austenite boundaries, and then appear near the needle-shaped microtwins in the ferrite phase, because of the difference in physicochemical properties of hydrogen-induced phase transformation microstructures.

In situ study of phase transformations in Ti–6Al–4V–xH alloys

The hydrogen-induced microstructure evolution and phase transformations in Ti–6Al–4V alloy during heating and cooling were studied. The specimens were heated to 1273 K and subsequently cooled to room temperature. The hydrogen content is up to 0.8% (mass fraction). The hydrogen-induced dynamic phase transformations and the corresponding mechanisms were analyzed. When the hydrogen content increases, the β transus temperature significantly decreases and the magnitude decreases, and the volume fraction of β phase increases. During heating, the phase transformations in hydrogenated Ti–6Al–4V alloys can be divided into three stages, and the phase transformation order is δ→α+H2↑⇒δ+α'→βH⇒α'→αH+βH⇒αH→α+H2↑⇒α→β⇒βH→β+H2↑. In addition, the relationship among hydrogenation and Ms and Mf of α' martensite were determined.

Hydrogen-induced phase transformations in foils of palladium–indium–ruthenium alloy

Precision X-ray diffractometry of the structure–phase state of the Pd–5.3 at % In–0.5 at % Ru alloy foil was performed after the foil had been hydrogenated in different ways. Nonmonotonic changes in the phase composition of the alloy during the relaxation process were observed. Processes of separation of coexisting phases were found to depend on the direction of hydrogen entry into the foil and on the diffraction orientation of the coherent scattering domains (CSDs).

DFT calculations of hydrogen diffusion and phase transformations in magnesium

To describe microscopic hydrogen migration processes and steps of hydride formation we have carried out a theoretical study of the hydrogen diffusion in hexagonal closed packed (hcp), body-centered cubic (bcc), and face-centered cubic (fcc) Mg-lattices. To determine the preferable hydrogen diffusion pathways the activation energy Ea along the minimum energy path between two hydrogen positions were calculated. The hydrogen diffusion coefficient at 673K for hcp-Mg was found to be equal to 1.11×10−8m2/s that is in fair agreement with experiment data. The calculations of the hydrogen migration in Mg lattices have shown that bcc-Mg exhibits the lowest activation energy and, as the result, the highest diffusion coefficient. Taking into account that the bcc-Mg structure does not share the so-called blocking layer effect, we consider that this structure should promote fast hydrogen diffusion. On the bases of the present theoretical study, as well from previous calculations and experimental data we propose a scheme of structural hydrogen induced phase transformations in magnesium.

Kinetic model for hydrogen-induced direct phase transformations in R2Fe17 (R - Sm, Y) type alloys

Kinetic model for hydrogen-induced direct phase transformations in R2Fe17 (R - Sm, Y) type alloys

Kinetics of hydrogen-induced reverse phase transformation in Y2Fe17 hard magnetic alloy

Kinetics of hydrogen-induced reverse phase transformation, i.e. recombination process of decomposed of YH₂ hydride phase and alpha-Fe phase of iron in initial phase of Y₂Fe₁₇ hard magnetic alloy has been studied. It has been established that, as the temperature increase from 650 to 750˚С, a reverse phase transformation significantly accelerates. The activation energy of phase transformation process has been determined from kinetic data (227÷383 kJ/mol) that it is correspond to the values of activation energy of the Fe atoms diffusion in R–T type alloys. It has been shown that the kinetics of hydrogen-induced reverse phase transformation in Y₂Fe₁₇ hard magnetic alloy is controlled by the Fe atoms diffusion to growing new Y₂Fe₁₇ phase centers.