TiH2 Research Articles

Correlating the microstructure of titanium hydride with its hydrogenation conditions via thermodynamics and kinetics

Titanium hydride is widely considered as an important hydrogen storage material due to its high capacity, stability and low cost. The microstructure of titanium hydride, which is usually induced by the hydrogenation of titanium, strongly influences its storage performance. However, the relationship between the hydrogenation conditions and the derived microstructure of titanium hydride is still unclear, limiting the development of its structure design strategy. In this work, the microstructure of titanium hydride is correlated with its hydrogenation conditions through the thermodynamics and kinetics. By evaluating the effect of the hydrogenation temperature, pressure and cooling rate, three classes of the hydrogenation processes were clarified as kinetic-limited, continuous and stepwise. Besides, according to the SEM and FIB-STEM results, these different processes are confirmed to greatly vary the bulk microstructure. It is concluded that a fast and continuous transition induces a broken bulk morphology, while a stepwise process leads to a larger bulk grain size. In summary, this study presents a framework for designing titanium hydride structures by modifying phase transition processes through careful adjustment of hydrogenation parameters, namely, a condition-process-structure relationship. This relationship offers crucial guidance in managing the grain refinement or coarsening of hydrides within hydrogen storage components and metallurgical applications.

MICROSTRUCTURE AND WEAR BEHAVIOR CHARACTERIZATION OF STIR-CAST ALUMINUM 6063 ALLOY AND ITS FOAM: A COMPARATIVE STUDY

In this research work, aluminum 6063 alloy foam was synthesized through stir casting using varying amounts of titanium hydride as a foaming agent. A comparative microstructural and wear behavior analysis of stir-cast aluminum 6063 alloy and its foam is presented. The oxide and aluminide phases present as intermetallic compounds in the stir-cast aluminum 6063 alloy foam ensure the stability of the synthesized foam. In order to optimize the effect of wear parameters on the tribological properties of stir-cast aluminum 6063 alloy foam, the L933 orthogonal array (OA) was designed and modeled statistically. The tribological properties of stir-cast aluminum 6063 alloy and its foam were evaluated by pin-on-disc tests. The optimal tribological properties of the synthesized foam are 0.29 coefficient of friction, 8.6 N frictional force, and 0.00025 mm3/N-m specific wear rate. The tribological properties of stir-cast aluminum 6063 alloy foam are superior to those of aluminum 6063 alloy and comparable to other aluminum foams described in the literature.

Covalent Carbide Interconnects Enable Robust Interfaces and Thin SEI for Graphite Anode Stability under Extreme Fast Charging.

Carbonaceous and carbon-coated electrodes are ubiquitous in electrochemical energy storage and conversion technologies due to their electrochemical stability, lightweight nature, and relatively low cost. However, traditional reliance on conductive additives and binders leads to impermanent electrical pathways. Here, a general approach is presented to fabricate robust electrodes with a progressive failure mechanism by introducing carbide-based interconnects grown via carbothermal conversion of (5 wt%) titanium hydride nanoparticles. This method concurrently enhances both electrical and mechanical properties within the electrode architecture. The resulting chemical bonding between active materials establishes a novel mechanism to maintain stable electrical pathways during cycling. Employed as Li-ion battery anodes, these electrodes exhibit improved cyclability, achieving 80% capacity retention after 800 fast-charge cycles at moderate loading (1 mAh cm- 2). High loading cells with areal capacity of 3 mAh cm-2 show significantly improved cycle lifeover the same number of cycles. This performance improvement is attributed to the absence of significant impedance growth and a thinner solid electrolyte interphase (SEI) layer formed at high current densities (4C) as demonstrated by X-ray photoelectron spectroscopy and transmission electron microscopy studies. The enhanced conductivity facilitates SEI formation, lowering ionic impedance and mitigating lithium plating, ultimately leading to the reported extended cycle life.

Mechanical energy absorption capacity of the porous Ti-6Al-4V alloy under quasi-static and dynamic compression

Porous materials are very efficient in absorbing mechanical energy in different applications. In the present study, porous materials based on the Ti-6(wt.%)Al-4V alloy were manufactured with the use of two different powder metallurgy methods: i) blended elemental powder approach using titanium hydride (TiH2) as well as V-Al master alloy powders and ii) using hydrogenated Ti-6-4 pre-alloyed powder. The powder compacts were sintered with additions of ammonium bicarbonate as a pore-holding removable agent. The emission of hydrogen from hydrogenated powders on vacuum sintering and the resulting shrinkage of powder particles permitted the control of the sintering process and creation of anticipated porous structures. Mechanical characteristics were evaluated under quasi-static and dynamic compressive loading conditions. Dynamic compression tests were performed using the direct impact Hopkinson pressure bar technique. All investigations aimed at characterizing the mechanical energy-absorbing ability of the obtained porous structures. The anticipated strength, plasticity, and energy-absorbing characteristics of porous Ti-6-4 material were evaluated, and the possibilities of their application were also discussed. Based on the obtained results, it was found that porous Ti-6-4 material produced with a blended elemental powder approach showed more promising energy absorption properties in comparison with pre-alloyed powder.

Ti-decorated B-doped biphenylene: A hydrogen storage sponge at room temperature

It has always been a difficult problem to design Kubas-type hydrogen storage materials that make hydrogen molecules activate but do not dissociate. Density functional theory calculations confirm that Ti decorated B-doped biphenylene is the first two-dimensional room-temperature hydrogen storage sponge reported so far. The hydrogen adsorption energy of Ti-decorated B-doped biphenylene is in the range of −0.29 ∼ −0.51 eV with hydrogen storage capacity is 6.58–9.13 wt%. What's very interesting and exciting is that the Kubas interaction between Ti sites and H2 can be regulated by changing the electron occupation state of the dx2-y2 orbitals of Ti, and this regulation can be realized through the uniaxial lattice strain in the XOY plane. The average hydrogen adsorption energy of tensile Ti-decorated B-doped biphenylene ranges between −0.34 ∼ −0.66 eV, while −0.22 ∼ −0.48 eV for compressed one. This process makes the reversible hydrogen storage as simple and feasible as a sponge absorbing water. This study provides clear structural for the transformation mechanism to form a folded structure and also provides an important precedent model.

Operando visualization of porous metal additive manufacturing with foaming agents through high-speed x-ray imaging

Porous metals find extensive applications in soundproofing, filtration, catalysis, and energy-absorbing structures, thanks to their unique internal pore structure and high specific strength. In recent years, there has been an increasing interest in fabricating porous metals using additive manufacturing (AM), leveraging its unique advantages, including improved design freedom, spatial material control, and cost-effective small-batch production. In this study, we conducted pioneering operando visualization of AM porous metal using a laser powder bed fusion (L-PBF) setup combined with a high-speed synchrotron x-ray imaging system. Single track printing experiments using Ti6Al4V (Ti64) combined with titanium hydride (TiH2) and sodium carbonate (Na2CO3) as foaming agents, with varying mixing ratios were performed under different processing conditions. The results elucidate the dynamic development of porosity formation. The average pore size is significantly influenced by the particle size of foaming agents when pore coalescence is absent. For all foaming agent content tested in the current study, the number of pores is found to be more sensitive to changes in laser power than in laser scanning speed. Increasing linear energy density (increasing laser power or reducing laser scanning speed) promotes the foaming agent activation thereby porosity formation. However, high linear energy density skews pore distribution towards the surface despite forming deeper melt pools. In addition, the impact of additional factors including foaming agent’s laser absorptivity and decomposition kinetics with respect to AM time scales should be carefully considered to avoid ineffective activation of foaming agents during the AM of porous metals.

Enhancing hydrogen storage properties of titanium hydride TiH2 with vacancy defects and uniaxial strain: A first-principles study

The structural, electronic, and thermodynamic properties of titanium hydride TiH2 have been investigated using the principles of density functional theory based on the coherent potential approximation (CPA) integrated in the AkaiKKR package, with the aim of reducing the high stability and decomposition temperature to create an ideal material for hydrogen storage, thereby meeting DOE (the U.S. Department of Energy) standard conditions (formation enthalpy ΔH = −40 kJ/mol.H2 and a decomposition temperature Tdes from 289 to 393 K).This study focuses particularly on the effects of titanium vacancy creation and the application of uniaxial deformation on the compound. The results show that with 12% of titanium vacancies, the formation energy increase from −122.34 kJ/mol.H2 to −40.97 kJ/mol.H2 and the temperature of desorption decreases from 936.045 K to 313.49 K, while −8% compressive uniaxial deformation achieve the value −41.42 kJ/mol.H2 and 316.97 K.

Compressor‐Driven Titanium and Magnesium Hydride Systems for Thermal Energy Storage: Thermodynamic Assessment

ABSTRACTMetal hydrides enable excellent thermal energy storage due to their high energy density, extended storage capability, and cost‐effective operation. A metal hydride‐driven storage system couples two reactors that assist in thermochemical storage using cyclic operation. Metal hydride reactors, operating at both low and high temperatures, serve for the storage of hydrogen and thermal energy, respectively. The integration of efficient thermal energy storage technology is known to enhance the efficiency of solar thermal systems. In this regard, during the peak hours of solar energy, the high‐temperature supply heat can be utilized to store hydrogen gas in the low‐temperature reactor, which simultaneously facilitates energy storage in the high‐temperature reactor. Moreover, the temperature and energy released from the reactors are highly dependent on the pressure of the gas. As a result, installing a compressor between the low and high‐temperature metal hydride reactors can help generate additional outputs, such as a cooling effect. This paper conducts a thermodynamic analysis to assess the system's performance, considering parameters such as thermal storage efficiency, coefficient of performance (COP), and COPCCH (combined cooling and heating based COP). Moreover, the performance analysis was carried out for two cases, that is, high‐temperature titanium hydride (TiH2) and magnesium hydride (MgH2). The results show that MgH2 and TiH2 achieve a maximum COPCCH of 1.08 and 0.9, respectively, and system storage efficiency of 76.15% and 74.34%, respectively. In spite of having lower efficiency than MgH2, the TiH2‐based system has the ability to recover heat at a very high temperature.

Coating Titanium Transport Layers for Enhanced Short and Long Term Performance in PEM Water Electrolysis Cells

The ageing of titanium porous transport layers (PTL) affects the longevity of polymer electrolyte membrane water electrolysis systems by the formation and thickening of a weakly conductive oxide layer. The evolving oxygen on the anode in combination with the high electrical potential provides a challenging environment in which a low electrical contact resistance to the catalyst layer (CL) needs to sustain over a long period. One previous approach is a precious metal coating of the PTL with platinum or iridium [1], which can prevent oxidation of the titanium. A cost-effective alternative is a hydrochloric acid treatment, which forms titanium hydride (TiH). [2] Titanium nitride (TiN) promises better electrical conductivity with similar corrosion resistance, but has not yet been tested as a coating. The same is true for niobium, which performed well as a coating for stainless steel bipolar plates. [3] In our studies, all these candidates face the same conditions aside an untreated PTL in one unique series of measurements. All types are repeated three times to ensure reproducibility. The mean polarization curves (figure 1 left) show that the two precious metals, iridium and platinum, significantly improve the cell voltage at a current density of 2A/cm² by 80mV and 95mV, respectively. Titanium hydride achieves an even higher reduction of 100mV without the use of precious metals. The high frequency resistance plots in figure 1 right determine that the improvement of up to 20% or 30mΩ∙cm² is attributable to a reduced contact resistance to the CL. This assumption is proven by the fact that a PTL that is only coated with iridium on the side facing the CL behaves identically to a PTL that is coated on both sides. The ohmic losses in contact with the BPP are therefore negligible in these experiments. Titanium nitride and niobium coated PTLs behave in the same way as the reference measurement. In addition to the measurements shown, long-term experiments (>1000 hours) are currently running for all types of coating, and corrosion measurements in sulphuric acid are being carried out in parallel. The joint results will determine the suitability of the various coatings for long-term use in electrolysers.[1] Liu, C., Shviro, M., Gago, A. S., Zaccarine, S. F., Bender, G., Gazdzicki, P., Morawietz, T., Biswas, I., Rasinski, M., Everwand, A., Schierholz, R., Pfeilsticker, J., Müller, M., Lopes, P. P., Eichel, R.-A., Pivovar, B., Pylypenko, S., Friedrich, K. A., Lehnert, W., Carmo, M., Exploring the Interface of Skin-Layered Titanium Fibers for Electrochemical Water Splitting. Adv. Energy Mater. 2021, 11, 2002926. https://doi.org/10.1002/aenm.202002926[2] Bystron, T., Vesely, M., Paidar, M. et al. Enhancing PEM water electrolysis efficiency by reducing the extent of Ti gas diffusion layer passivation. J Appl Electrochem 48, 713–723 (2018). https://doi.org/10.1007/s10800-018-1174-6[3] Stiber, S., Hehemann, M., Carmo, M., Müller, M., Ayers, K.E., Capuano, C., Danilovic, N., Morawietz, T., Biswas, I., Gazdzicki, P., Heger, J.-F., Gago, A.S. and Friedrich, K.A. (2022), Long-Term Operation of Nb-Coated Stainless Steel Bipolar Plates for Proton Exchange Membrane Water Electrolyzers. Adv. Energy Sustainability Res., 3: 2200024. https://doi.org/10.1002/aesr.202200024 Figure 1

Elastoplastic and Electrochemical Characterization of xTiB2 Strengthened Ti Porous Composites for Their Potential Biomedical Applications

The microstructure, elastoplastic properties, and corrosive response of induced porous Ti-TiH2 materials reinforced with TiB2 particles were investigated. Samples were fabricated using CP-Ti Grade1, Titanium Hydride (TiH2), TiB2 powders (0, 3, 10, and 30 vol.%), and ammonium bicarbonate salt (40 vol.%) as a space holder. Composites were fabricated using the Powder Metallurgy technique under high-vacuum conditions (HVS) at 1100 °C. Scanning electron microscopy, X-ray diffraction, nanoindentation tests, and electrochemical assays were used to investigate the pore formation, pore distribution, phase formation, elastoplastic properties, and electrochemical behavior of the compounds, respectively. With a mean pore diameter of 50–900 µm and Young’s modulus of less than 100 GPa, which is close to the properties of human bone, the pore structures of the compounds processed here are shown to be a potential biomaterial for osseointegration. In addition, their H/Er and H3/Er2 ratios for the reinforced samples are higher than those of the unreinforced sample (1.5 and 4 times higher than the unreinforced sample, respectively), suggesting a better wear resistance of the Ti-TiH2/xTiB2 composites. Electrochemical experiments demonstrated that the Ti-TiH2/xTiB2 composites exhibited superior passivation properties compared to the Ti-TiH2 sample. Additionally, the corrosion rates exhibited by the 3 and 10 vol.% of TiB2 samples were found to be within an acceptable range for potential biomedical applications (29.26 and 185.82 E-3 mm·y−1). The elastoplastic properties combined with the electrochemical behavior place the Ti-TiH2/3-10TiB2 composites as potential candidates for the biomedical application of CP-Ti.

A closed-loop patch based on bioinspired infection sensor for wound management

Pathogenic wound infections remain a major global health challenge. When skin tissue is wounded, some colonizing pathogenic bacteria secrete large amounts of hyaluronidase (HAase) to degrade the extracellular matrix (ECM), enabling their diffusion and proliferation. Current clinical techniques for detecting infection rely on basic visual cues or analysis of wound fluids, enabling only reactive treatment once severe signs manifest. Herein, we present the use of a closed-loop patch (CLP) for advanced wound management. The patch contains a bio-inspired sensor based on an engineered hyaluronic acid (HA) hydrogel that detects HAase secreted by invading pathogens. This detection triggers a quantifiable change in the capacitance of the interdigital electrode and achieving in-time detection of Staphylococcus aureus (S. aureus) infection in a murine wound model before obvious visual signs appear. In response, the release of titanium hydride (TiH1.924) nanodots from hydrogel decomposition can eradicate pathogens under ultrasound (US) irradiation. Concurrently, the product, HA fragments further promoted cell migration and angiogenesis to significantly improve wound healing by ∼14.6-fold rates in 12 days. Overall, the CLP provides a sensory act-treat effect according to the wound status, which facilitate the closed-loop integration of monitoring and rehabilitation. This strategy may provide additional possibilities for the design of future wound management systems.

The Effect of Implantoplasty on the Fatigue Behavior and Corrosion Resistance in Titanium Dental Implants.

Implantoplasty is a technique increasingly used to remove the biofilm that causes peri-implantitis on dental implants. This technique of mechanization of the titanium surface makes it possible to eliminate bacterial colonies, but it can generate variations in the properties of the implant. These variations, especially those in fatigue resistance and electrochemical corrosion behavior, have not been studied much. In this work, fatigue tests were performed on 60 dental implants without implantoplasty, namely 30 in air and 30 in Hank's solution at 37 °C, and 60 with implatoplasty, namely 30 in air and 30 in Hank's solution at 37 °C, using triaxial tension-compression and torsion stresses simulating human chewing. Mechanical tests were performed with a Bionix servo-hydraulic testing machine and fracture surfaces were studied by scanning electron microcopyElectrochemical corrosion tests were performed on 20 dental implants to determine the corrosion potentials and corrosion intensity for control implants and implantoplasty implants. Studies of titanium ion release to the physiological medium were carried out for each type of dental implants by Inductively Coupled-Plasma Mass Spectrometry at different immersion times at 37 °C. The results show a loss of fatigue caused by the implantoplasty of 30%, observing that the nucleation points of the cracks are in the areas of high deformation in the areas of the implant neck where the mechanization produced in the treatment of the implantoplasty causes an exaltation of fatigue cracks. It has been observed that tests performed in Hank's solution reduce the fatigue life due to the incorporation of hydrogen in the titanium causing the formation of hydrides that embrittle the dental implant. Likewise, the implantoplasty causes a reduction of the corrosion resistance with some pitting on the machined surface. Ion release analyses are slightly higher in the implantoplasted samples but do not show statistically significant differences. It has been observed that the physiological environment reduces the fatigue life of the implants due to the penetration of hydrogen into the titanium forming titanium hydrides which embrittle the implant. These results should be taken into account by clinicians to determine the convenience of performing a treatment such as implantoplasty that reduces the mechanical behavior and increases the chemical degradation of the titanium dental implant.

Reaction path search for propane dehydrogenation and metathesis on titanium hydride

Abstract The reaction mechanisms of C3H8 dehydrogenation and subsequent metathesis on a TiH2 surface were investigated using an automated reaction path search method. The predicted pathway suggests that dehydrogenation could occur at the H vacancies present on the TiH2 surface. The propylidene species formed by the initial dehydrogenation reaction are crucial intermediates in butane formation; this compound is in turn required by the subsequent metathesis reaction on the surface of the metal hydrides.

Insights into the formation of titanium hydrides from first principles calculations

Hydride induced embrittlement is one of the main causes for the stress corrosion crack of titanium alloys. The formation process of the hydride at atomic level remains controversial as it is not accessible directly by experiments. In the present work, the formation of titanium hydride is investigated by using a first principles method. By calculating the formation energies of TiHm with various types of H occupation state in HCP and FCC Ti matrix, we show that there exists phase equilibrium between α-Ti-H solid solution and δ/ε hydrides with χ and γ hydrides appearing as metastable phases. The energy barrier for the HCP → FCC structure transformation for the formation of the hydrides decreases with increasing H:Ti ratio m. The structure transformation is accompanied with spontaneous shift of half amounts of the H atoms from the octahedral to the tetrahedral interstices. The energy barrier for the migration of the remaining half of the H atoms from the octahedral to the tetrahedral interstices reduces with increasing m and approaches to a constant value. The hydrostatic compressive stress acting on TiHm, resulted from the H-induced lattice dilation tendency and the constraint of the surrounding matrix, does not facilitate the HCP → FCC transformation and the migration of the H atoms. The present work sheds light on the hydride formation process at atomic level.

Hydrogen storage properties of Ti-doped C20 nanocage and its derivatives: A comprehensive density functional theory investigation

We have innovatively designed and assessed the hydrogen storage properties of a single Ti-doped C20 nanocage and its B, N or B and N heteroatom substituted derivatives with the help of the density functional theory approach for the first time to the best of our knowledge. Out of fifteen designed structures by substituting heteroatoms, only 8 structures are found to be suitable for Ti doping and H2 adsorption viz. C20, C12N8, C12B8, C12B4N4, C10B5N5, C10B10, B10C10 and B10N10. Their formation energy and cohesive energy values confirm the stability of all the structures. Ti atom binds more strongly with B, N or B and N heteroatom substituted C20 nanocage than unsubstituted C20 nanocage which is necessary to avoid clustering for multiple Ti doped nanocages. Among the eight Ti doped nanocages considered, H2 molecules strongly interact with Ti-doped C12B4N4 nanocage. The binding strength of Ti atom with nanocages decreases during the hydrogen adsorption process. The obtained H2 adsorption energy values for all the structures are within the range of 0.2–0.7 eV, which is conducive to reversible hydrogen storage. Calculated Gibbs free energy-corrected H2 adsorption energy values at different temperature and pressure indicate favorable H2 adsorption over the entire pressure range at room temperature. For Ti doped C20, C12N8, C12B8, C10B5N5, C10B10, B10N10, and B10C10 nanocages, H2 adsorption is thermodynamically favorable below 360, 350, 300, 285, 235, 277, and 251 K, respectively at 1 atm pressure. From PDOS analysis, it is observed that the 1s orbital of H overlaps with 3d orbital of Ti atom. The highest H2 desorption temperature is observed for the C12B4N4Ti nanocage, and the lowest for C10B10Ti, aligning well with the calculated adsorption energy values. As H2 desorption temperature is high, the stability of Ti-doped nanocages at high temperature (634 K) is confirmed using ab initio molecular dynamics simulations. Bader's quantum theory in atoms in molecules is used to prove the weak non-covalent interaction between all the atoms.

Energy release characteristics of PTFE/Al/TiH2 reactive jet with different TiH2 content

Titanium hydride (TiH2), a promising high-energy additive, is doped into PTFE/Al to optimize the energy output structure of the reactive jet and strive for better aftereffect damage ability to the target. Six types of PTFE/Al/TiH2 reactive liners with different TiH2 content are prepared by the molding and sintering method. The energy release characteristics of PTFE/Al/TiH2 reactive jet are tested by the transient explosion energy test, and are characterized from pressure and temperature. The reaction delay time, pressure history, and temperature history of the energy release process are obtained, then the actual value of released energy and reaction efficiency of the reactive jet are calculated. The results show that the peak pressure and temperature of the PTFE/Al/TiH2 jet initially increase and then decrease with increasing TiH2 content. When the TiH2 content is 10%, the actual value of released energy and reaction efficiency increased by 24% and 6.4%, respectively, compared to the PTFE/Al jet. The reaction duration of the reactive material is significantly prolonged as the TiH2 content increased from 0% to 30%. Finally, combined with the energy release behaviors of PAT material and the dynamic deformation process of liner, the enhancement mechanism of TiH2 on energy release of the reactive jet is expounded.

Prior implantation of hydrogen as a mechanism to delay helium bubbles, blistering, and exfoliation in titanium

This study explores the delaying of the formation of helium bubbles and blisters in pure titanium by hydrogen pre-implantation. Titanium, implanted with helium (40 KeV, 5 × 1017 ions/cm²), exhibited large bubbles that cause exfoliation after heat treatment, whereas hydrogen pre-implantation inhibited bubble growth at room temperature and reduced the exfoliation after heat treatment.In the samples pre-implanted with hydrogen, we found evidence of helium diffusion delay by: (a) a fourfold reduction in bubble pressure (b) faceted cavities in the samples (c) a smaller increase in titanium lattice parameters (d) a 16-fold reduction in average bubble size and a sixfold reduction in bubble area fraction (e) a more than twofold decrease in exfoliation (f) a tendency toward the formation of larger bubbles as a result of heat treatment. We believe that it is reasonable to assume that the inhibition of helium diffusion between tetrahedral interstitial lattice sites takes place because of the occupation of the intermediate octahedral sites by hydrogen atoms.Evidence for the opposite effect, that is inhibition of the diffusion of hydrogen in the presence of helium, is found in the retention of hydrogen in the specimens at elevated temperatures. This retention allowed the existence of titanium hydride after heat treatment at 680 °C. The present study sheds light on the intricate interplay between hydrogen and helium in titanium, providing insights into mechanisms that can potentially mitigate helium-induced damage in materials.

First-principles investigation of the structure and thermodynamic properties of titanium hydrides

The formation of titanium hydrides is closely related to the hydrogen embrittlement in titanium and its alloy. In this work, a comprehensive study on the stable and metastable structures of TiHx (x=0.5, 1, 1.5 and 2) has been performed by means of first-principles calculations in combination with the evolutionary algorithm. The structural features, dynamical stability and elastic properties of these searched titanium hydrides have been systematically investigated. The influence of temperature on the structure stability and phase transition of the Ti2H, TiH, Ti2H3 and TiH2 phases is considered using the quasi-harmonic approximation theory. Our calculations predict that Ti2H, Ti2H3 and TiH2 undergo temperature-driven phase transitions. The reason that the theoretical calculation overestimates the critical temperature of the tetragonal to cubic phase transition in TiH2 within the quasi-harmonic approximations approach is also explored and discussed. The thermodynamic stability of these TiHx (x=0.5, 1, 1.5 and 2) structures is further analyzed over the temperature range of 0–1100 K. TiH2 is the only stable structure in the Ti–H system and Ti2H, TiH, and Ti2H3 are thermodynamically metastable within 900 K.

Robust training of machine learning interatomic potentials with dimensionality reduction and stratified sampling

Machine learning interatomic potentials (MLIPs) enable accurate simulations of materials at scales beyond that accessible by ab initio methods and play an increasingly important role in the study and design of materials. However, MLIPs are only as accurate and robust as the data on which they are trained. Here, we present DImensionality-Reduced Encoded Clusters with sTratified (DIRECT) sampling as an approach to select a robust training set of structures from a large and complex configuration space. By applying DIRECT sampling on the Materials Project relaxation trajectories dataset with over one million structures and 89 elements, we develop an improved materials 3-body graph network (M3GNet) universal potential that extrapolates more reliably to unseen structures. We further show that molecular dynamics (MD) simulations with the M3GNet universal potential can be used instead of expensive ab initio MD to rapidly create a large configuration space for target systems. We combined this scheme with DIRECT sampling to develop a reliable moment tensor potential for titanium hydrides without the need for iterative augmentation of training structures. This work paves the way for robust high-throughput development of MLIPs across any compositional complexity.

Multiscale evaluation of hydrogen-assisted mechanical degradation in grade 2 titanium

The mechanism of hydrogen embrittlement in grade 2 titanium was studied by a combined micro and nanoscale approach. Insights into the hydrogen related deformation and degradation mechanisms were acquired via a unique characterisation procedure, correlating the topological observations on the fracture surface after tensile testing to the dislocation arrangement underneath. Focused ion beam lift-out was employed to extract a subsurface slice perpendicular to the fracture surface. High resolution characterisation provided information about the dislocation configuration and active hydrogen-assisted degradation mechanism. A high hydrogen content forms a brittle titanium hydride phase, failing via hydride cleavage. At a moderate hydrogen quantity, hydrogen in solid solution is responsible for a ductility decrease via the hydrogen-enhanced localised plasticity mechanism.