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.

The loss of ductility of a Ti-48Al-2Cr-2Nb (at%) near-gamma alloy after exposure to 80 % Ar-20 % O2 at 700 degrees C for 1 h and 100 h has been investigated. In addition to marked decreases in ductility classically reported, increases in yield stress reaching +25 % have been observed. SIMS depth profiles enabled the determination of the 18O isotope penetration depth via volume diffusion, which reached about 10 mu m after 100 h of exposure. Site-specific lift-out by FIB, along with EBSD and TEM characterizations, were employed to examine the local transformations occurring in the diffusion zone and within the volume of exposed tensile specimens. In-situ TEM straining experiments were carried out on exposed and un-exposed specimens to study the influence of exposure on the microscopic mechanisms occurring in the bulk material. Results revealed the formation of chromium-rich precipitates and a recrystallized zone extending up to a depth of 15 mu m from the surface after exposure. The dislocations in the subsurface layer were non-linear, likely due interactions between dislocation lines and oxygen solutes from the exposure atmosphere which potentially contribute to strengthening of the subsurface layer and ultimately to premature failure in this region. The elevated scattering in the in-situ TEM data did not allow to correlate the dislocation jump distances to the increase in yield stress.

The evolution with oxidation time of the chemical composition of the oxide scale of In625 was experimentally studied at 900 degrees C under an atmosphere of N-2-14.7%O-2-30%H2O (vol%). As time goes by, the initial chromia scale was progressively replaced by other oxides rich in Ni, Nb, Ti or Mn, decreasing the activity of chromia. By developing a new transient CFD model, this temporal evolution in chromia activity was simulated for different superficial gas velocities. Simulation results well represented the changes observed experimentally with a decrease in volatilization rate as chromia was progressively consumed by volatilization. The simulation provided new insights on the coupled influence of local gas velocity and composition on the heterogeneities in volatilization rate and chromia coverage evolving with time, as observed on samples.

The precise quantification of oxygen concentration in titanium is crucial for various high-performance applications. In this study, we developed reference samples to calibrate high-resolution micro laser-induced breakdown spectroscopy (HR-mu LIBS) for oxygen analysis in titanium. Two fabrication methods were employed: (i) oxidation of titanium powder at high temperature in a controlled atmosphere with oxygen, followed by spark plasma sintering (SPS), and (ii) blending titanium powder with TiO2 powder before sintering by SPS. Homogenized samples were then used to establish a calibration line correlating HR-mu LIBS intensity with known oxygen concentrations. To evaluate the HR-mu LIBS method, a titanium sample oxidized 500 h at 650 degrees C in Ar-20 %O-2 was analyzed using both HR-mu LIBS and electron probe microanalysis (EPMA). The results demonstrate a strong agreement between the two methods, with HR-mu LIBS offering superior speed and improved accuracy at low oxygen concentrations (< 3 at. %).

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.

Pt-rich gamma-gamma' bond-coatings for thermal barrier coating systems may lead to detrimental pore formation. An extensive analysis of literature on voids nucleation and growth in the different metallic components of several kinds of TBC systems was performed. The study then focused on Pt-rich gamma-gamma' bond-coating/Ni-based superalloy systems employing an original statistical analysis of voids formation and spatial distribution through secondary electron microscopy with focused ion beam sequential cross sectioning and synchrotron X-Ray tomography. It was found that pores at the Pt-rich gamma-gamma' bond-coating/superalloy interface form due to the Kirkendall effect, with the highest concentration located at the Pt diffusion front.

High-temperature oxidation of titanium leads to the formation of an external oxide scale and oxygen ingress into the metallic titanium material. Oxygen ingress can be significant due to the high solubility of O within Ti. An oxygen-rich layer (ORL) thus forms beneath the external oxide scale, exhibiting a brittle behavior. Microtensile specimens were used in order to exacerbate surface effects, i.e., surface reactivity in the case of the oxidation of titanium. Playing with the specimen thickness and pre-oxidation durations, it was possible to evaluate the evolution of tensile strength as well as the reduction in ductility for deep extensions of ORL relative to the specimen thickness (high fraction of ORL). In addition, ultrathin specimen extraction at different locations within the ORL depth aimed at better identifying the gradient of properties within the ORL. This micromechanical approach was applied to a commercially pure titanium (CP-Ti grade 2) and to a structural titanium alloy (Ti6242s). Both strengthening and loss of mechanical properties (yield strength and ductility) were observed depending on the material and oxygen ingress. While CP-Ti demonstrated an increase in mechanical strength up to ORL representing 80 pct of the gage section, Ti6242s experienced a loss of mechanical resistance even for the shortest exposure times (the ORL representing 10 pct of the gage section).

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.

Surface effects were investigated using ultrathin specimens with thicknesses in the order of the grain size of the material. The candidate material was a polycrystalline Ni-based superalloy (Alloy 718) purposely heat treated to document both the effects of the grain size and the metallurgical state, i.e., solid solution and precipitation hardened state, on the polycrystalline-tomulticrystalline behavior. Ultrathin tensile specimens were prepared with a dedicated technique to obtain specimens with thicknesses ranging between 20 and 550 mu m, then tensile tested at room temperature. The polycrystalline-to-multicrystalline transition (PMT) was found to depend on the material grain size relative to the specimen thickness and to impair severely the tensile strength of the material. The yield strength, ultimate tensile strength (maximal stress on the stress-strain curve) and strain-to-failure severely dropped for specimens thinner than approximately two times the grain size of the material regardless of the metallurgical state. Such a decrease in tensile properties is mainly attributed to free-surface effects acting as an escape sink of dislocations, thus leading to a significant decrease of the primary dislocations density within the surface grains in comparison with the core grains. Interestingly, difference in work-hardening behavior with size reduction was found between both precipitation states, the solid solution state being more sensitive with the size reduction. The decrease in tensile properties was not found as expected from the commonly reported "thickness/grain size (t/D)"ratio. Therefore, a numerical approach using a modified Berveiller-Zaoui self-consistent model based on a continuum crystal plasticity approach was conducted in the present paper to distinguish microstructural features acting as strengthening (dislocation accumulation) and softening (dislocation escape at the free-surface) features. 3D numerical materials were produced using Voronoi tessellation methods to represent the fraction of "core grains" versus "surface grains". These fractions were then used as microstructural parameters for the identification of a crystal plasticity model using mean-field homogenization with different populations of grains, i.e., core versus surface features. The present work aimed at distinguishing the mechanical behavior of surface grains from core grains in Alloy 718 Ni-based superalloys using various thicknesses of specimens and different microstructure and metallurgical state variants.

Corrosion mechanisms in the presence of molten salts and under deposits have been studied for over 50 years.Still, they remain an important topic because of their relevance in various technological sectors, including energy production and storage, aeronautics, the glass industry, etc.The degradation in this form of corrosion is often quite significant.It occurs over a wide temperature and pressure range with very complex phenomena clearly influenced by the presence of (even at the ppm level) corrosive species.

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.

Ti–6Al–4V alloys manufactured by laser or electron powder bed fusion (L-PBF and E-PBF) with or without hipping treatment have different microstructures from foundry alloys. Their oxidation kinetics at high temperatures between 500 and 600 °C for durations up to 2,000 h were compared. The effect of oxidation on their room temperature tensile embrittlement was quantified. It was shown that the growth kinetics of the brittle fracture zone, of the zone with cracks at 1

Ni–30Cr model alloy was used to study chromia-scale formation behaviour. During its formation the mass variation signal is weak and it is important to take specific care to measure oxidation kinetics by thermogravimetric analysis. Symmetrical design allows the determination of very small mass changes in compensating buoyancy effects and limiting measurement drift. The mass variation measurement comprises noise coming from different sources that affects the electronic signal. It must be minimised to improve accuracy. This involves keeping the room-temperature constant, strictly balancing the beam and minimising buoyancy effects. By this way it was possible to acquire kinetics and to measured rate constants, kp, in a range from 3.10–5 to 3.10–10 mg2.cm−4.s−1 equivalent to 10–12 down to 10–17 cm2.s−1. Oxygen partial pressure ( P_O_2 ) was monitor and revealed an abnormal consumption of oxygen at the beginning of the thermal exposure. Experiments with an inert material showed parasitic reactions identified by mass spectrometry as combustion of impurities. As oxygen consumption is not only due to oxidation of the sample, corrosion kinetics can’t be deduced from it. Hence, to determine whether the oxygen supply from the gas is a limiting parameter, a model, which quantifies oxygen consumption by sample oxidation within the thermobalance, is proposed.

The influence of low and steady loading on the oxidation behaviour and oxygen diffusion within Ti6242S was investigated at 650 degrees C in air. Electron probe microanalyser (EPMA), secondary-ion mass spectrometry (SIMS) and high energy synchrotron X-ray diffraction (S-XRD) were used to quantify the oxygen distribution within the oxygen-enriched layer beneath the external oxide scale. Rietveld and peak by peak methods were used to evaluate the average response versus the crystal-oriented response of this diffusion process under load. Interestingly, a thermo-mechano-chemical coupling occurs during the creep-oxidation experiment even for moderate applied stresses (25-70 MPa) and demonstrates: (i) a decrease of the oxygen concentration at the metal/oxide interface, and (ii) a curvature change of the oxygen diffusion profile with load application. A qualitative thermomechano-chemical approach is proposed to model the modification of the diffusion law of the oxygen within Ti6242S due to application of a mechanical loading to explain observed experimental results.

Heat treatments and oxidation tests were performed on systems composed of a single-crystal AM3 superalloy with a NiCoCrAlYTa+Pt coating to investigate the effect of Pt on Ta and Ti diffusion at high temperature. Experimental results were compared to an AM3 superalloy directly coated with Pt. For all the studied systems, the effect of Pt on elemental activities was evaluated through thermodynamic calculations. Pt diffusion was found to be faster in the NiCoCrAlYTa coating than in AM3. High Ta contents were measured in the Pt-rich γ′ phase below the surface and significant Al and Cr transport toward the surface was observed, in agreement with thermodynamic calculations which predicted an important decrease in their activities in the presence of Pt. An outward diffusion of Ti was also noticed, whereas calculations did not show a decrease in Ti activity due to Pt. Other discrepancies between experiments and thermodynamic calculations were noted and are discussed in this work.

This work introduces a new high-throughput method to characterize the oxidation behavior of chemically graded Ni-based alloys in order to feed databases destined to numerical metallurgy approaches. A Ni–wCr–3Al (w ∈ [0, 30]) chemically graded material was obtained from two homogeneous samples by a diffusion couple method at 1300 °C for 100 h. The composition range was selected in order to observe the three types of oxidation behavior identified in the reference work of Giggins and Pettit (Giggins and Pettit in Journal of The Electrochemical Society 118:1782, 1971). The excellent agreement between simulated and experimental diffusion profiles validated the experimental method used to manufacture the chemically graded material (CGM). The CGM was then oxidized at 1200 °C in air. Surface and cross-section characterization was conducted by SEM/EDS and Raman spectroscopy to identify the oxides formed on the CGM. To accelerate the Raman characterization treatment, a method linking principal component analysis and K-means unsupervised clustering algorithm was developed. It allowed for the identification of the oxide type without peak indexation issues and is well suited for CGM. These results show that results similar to well-recognized reference experiments (Giggins and Pettit in Journal of The Electrochemical Society 118:1782, 1971) can be achieved using only one CGM.

The effect of increased productivity in L-PBF manufacturing on the metal dusting resistance of Inconel 625 was studied at 610 °C. Two sets of processing parameters were tested: the standard parameters given by the machine manufacturer and degraded parameters with increased scan speed and hatching distance. The materials obtained with degraded parameters presented numerous lacks of fusion at the surface and exhibited significantly shorter incubation times before corrosion. Carburisation was preferentially localised around the defects where the oxide scale was less protective and where the confinement of the atmosphere could result in an increase of the local carbon activity.