While the Ti6242S(Ti-6.0Al-1.8Sn-4.0Zr-2.1Mo-0.1Si, wt. %) is a leading high-temperature titanium alloy, the underlying causes of its outstanding oxidation resistance have remained elusive. The metal/oxide scale interface of a Ti6242S sample oxidized in air at 650 degrees C was analyzed at the atomic scale using high-resolution scanning transmission electron microscopy. Beneath the oxide layer-primarily composed of rutile TiO2-a nitrogen-and aluminum-rich region was observed, which contained the MAX phase Ti4AlN3 along with a layer of twinned titanium nitride (TiN). It is proposed that the MAX phase forms via nitridation of the alloy by diffusion of nitrogen across the external TiO2scale and acts as a precursor in a mechanism leading to the formation of TiN. This TiN layer serves as an effective barrier to oxygen diffusion, thereby contributing to the alloy's resistance to oxidation-induced embrittlement.

This work presents an overview of hydrogen insertion and diffusion in delta-TiN, epsilon- and delta'-Ti2N systems. Hydrogen insertion and diffusivity were studied in detail taking into account the effects of binary stoichiometry. address these questions, hydrogen insertion energies and migration barriers were calculated using principles methods. The DFT energies were then integrated into a thermodynamic model to determine preferred sites for hydrogen insertion (interstitial or substituted sites), and to calculate the concentrations hydrogen and point defects. In nitrogen sub-stoichiometric regimes, H atoms preferentially occupy substituted sites (those of nitrogen). This behavior is observed in all phases studied. Conversely, under nitrogen conditions, hydrogen atoms prefer interstitial incorporation. The hydrogen solubility in such systems is discussed. Despite the variation in site preference due to stoichiometry, hydrogen diffusion is, in all dominated by interstitial mechanisms. The diffusion coefficients are therefore presented and discussed detail.

The insertion and diffusivity of nitrogen and carbon atoms in the a2-Ti3Al D019 system have been studied using first-principles calculations. The case of oxygen is also reexamined and compared with other interstitial species. The results show that the octahedral site (2a positions) composed only of Ti atoms is always found to be the most stable position. The other sites (tetrahedral, hexahedral or second octahedral sites) show significantly higher energies than the 2a sites regardless of the interstitial species. This high difference in energy also explains the slow diffusivity of the interstitial species in the a2 system. Elastic effects and the chemical environment explain part of this remarkable stability: the 2a sites offer the lowest elastic deformation. The diffusivity is then discussed in terms of trajectories, diffusion coefficients and anisotropy. The results are compared with experimental values where available.

In this study, we investigate the self-diffusion of nitrogen (N) and titanium (Ti), as well as the diffusion of oxygen (O), within the delta-TiN and epsilon-Ti2N anti-rutile phases. Our approach combines density functional theory (DFT) calculations, to analyse the fundamental diffusion processes of these species with the KineCluE code to calculate the diffusion coefficients. We first identify the dominant defects in these systems, including titanium and nitrogen vacancies, as well as interstitial sites. In particular, oxygen shows a similar diffusion behaviour to that of nitrogen in these structures: coupling interstitial and vacancy diffusion mechanism. Atomic-scale analysis reveals that the diffusion pathways for nitrogen and titanium are decoupled, highlighting the unique dynamics within the NaCl-type lattice structure. Our calculations of the diffusion coefficients for nitrogen and titanium reveal significant asymmetries influenced by the alloy stoichiometry. The relatively low concentration of titanium vacancies, compared to the higher concentration of nitrogen vacancies, results in pronounced differences in the diffusion rates of the two elements. Finally, we investigate how diffusion mechanisms vary as a function of stoichiometry, providing new insights into the diffusion behaviour of nitrogen, titanium, and oxygen in key titanium nitride compounds. This work deepens our understanding of atomic-scale diffusion in these technologically important materials.

The insertion and diffusivity of interstitial species in metallic beryllium are discussed in this work using a multi-scale methodology, coupling first-principles calculations and a multi-site approach. Emphasis is placed on the main interstitial species, i.e., H, C, N and O atoms. The results show that the most stable site is strongly dependent on the nature of the interstitial atom. Indeed, carbon is most stable in octahedral sites and hydrogen tetrahedral sites, while oxygen and nitrogen are most stable in basal tetrahedral sites. From the stable insertion sites and symmetrical saddle points identified, a number of migration pathways were mapped. The diffusion pathways were then completed using nudged elastic band (NEB) calculations. The diffusivity of the atoms shows an isotropic behavior as expected for carbon, which shows a strong anisotropic behavior with faster diffusivity along the basal plane. These theoretical results are in agreement with known experimental data, especially for hydrogen diffusion. The discrepancy between theory and experiment is corrected by the vacancy trapping effect. Finally, this paper theoretically determines the Arrhenius parameters of each diffusing species, in particular carbon and oxygen, for which no data were available.

First reliable experimental oxygen diffusion coefficient data have been obtained in a Ti 48.3 Al 47.7 Cr 1.9 Nb 2.1 near-gamma GE alloy through secondary ion mass spectrometry (SIMS) depth profiling measurements of 18 O isotopes between 500 degrees C and 700 degrees C. The following expression of diffusion coefficient D has thus been derived: D (m 2 /s) = ( ) 10 - 10 . 6 +/- 1 center dot exp - 107 +/- 10 kJ .mol - 1 / RT . These data have been compared with theoretical calculations from literature, showing reasonable agreement concerning the activation energy, but significant discrepancy regarding the D values.

The role of nitrogen in the oxidation of Ti-2W, Ti-10Al-2W (at.

A detailed overview of oxygen insertion in titanium nitrides is presented. Our atomic-scale approach is based on DFT point defect energetics and thermodynamics based on the Independent-Point-Defect Approximation (IPDA). In the first part, the study of intrinsic defects (vacancies, anti-sites, interstitials, and dumbbells) is carried out in the delta-TiN, & varepsilon;-Ti2N, and delta '-Ti2N binary compounds. It is shown that nitrogen vacancies are the main point defects in all of the Ti-rich nitrides. The N-rich phases show a more complicated behavior, with either interstitial N or Ti vacancies as the predominant defects. Noticeably, while the stability of delta ' is pointed out as controversial, the thermodynamic modeling of & varepsilon;- and delta '-Ti2N shows that a proper identification of the relevant interstitial sites is crucial for understanding the physico-chemistry of these two nitrides. The thorough investigation of the Ti-N phase equilibria resulting from point defect thermodynamics confirms the validity of the chosen IPDA approach, which provides a solid basis for further studies, forming the second part of this work, on oxygen effects in Ti nitrides. The effect of O on the point defect structures of Ti nitrides is studied, and it is shown that this element has a strong tendency to substitute nitrogen, explaining the diffusion barrier effect of the nitrides. Finally, to emphasize the far-reaching practical implications of these atomic-scale results, we consider the oxygen behavior and related oxidation trends of Ti-based alloys under air conditions. We show how ab initio IPDA helps to rationalize the identification of the relevant Ti-N-O system at stake for oxidation, the roles of the phases, and the external conditions to which it is subjected as it relaxes toward equilibrium. We also show how the thermodynamic criteria derived from IPDA can help unravel the ambiguous experimental links between nitride formation and lower amounts of O dissolved in the metal of preoxidized Ti-based alloys.

The effect of aluminium, tungsten, tantalum and hafnium on the oxidation behaviour of titanium was investigated. Model alloys were oxidized in air for 5000 h at 650 degrees C. All alloying elements decreased oxide growth and oxygen dissolution in the metal; tungsten was the most efficient. The oxygen diffusivity decreased with aluminium. Tungsten enhanced the formation of Ti2N at the oxide/metal interface which decreased oxygen dissolution in the alloys. Experiments in Ar-20%O2, where Ti2N could not form, confirmed the major role of nitrogen on the oxidation resistance of tungsten-containing alloys. The ternary model alloy Ti-10Al-2W outperformed the high-temperature alloy Ti6242S.

We analyse herein the interfacial region formed between a borosilicate glass coating and Ti-6Al-4V alloy after a transient of 8 h at 1150 degrees C. The correlation of two optical and electron microscopy methods (Raman and Backscatter Electron Diffraction imaging) enabled an accurate description of this particular interface. This interface is composed of five different phases, namely Ti2O3, TiB2, TiB, Ti5Si3 and Ti6Si2B, and it proves to be protective against oxygen permeation. Finally, this study also enabled the development of a Raman database for Ti borides, silicides, and borosilicides, in part from experimental and more completely from theoretical (DFT computations) data.

This work revisits hydrogen insertion in the wurtzite beryllium oxide in order to fill the gap in scientific knowledge regarding the diffusion coefficients of the different chemical states of hydrogen (neutral, charged, or molecular). Both first-principle and macroscopic models were used to this end. In the former, two exchange-correlation functionals (PBE and SCAN) were used to compute the properties of interest and accuracy was then discussed. Regarding the behavior of interstitial insertion of hydrogen, this work is slightly different from previous works; hydrogen was mainly found in the form of charged H- or H+ ions, depending on experimental conditions. In regard to diffusivity properties, a complex migration pathway was found for hydrogen cation. Finally, the present study succeeded in producing a reliable set of diffusion coefficients for neutral, charged, and molecular hydrogen.

The purpose of the present study is to investigate the effect of post heat treatment as well as the manufacturing strategy on the microstructure evolution during martensite decomposition of Ti–6Al–4V manufactured by laser powder bed fusion (LPBF). The microstructural evolution was tracked using in situ high-energy synchrotron X-ray diffraction and differential thermal analysis. The phase fraction, the mean lattice parameters, d-spacing and the Full Width at Half Maximum (FWHM) variations were determined by Rietveld refinement from XRD patterns during continuous heating up to the single β phase domain with different heating rates and manufacturing strategies. The complementarity of characterization tools clearly evidences a shift of the α/α' → β transformation kinetics toward higher temperatures as the heating rate increases. The variations in transformation kinetics are discussed with regard to the predicted values at thermodynamic equilibrium. In addition, the combined analysis of d-spacing and FWHM shows the deviations from linearity at intermediate and high temperatures that are related to the heating and manufacturing strategy. These variations were analyzed in regard of internal stress relaxation or changes of chemical composition. Moreover, the anisotropic elastic distortions in martensite α' caused by the manufacturing strategy were examined.

This work reexamines the insertion of O atoms in the L10 γ-TiAl system using first-principles calculations and thermodynamic modeling in the independent point defect approximation. It includes a study of intrinsic point defects, the insertion of many alloying elements (more than twenty were considered), as well as a study of their interaction with oxygen. The formation of complex defects composed of either vacancies, anti-sites or solute elements is then studied. Results at the atomic scale show a high segregation of oxygen in titanium-rich environments: oxygen easily segregates onto Ti anti-sites (TiAl) and alloying elements are located in the vicinity of Al sub-lattices. DFT point-defect energetics shows that there is a clear correlation between the nature and site preference of an alloying element, and the oxygen segregation energy in the vicinity of this solute. The thermodynamic model shows that at equilibrium, oxygen does not occupy isolated interstitial sites but prefers to be located in the vicinity of Ti anti-sites or alloying elements. The effect of this strong segregation on oxygen diffusivity is discussed hereinafter. Results show a strong slowdown in oxygen diffusivity due to intrinsic defects. For Ti/Al >0.5 ratios, the traps for O diffusion are mainly constituted by Ti anti-sites, and the addition of solutes does not contribute much to the trapping of diffusing O atoms. For Ti/Al < 0.5 ratios however, the contribution of solutes to trapping phenomena can be very important, and a decrease by 1–2 orders of magnitude of effective O diffusion coefficients can be observed for temperatures around 800–1100 K.

Nitrogen and carbon play an important role in the oxidation of Ti-Al intermetallic alloys in air. The insertion and diffusion of these elements in the γ-TiAl phase are investigated by first-principle computations. The accommodation of C and N atoms in several likely interstitial positions has been evaluated using density functional theory (DFT) calculations. The results show that both carbon and nitrogen prefer Ti-rich environments. Then, considering the possible jumps among the stable and metastable interstitial sites, the diffusion coefficients have also been obtained from ab initio calculations. According to the Transitional State Theory, in order to compute atomic jump rates, diffusion energy barriers and vibrational modes need to be known. Herein, barrier energies are obtained using the Climbing Image Nudged Elastic Band method. Vibrational properties are computed using the finite displacement method. Finally, diffusion coefficients are obtained solving the transport equation in the infinite time limit, using an analytical approach. The obtained results are compared to the diffusion of oxygen in γ-TiAl, investigated in previous studies. An anisotropic diffusion is obtained for all the interstitial species.

The goal of this study is to investigate the mechanical and elastic characteristics of the Mn15Si26 compound via experimental nanoindentation measurements and ab-initio calculations. The mechanical properties such as Young's modulus (E) and nanohardness are important inputs for improving the design and mechanical reliability of thermoelectric modules. The high-energy X-ray diffraction pattern of Mn15Si26 has been indexed with the Miller indices of a tetragonal crystalline structure whose cell parameters are the following: a = b = 5.535(3) angstrom and c = 65.552(4) angstrom. Nanoindentation measurements, with a Berkovich indenter tip have been performed on higher manganese silicide (HMS) compound mainly composed of Mn15Si26 grains. For the first time ever, it has been evidenced that both elastic modulus and nanohardness of the latter varied significantly depending on their crystallographic orientations provided by electron backscatter diffraction. Nanohardness and Young's modulus along the < 001 > orientations are higher than the < 100 > ones. The nanohardness value of Mn15Si26 ranges from 16 GPa to 20 GPa and the Young's modulus measured varies between 234 GPa and 300 GPa. The stiffness tensor (S-ij = (C-ij)(-1) of Mn15Si26 has been deduced from these experimental measurements as well as calculated using Ab-initio calculations. The macroscopic elastic modulus (E, G, B) and Poisson's coefficient have been examined and discussed and their 3D-representation has been plotted. The mechanical anisotropy hereby evidenced as the existence of anisotropy of the thermoelectric properties could be a significant factor for the mechanical reliability of thermoelectric modules which consisted of Mn15Si26 legs with a possible preferred crystallographic orientation induced during their fabrication. (C) 2021 Elsevier B.V. All rights reserved.

Although many materials are used under extreme conditions, the effects of stress and strain are often neglected in material studies, especially when studying the solubility or calculating the diffusivity of substitutional or interstitial species. In this work, a general method based on first-principles calculations and elasticity theory in fcc systems is presented to fill this gap. The case of hydrogen in aluminum is investigated in detail as an application sample by comparing results from the density functional theory (DFT) and the elasticity theory. Additional systems, Ni, Cu and Pd, are also examined for hydrogen but in the framework of the elasticity theory only. Different types of stresses, i.e. hydrostatic, multi-axial and shear stress, are investigated for comparison purposes. The symmetry break induced by the loading is analyzed at the atomic scale by calculating the jump rates, at macroscopic scale from computed diffusion coefficients. Equations of diffusion are developed for each loading. Results show that the effect of loading on atomic parameters - insertion energies, energy barriers, etc. - can be accurately captured by the elasticity theory in terms of elementary parameters calculated using the DFT. Results show that the effect of stress is weak in the case of hydrogen. (C) 2021 Elsevier B.V. All rights reserved.

The segregation of hydrogen and vacancies at the Σ5(210)[001] symmetric tilt grain boundary (GB) was studied by atomic scale simulations in Ni. First, the hydrogen segregation energies and hydrogen–hydrogen pair interaction energies were calculated on every interstitial site of the GB. The vacancy–hydrogen clusters’ formation energies were also determined on the most favorable site. All these calculations were done using the density functional theory. Second, based on these elementary energies, a free energy functional was built to determine the concentration of segregated hydrogen and of vacancy-hydrogen clusters, as a function of the bulk hydrogen concentration and the temperature. It was found that two configurations exits in typical conditions where embrittlement is observed experimentally: H segregation only, with up to 3 hydrogen atom per structural unit or 50% occupancy by VH 5 clusters (1 cluster every two structural unit). The cohesive stress and ideal work of fracture were evaluated by fracturing the GB with different degrees of hydrogen and vacancy segregation. H segregation alone (no vacancy) decreased the work of fracture by 25%. A significantly larger decrease of cohesion was obtained when considering vacancy-hydrogen clusters. A maximum drop of the cohesive stress, of a magnitude of 40%, was obtained when every structural unit was hosting a VH 4 cluster. Finally, these data were transformed into cohesive stress models. They were used to evaluate the degree of localization of the shear displacement at the crack tip. The conclusion is that, even if cohesion is very significantly decreased, shear localization is still effective, meaning that dislocation emission should occur at the expense of crack propagation. The comparison with other grain boundaries in the literature shows that the GB studied is almost an ideal sink and therefore is very favorable for the formation of equilibrium VH n . It represents more an upper bound of the effect. Therefore, extra ingredients should be considered to explain the embrittlement observed experimentally.

This work is a first-principles study of the insertion and diffusivity of oxygen in the [Formula: see text]-TiAl L10 system. Five interstitial positions were identified as stable. One, however, the 2h site a pyramid composed of a Ti square topped by an Al atom, was found more stable than the others. The oxygen interactions with the TiAl system were thus studied and analyzed in detail using vibrational, elastic and electronic properties. The results show that the O atom prefers to be surrounded by Ti atoms and tries to minimize the number of bonds with aluminum. The diffusion mechanism is subsequently studied at the atomic scale, by analyzing displacements between stable interstitial sites. The oxygen diffusivity is found to be anisotropic and the components in the x and z direction, D x and D z , are then calculated and compared with those of O diffusion into other Ti-Al alloys. The analysis of results shows two effects. First, the stability of sites is related to the number of O-Al bonds, the fewer there are, the more stable the site is, and second, the diffusion is faster when the content of interstitial sites composed of many Ti atoms is low.

This work is a first-principles study of the insertion and diffusivity of oxygen in the [Formula: see text]-TiAl L10 system. Five interstitial positions were identified as stable. One, however, the 2h site a pyramid composed of a Ti square topped by an Al atom, was found more stable than the others. The oxygen interactions with the TiAl system were thus studied and analyzed in detail using vibrational, elastic and electronic properties. The results show that the O atom prefers to be surrounded by Ti atoms and tries to minimize the number of bonds with aluminum. The diffusion mechanism is subsequently studied at the atomic scale, by analyzing displacements between stable interstitial sites. The oxygen diffusivity is found to be anisotropic and the components in the x and z direction, D x and D z , are then calculated and compared with those of O diffusion into other Ti-Al alloys. The analysis of results shows two effects. First, the stability of sites is related to the number of O-Al bonds, the fewer there are, the more stable the site is, and second, the diffusion is faster when the content of interstitial sites composed of many Ti atoms is low.

Martensite is a supersaturated solid solution of carbon in body-centered iron wherein interstitial carbon atoms preferentially occupy a single octahedral sublattice. Despite a century of research, the mechanism of this long-range ordering is still a subject of debate. Recently, Zener's theory of ordering was challenged both experimentally and theoretically. In an attempt to settle the controversy, we investigated by density functional theory the ground states of Fe-C configurations having various degrees of order. We conclude that the fully Zener-ordered configurations are always the most stable energetically, thus confirming Zener's theory. Comparison with mean-field elasticity and Ising-type modelling supports the elastic origin of Zener ordering. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.