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.

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 development of the Kirkendall porosity was studied in fcc Ni-30Cr/Ni-10Si diffusion couples at 1176 degrees C using X-ray tomography, optical and scanning electron microscopy, and multicomponent diffusion simulation. Diffusion experiments were interrupted multiple times to monitor the porosity ex situ by tomography. This allowed tracking the position and size of thousands of pores over tens of hours. Pores detected by tomography were also observed by microscopy to determine the surrounding grain structure. The porosity depth profiles (number density, equivalent diameter, area/volume fraction) derived from 2D cross-sectional observations and 3D tomography were compared and the benefits of both methods discussed. Alloys of different grain sizes were used as starting materials to study the influence of the grain boundary density on the spatial pore distribution. Local analysis showed that pore nucleation was not significantly accelerated on grain boundaries compared to the grain interior, but that pore growth was faster along grain boundaries. The time-resolved pore distribution data indicated that the porosity evolved through both pore movement and growth-shrinkage. These mechanisms were discussed in view of the simulated vacancy flux profile.

The diffusion properties of polycrystalline materials depend on their grain shape and size, which determine the spatial distribution of grain boundaries. These morphological characteristics are of interest when evaluating an alloy ability to form a protective oxide scale by selective oxidation at high temperature. The composition changes induced by selective oxidation in 2D polycrystals were studied by finite element simulations. We examined the effect of the grain boundary orientation in lamellar polycrystals, and the effects of the grain size distribution in random equiaxed polycrystals. Fine-grained polycrystals were found to behave as uniform media. The effective diffusivity of fine lamellar polycrystals depends on the grain boundary orientation and is bounded by the upper and lower composite diffusivities, while the effective diffusivity of fine equiaxed polycrystals can be estimated by a modified Hart equation. The behavior of coarser equiaxed polycrystal was shown to vary according to the local grain size: the concentration at the alloy-scale interface is fully determined by the local grain size in larger grains, while it is affected by the surrounding grains in finer grains. Increasing the grain size dispersion led to a more scattered response and shifted the minimum interface concentrations toward lower values, which is expected to have a detrimental effect on the oxidation resistance.

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

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.

Grain-boundary diffusion affects composition profiles developed during selective oxidation in polycrystals. It is therefore desirable to consider the grain size when choosing an alloy or setting acceptable operating conditions for an application where high-temperature corrosion resistance is important. This paper aims to provide guidelines to evaluate subsurface composition changes as a function of grain size and oxidation conditions. We show that depletion profiles around isolated grain boundaries of Ni-based Cr2O3-forming alloys oxidized at 1000 ^∘ C can be described by a 2D diffusion model, where fast diffusion at the alloy–scale interface is considered in addition to grain-boundary diffusion. We then illustrate the general features of depletion profiles in polycrystals, in comparison with single crystals. Finally, we use a set of simulations covering a wide range of times, temperatures and grain sizes to identify regimes where a 1D model can be used, either because grain-boundary diffusion can be neglected or because it produces a uniform depletion front.

In an alloy subject to vacancy-mediated diffusion, differences in intrinsic diffusivities tend to produce vacancy excess and deficit. These are accommodated by mechanisms such as dislocation climb and pore formation. This is of concern in high temperature alloy-coating systems used in industrial applications, as pores may develop at the interface between the alloy and the coating, which is undesired. This paper presents a multicomponent diffusion model with two types of vacancy sinks/sources: one is associated with dislocation climb and generates lattice shift, the other one is associated with porosity increase/decrease. The model is designed toward a 1D implementation, and porosity is described with a local average volume fraction. Thermodynamic properties and mobility are modeled according to the Calphad method to allow future application to engineering materials. Finite-difference simulations run on two binary systems, NiCr and NiSi, illustrate the role of the two types of sinks in interdiffusion and pore development. Diffusion is found to be more sensitive to the sink strengths in the NiSi system, where intrinsic diffusivities have a stronger composition dependence. This work provides a basis for the evaluation of the parameters involved in vacancy generation/annihilation (e.g. dislocation density) from experimental data, such as concentration profiles obtained from diffusion couple experiments, and for the prediction of porosity in engineering materials.

The paper demonstrates how defects inherited from the deposition processes can severely impair the lifetime of MCrAlY coatings in service. The oxidation behavior of two NiCoCrAlY coatings was investigated at 1150 degrees C up to 500 h. The coatings had the same nominal composition but were processed by two different projection techniques: air plasma spray (APS) and high velocity oxy fuel (HVOF). Freestanding coating specimens were extracted from the coated system and thinned down to different thicknesses ranging from 520 to 15 mu m in order to investigate size effects inherent to the oxidation response. The oxidation rate of the APS coating was found to be insensitive to the specimen thickness, while that of the HVOF coating increased with the specimen thickness, due to greater intersplat oxidation. APS specimens thinner than 60 mu m experienced intrinsic chemical failure (InCF) due to Al consumption to form the Al2O3 scale. In comparison, HVOF specimens with a thickness of 367 mu m were subject to InCF after 250-350 h oxidation. This first stage of InCF resulted in the formation of a Cr2O3 layer at the Al2O3/metal interface once Al activity in the MCrAlY coating was low enough to thermodynamically allow Cr2O3 to form. In addition, thick HVOF specimens developed mechanically induced chemical failure (MICF) resulting in the formation of (Ni,Co)(Cr,A1)(2)O-4 spinels on top of the Al2O3 scale and within oxide intrusions. The occurrence of MICF was associated with the concomitant effects of Al consumption due to intrusive oxidation and the spallation of the external Al2O3 scale.

Interdiffusion coefficients were measured in NiSi and NiCr systems from diffusion couples under hydrostatic pressures between 50 and 326 MPa, at 1200 °C. Uniaxial compression creep tests were also carried out on Ni–Cr diffusion couples at 885 °C, 940 °C and 1000 °C with stress values between 0 and 25 MPa, to observe the effect of applied stress on diffusion. A numerical inverse method was elaborated to determine interdiffusion coefficients that takes into account the plastic strain. The comparison of all the diffusion coefficients shows no significant effect of stress for the creep tests and a slight decrease of the interdiffusion coefficients for the hot isostatic pressing (HIP) tests in both systems when the compressive stress increases. This variation is less than twofold, thus the effect is negligible compared to the scatter observed for one diffusion coefficient from different sources of the literature.

The high temperature oxidation behavior of an APS processed ??? NiCoCrAlY coating was investigated at 1150 ?C. Ultrathin freestanding coating specimens ranging from 16 to 240 ?m in thickness were tested for short-term oxidation time (< 300 h). Intrinsic chemical failure (InCF) occurred after the full consumption of Al, and resulted in Cr2O3 growth and CrN precipitation at the Al2O3/alloy interface. Mass gain slowed down after InCF, even leading to a mass loss associated with the volatilization of Cr2O3. InCF occurred after [10?20] h and [20?100] h for the - 21 ?m and - 31 ?m-thick specimens, respectively. Severe metal recession was observed after InCF.

This paper examines the oxidation behavior of thin specimens of cast NiCoCrAlY alloys at 1150  $$^\circ {\rm C}$$ through successive stages, from $${\rm Al}_2{\rm O}_3$$ growth to complete alloy conversion to oxide. Five alloy compositions were used, with varying fractions and compositions of $$\gamma$$ and $$\beta$$ . The time evolution of the alloy composition during $${\rm Al}_2{\rm O}_3$$ growth was simulated using the DICTRA module of Thermo-Calc and calculated analytically in the approximation of flat profiles. Simulated and experimental profiles were found to be in good agreement, indicating that the phase equilibrium and mass balance were correctly reproduced in the simulations. Local variations of alloy composition were observed in thinner specimens and found to be comparable with the variations expected from the uncertainty on the initial specimen thickness. The variations observed in the time-to- $${\rm Al}_2{\rm O}_3$$ failure were greater than expected on this basis, suggesting that additional sources of variability were in effect. Alumina failure was followed by the growth of a $${\rm Cr}_2{\rm O}_3$$ layer at the alloy–scale interface. Similarly, Cr consumption eventually led to $${\rm Cr}_2{\rm O}_3$$ failure, and Ni- and Co-containing spinel oxide formed, converting the $${\rm Cr}_2{\rm O}_3$$ at the alloy–scale interface and the $${\rm Al}_2{\rm O}_3$$ at the scale–gas interface. The remaining NiCo alloy was then converted to (Ni,Co)O. This sequence occurred without abrupt increase in the mass gain, due to the continued presence of the remnant $${\rm Al}_2{\rm O}_3$$ layer, and to the small amount of metal left to oxidize when the (Ni,Co)O eventually broke through the scale. The evolution of the scale composition throughout the oxidation stages is discussed based on an analysis of the thermodynamic conditions at the alloy–scale interface.

The effect of surface deformation on selective oxidation is studied on a Ni-30Cr alloy prepared by grinding (SiC paper) vs. polishing (diamond suspensions), and oxidized in air at 340-600 degrees C. Imaging and subsurface composition profiling by transmission electron microscopy are combined to investigate the relationships between microstructure, diffusion and oxidation. The depth of deformed material is found to be critical to selective oxidation. Atypical Cr depletion profiles reflect an abrupt change in diffusion properties between the severely deformed subsurface and the bulk. The relative contributions of grain boundary, dislocation and bulk diffusion to the Cr flux are examined based on short-circuit diffusion theory.

Spark plasma sintering (SPS) enables the manufacturing of TiAl alloys with an exceptional combination of low density and mechanical properties such as acceptable ductility at room temperature and high strength at high temperature. However, TiAl alloys are known to exhibit low oxidation resistance above 700 °C. The oxidation of a Ti–48Al–2W–0.1B (at. %) processed by SPS was investigated at 800 °C. Coupons were oxidized in air and in Ar-21O2 (vol%), in isothermal and cyclic tests. In air, the alloy formed a mixture of Ti and Al oxides, but oxidation was slower than typically observed for W-free alloys. The oxide scale and underlying alloy were characterized by X-ray diffraction and electron microscopy in order to examine the beneficial role of W in the oxidation resistance. The main constituents of the reaction product after reaction in air may be described as follows (from gas to alloy): TiO2/porous Al2O3/TiO2 + Al2O3/TiN + Al2O3 + W/TiAl2 + W/TiAl. Close examination suggests that the relatively good oxidation resistance of the alloy is related to W doping of TiO2 in the mixed Al2O3 + TiO2 layer. The alloy formed an Al-rich oxide scale with much slower kinetics in Ar-21O2, confirming the detrimental role of N in the oxidation process.

The grain boundary diffusion of chromium in polycrystalline nickel was studied by means of tracer experiments at 346-668 degrees C. Intensity-depth profiles were recorded by secondary ion mass spectrometry (SIMS), which allowed short diffusion distances, and therefore relatively low temperatures, to be examined. Individual grain boundaries in coarse-grained substrates produced profiles with a variety of shapes, reflecting the variability of diffusivities in a polycrystal, and even within a grain boundary. Average diffusivities were also measured using cold-rolled substrates, which rapidly recrystallized and provided finer microstructures. The simplifying assumptions usually made in processing B regime data were found not to be applicable here, because of the shallow measurement depths. The errors made by using approximate solutions to the diffusion equation are discussed, and a data processing method adapted to the conditions of SIMS measurements is used. A method to study diffusion tails with smooth slope variations is also demonstrated. The measured diffusivities were found to be in agreement with higher temperature data from the literature.

The ability to simulate the oxidation behavior of multicomponent alloys is a powerful tool for alloy development and oxidation research. The present work shows how the DICTRA module of Thermo-Calc can be applied to reproduce composition profiles in NiCoCrAl alloys during Al 2 O 3 scale growth when used in conjunction with appropriate Calphad thermodynamic and diffusion mobility databases. Profiles were calculated in pure oxidation and oxidation–dissolution regimes to simulate reaction in air and in a molten silicate. For each regime, a simple analytical expression was used to set the outward Al flux at the alloy surface as a boundary condition. The simulations were performed using different combinations of thermodynamic and diffusion mobility databases, which demonstrated the relative importance of the thermodynamic and kinetic contributions to the interdiffusion coefficients, and in turn to the concentration profiles. The simulations done with the developmental NISTCoNi-mob mobility database were found to be in good agreement with experimental data, while those done with MOBNI4 significantly underestimated Al depletion (and Co, Cr and Ni enrichment), due in part to an incomplete description of diffusivity in the Co–Ni binary system. All the tested thermodynamic databases yielded similar results. This work provides a quantitative illustration of the importance of critically assessed diffusion mobility descriptions in designing oxidation-resistant materials.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Revisiting the role of substrate microstructure and short-circuit diffusion in the oxidation of Ni-Cr alloys Thomas Gheno, Clara Desgranges, Laure Martinelli