Oxide Growth Stresses Research Articles

A novel NiCoCrAlPt high-entropy alloy with superb oxidation resistance at 1200 °C

A new Y-Hf co-doped NiCoCrAlPt high-entropy alloy (HEA) was developed and subjected to a study on its long-term oxidation behavior at 1200 °C. The alloy shows a two-phase eutectic microstructure that comprises a γ' matrix and β’ martensite precipitates. It is indicated that the alloy has superb oxidation resistance by forming an exclusive, slow-growing Al2O3 scale that remains intact after oxidation at 1200 °C for 1000 h. This exceptional performance of the alloy is attributed to the refined microstructure, increased Y/Hf solubility, low oxide growth stress, and a Pt,Al-rich oxide/alloy interface without S segregation. The findings offer scientific insights for the design of Pt-containing oxidation-resistant HEAs.

Effect of interface undulation on the high temperature oxidation behaviors of grit-blasted and coated zircaloy in pressurized water

Effect of interface undulation on the high-temperature oxidation behaviors of grit-blasted and coated zircaloys in pressurized water was investigated. The grit-blasted and the coated zircaloy substrates present significantly higher corrosion rate than the as-received flat substrate during the early stage of exposure. Numerical analysis suggests the interface undulation results in redistribution of the oxide growth stress of zircaloy, as well as varying the local stress concentration, which induces initiation of cracks and therefore increases the corrosion rate of zircaloy. Propagation of cracks along the interface drives delamination of the protective coating. Results of this work not only promote understanding of the coating/substrate interface evolution in high-temperature pressurized water, but also provide a guide to design corrosion-resistant zirconium alloys and/or coatings for fuel cladding.

The effects of TGO growth stress and creep rate on TC/TGO interface cracking in APS thermal barrier coatings

To design reliable thermal barrier coatings (TBCs), it is essential to elucidate the interface failure mechanism and establish a better life-prediction model. Up to now, how the variations of thermally grown oxide (TGO) growth stresses and creep rates affect the interfacial cracking behavior between top coat (TC) and TGO has not been clearly studied. In this work, the thermos-mechanical model with simplified cosine curve interface morphology in TBCs is developed utilizing numerical simulation software (ABAQUS). According to the calculated stress states of TC/TGO interface, the possible damage scenarios are inferred. On this basis, the interface crack nucleation and propagation is simulated by the surface-based cohesive behavior. General features of the interface crack are in good coincidence with previous conjectures. Furthermore, large TGO growth stresses will trigger off premature cracking, conversely, the formation of TC/TGO interface crack can be postponed due to large TGO creep rates. In addition, the dominant fracture mode of TC/TGO interface is impacted by the variations of TGO growth stresses as well as its creep rate. The position of interface fracture mode transition has been shifted to a certain extent.

Discontinuities in Oxidation Kinetics: A New Model and its Application to Cr\u2013Si-Base Alloys

Some alloys such as many Cr-based systems show mass gain discontinuities during thermogravimetric measurements which strongly affect the oxidation kinetics. The behaviour cannot be described by the current models available in the literature. Thus, a novel k_mathrm{para}–k_mathrm{lin}-P-model was developed to describe oxidation kinetics during the isothermal exposure of materials which show such behaviour. Beside the parabolic rate constant k_mathrm{para} and the linear mass loss constant k_mathrm{lin}, the P-value and f_P are introduced to take into account spontaneous rapid mass gains due to local oxide scale failure. The parameter P serves as a measure for the mass gain due to discontinuous events and f_P is the frequency of such events. The both parameters can be related to oxide scale detachment and growth stresses. The application of the model is demonstrated for the oxidation of Cr–Si-based alloys in synthetic air at 1200^{circ }hbox {C} for 100 h. For these alloys, the origin of the mass gain discontinuities is discussed and the meaning of P and f_P is explained in more detail. Using this newly developed model, an insight into growth and nitridation resistance of oxide scales as well as scale adhesion is gained.

Effects of Roughness on Stresses in an Oxide Scale Formed on a Superalloy Substrate

The effects of surface roughness on the stresses in an alumina scale formed on a Fecralloy substrate are investigated. Spherical indenters were used to create indents with different radii and depths to represent surface roughness and then the roughness effect was studied comprehensively. It was found that the residual stresses in the alumina scale formed around the rough surface are almost constant and they are dominated by the curvature rather than the depth of the roughness. Oxidation changes the surface roughness. The edge of the indent was sharpened after oxidation and the residual stress there was released presumably due to cracking. The residual stresses in the alumina scale decrease with increase in oxidation time, while the substrate thickness has little effect, given that the substrate is thicker than the alumina scale. Furthermore, the effect of roughness on the oxide growth stress is analysed. This work indicates that the surface roughness should be considered for evaluation of stresses in coatings.

Water vapor volatilization and oxidation induced surface cracking of environmental barrier coating systems: A numerical approach

A numerical model is developed for surface crack propagation in brittle ceramic coatings, aiming at the intrinsic failure of rare-earth silicate environmental barrier coating systems (EBCs) under combustion conditions in advanced gas turbines. The main features of progressive degradation of EBCs in such conditions are captured, including selective silica vaporization in the top coat due to exposure to water vapor, diffusion path-dependent bond coat oxidation, as well as crack propagation during cyclic thermal loading. In light of these features, user-defined subroutines are implemented in finite element analysis, where surface crack growth is simulated by node separation. Numerical results are validated by existing experimental data, in terms of monosilicate layer thickening, thermal oxide growth, and fracture behaviors. The experimentally observed quasi-linear oxidation in the early stage is also elucidated. Furthermore, it is suggested that surface crack undergoes rapid propagation in the late stage of extended thermal cycling in water vapor and leads to catastrophic failure, driven by both thermal mismatch and oxide growth stresses. The latter is identified as the dominant mechanism of penetration. Based on detailed analyses of failure mechanisms, the optimization strategy of EBCs composition is proposed, balancing the trade-off between mechanical compliance and erosion resistance.

A synergy of different corrosion failure modes pertaining to T91 steel impacted by extreme lead–bismuth eutectic flow pattern

This paper reports an attempt to speculate about corrosion resistance of T91 steel to extreme lead–bismuth eutectic (LBE) flow pattern. We build a modified dynamic corrosion apparatus permitting assemblages of oxidation, erosion or dissolution attack proceeding together within oxygen-saturated LBE. After corrosion test at 400℃ up to 1000 h, discrepancies of corrosion performance under various flow patterns were described by diffusion or mass transfer kinetics as well as erosion mechanics. In justification of dense Fe-O layer and cracks across oxide scales detected within typical erosion cavities, we put forward oxide growth stress enhanced by high surface curvature.

High temperature oxidation of Al4C3

Currently, high temperature corrosion resistant materials rely on the formation of an external surface oxide layer. This work is the first step in exploring an alternative coating concept in which one continuous phase in a two phase carbide–metallic binder coating system, preferentially oxidises internally within the composite. Controlled internal oxidation of the carbide would prevent oxide element consumption from the metallic binder, significantly improve adhesion of the oxide and reduce oxide growth stresses. Al4C3 is considered as the carbide phase in order to form a protective Al2O3 oxide. However, little has been presented on the high temperature oxidation mechanism of this compound. In this work the high temperature oxidation characteristics of Al4C3 powders were characterised by TGA, XRD and cross-sectional SEM. At 950 °C and 1100 °C the carbide powder showed rapid initial mass gains, before transitioning to slower mass gains indicative of protective oxide formation. A complex four-layer oxide scale formed and was correlated with the phase distribution predicted by equilibrium thermodynamic modelling to postulate the oxidation mechanism.

Corrosion of the bonding at FeCrAl/Zr alloy interfaces in steam

The interfacial bonding at protective coatings and Zr alloy substrates could have a significant effect on the corrosion behaviors of the coatings. In this work, FeCrAl-Zr couples were fabricated at 900 °C by spark plasma sintering (SPS), and the interfacial bonding of the couples was modified through the addition of Si and/or thermal diffusion at 1100 °C–1300 °C. Uphill diffusion of silicon occurred during SPS. The corrosion resistance of the couples in 400 °C/10.3 MPa steam, and the effects of the interfacial bonding characteristics were studied. Due to the presence of ZrSi2 in the interfacial layer, the interfacial bonding of the FeCrAl/Si-Zr couple had a better corrosion resistance than that of the FeCrAl-Zr couple, withstanding 14 days of corrosion in steam compared to 7 days for the FeCrAl-Zr couple. The results also suggested that bimetallic effect could accelerate the oxidation of Zr alloy at the interface of the couples, and galvanic corrosion significantly contributed to the serious corrosion of the interface of the diffusion bonded couples with thermal diffusion. The influence of oxide growth stress around interface on the bonding lifetime of the couples was also discussed.

In-mold oxidation behavior of Mg–4.32Y–2.83Nd–0.41Zr alloy

The oxidation behaviors of molten WE43 (Mg–4.32Y–2.83Nd–0.41Zr, wt%) in resin-sand mold atmosphere were systematically investigated. X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy analyses were employed to identify the phase compositions and microstructures of surface oxide scale. The initial oxide scale characterized by a three-layer structure mainly consisted of MgO, Y2O3, and ZrO2 and then transformed to Y2O3 layers covered by porous MgO after holding at high temperatures. CALPHAD method was used to analyze the oxidation process, and the calculated phase diagrams agreed well with the experimental results. A selective oxidation model was established based on thermodynamic and kinetic analysis to describe the structure transformation of oxide scale in detail. The initial oxidation was controlled by inward diffusion of oxygen due to the protection of surface Y2O3. However, the oxide scale was not protective after long-term oxidation or at temperatures higher than 1073 K due to the oxide fracture, which can be attributed to the increase in stress caused by inner Y2O3 layer growth. Based on experimental results, the oxide growth stress parameter C was estimated to be 0.0013 and 0.0026 for 1023 and 973 K, respectively. This work reveals the underlying oxide growth and fracture mechanisms of molten Mg–Y alloy in mold, and provides guidance for preventing the ignition of magnesium alloys during casting processes.

New Insights into Thermal Chromia Growth on Fe18Cr(10Ni) Model Alloys at 900°C: Scaling Kinetics and Microstructures

Specimen of Fe-18Cr(-10Ni) (wt.-%) were isothermally oxidized in dry and wet high p(O2) gas at 900°C. The morphology and microstructural evolution of the chromia scales formed were analysed by SEM-EBSD. At high pO2 test gas containing O2 and H2O, the pO2 is independent of the H2O content of the test gas. However, both types of oxygen species, i.e. O2 as well as H2O contribute to the chromia scaling reaction. The effect of specimen thickness on chromia scaling allows the change of the concentration of the intrinsic defects and their diffusivities in thermally grown chromia scales independently from the test gas applied. Chromia scales are under pressure during scale growth. For thermodynamic reasons the concentration and diffusivity of the intrinsic, native defects in oxides under hydro-static pressure varies with the amount of pressure applied. Ion beam polished oxide cross-sections were analysed in terms of oxide grain growth and nucleation rates of new oxide at the oxide-gas interface. Chromia scales formed on the various substrates at high p(O2) test gas grew at different rates, and microstructures formed on Fe18Cr(-10Ni) differ in grain shape and size. The chromia growth kinetic was found to be depending on the thickness of the specimens used, the H2O content of the gas, and its Ni content. The scale growth mechanism consists of two interlinked levels. At the meta-level oxide nucleation and oxide grain growth are affected by the cation flux in the oxide scale formed and the nature of the oxygen species in the gas. This fixes the number, size and shape of the oxide grains as well as the number of oxide grain boundaries available as short-circuit diffusion paths. The lattice level denotes the atomic level. Hereby the cation flux in the single grain by lattice diffusion and cation vacancy condensation are linked. The defect structure in the oxide lattice is important. The factors affecting it are the Cr diffusion in the substrate and the presence of Ni in the alloy, local p(O2), H-defects and oxide growth stress. The experimental findings will be discussed with strong focus on the oxide growth mechanism, the chromia microstructure of the scales formed under the various conditions and the concentration and mobility of the native defects in the chromia scales and its interactions with H-containing species originating from water vapour in the test gas. Furthermore, the role of Fe as base and Ni as alloying element in the metal substrate will be discussed. This is done by considering the differences between the Ni-Cr-O and Fe-Cr-O phase diagrams and the consequence of the existence of the Fe3+ ion on the miscibility of the oxides formed by the various cations.

The role of hydrogen in zirconium alloy corrosion

Hydrogen enters zirconium metal as a result of the corrosion process and forms hydrides when present in quantities above the solubility limit at a given temperature. Zircaloy-4 coupons of different thicknesses (0.4 mm-2.3 mm) but identical chemistry and processing were corroded in autoclave at 360 °C for various times up to 2800 days. Coupons were periodically removed and weighed to determine weight gain, which allows follow of the corrosion kinetics. Coupon thickness differences resulted in different volumetric concentrations of hydrogen, as quantified using hot vacuum extraction. The thinnest coupons, having the highest concentration of hydrogen, demonstrated acceleration in their corrosion kinetics and shorter transition times when compared to thicker coupons. Furthermore, it was seen that the post-transition corrosion rate was increased with increasing hydrogen concentration. Corrosion rates increased only after the terminal solid solubility (TSS) was exceeded for hydrogen in Zircaloy-4 at 360 °C. Therefore, it is hypothesized that the corrosion acceleration is caused by the formation of hydrides. Scanning electron microscope (SEM) examinations of fractured oxide layers demonstrate the oxide morphology changed with hydrogen content, with more equiaxed oxide grains in the high hydrogen samples than in those with lower hydrogen content. Additionally, locations of advanced oxide growth were correlated with locations of hydrides in the metal. A hypothesis is proposed to explain the accelerated corrosion due to the presence of the hydrides, namely that the metal, locally, is less able to accommodate oxide growth stresses and this leads to earlier loss of oxide protectiveness in the form of more frequent oxide kinetic transitions.

Microstructural analysis and thermal shock behavior of plasma sprayed ceria-stabilized zirconia thermal barrier coatings with micro and nano Al2O3 as a third layer

In this study, thermal shock behavior of three types of atmospheric plasma sprayed ceria-stabilized zirconia (CSZ) thermal barrier coatings was evaluated: usual CSZ, layer composite of CSZ/Micro Al2O3 and layer composite of CSZ/Nano Al2O3 in which Al2O3 was used as a top coat on CSZ layer. The thermal shock test was administrated by quenching the samples in the cold water with the temperature of 20–30°C from 1100°C. Microstructure evaluation, elemental and phase analysis of coatings before and after tests were performed using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometry, respectively. The experimental results revealed that the CSZ/Nano Al2O3 TBC has a superior thermal cycling behavior, in comparison to CSZ/Micro Al2O3 and usual CSZ TBCs. Also, failures of both usual CSZ and layer composite of CSZ/Micro Al2O3 TBCs mainly were associated with thermally grown oxide formation and growth stresses at NiCrAlY/CSZ interface, whereas, in CSZ/Nano Al2O3 TBC, the failure mechanism was coefficient of thermal expansion mismatch stress in NiCrAlY/CSZ interface.

An elastic phase field model for thermal oxidation of metals: Application to zirconia

A multi-phase field model was developed for non-selective oxidation of metals which captures both the oxidation kinetics and stress generation. Phase field formulation involved a non-conserved phase field variable as the marker for the metallic substrate, oxide scale, and a fluid phase containing oxygen, and a conserved phase field variable representing the concentration of oxygen. The evolution equations of the phase field variables were coupled to the mechanical equilibrium equations to investigate the evolution of stress generation in both the oxide scale and the underlying metal. The governing equations were solved in a finite element framework. This phase field model predicts the oxygen composition depth and stress profiles in the oxide layer and at the metal–oxide interface. The model was proven successful in predicting the observed evolution of oxide thickness and growth stresses for Zircaloy-4 oxidized at 900°C. The results of phase field simulations showed that the generation of stresses upon oxidation tends to slow down the oxidation kinetics, and this substantially improved the model predictability of experimental data.

Influence of humidity on high temperature oxidation of Inconel 600 alloy: Oxide layers and residual stress study

In order to understand the influence of humidity on high temperature oxidation of Inconel 600 alloy, in this work, water vapour (absolute humidity varying from 0% to 19%) was introduced in the thermal gravimetric analysis (TGA) system under artificial air between 600°C and 900°C. The oxides identification and the residual stress in the oxide layers have been studied by X-ray diffraction method in each of two oxide phases, simultaneously. The oxide surface morphology, cross-section microstructure and the chemical composition of the oxide layers were determined by FEG-SEM (Field Emission Gun Scanning Electron Microscope) observation and FEG-SEM EDS (Energy-dispersive X-ray spectroscopy) analysis. Depending on the oxidation temperature, the humidity and the oxidation duration, the oxide layer differed significantly. The residual stress levels in the different oxide layers (NiO-type layer and Cr2O3-type layer) have also been affected by the introduction of the water vapour. According to the analysis results, the residual stresses on oxide mainly came from the growth stress and thermomechanical stress; and the oxide growth stress was especially affected by humidity at high temperature.

The role and effect of residual stress on pore generation during anodization of aluminium thin films

The role and effect of residual stress on pore generation of anodized aluminium oxide (AAO) have been investigated into anodizing the various-residual-stresses aluminium films. The plane stresses were characterised by X-ray diffraction with sin2ψ method. The pore density roughly linearly increased with residual stress from 64.6 (−132.5MPa) to 90.5pores/μm2 (135.9MPa). However, the average pore size around 40nm was not changed significantly except for the rougher film. The tensile residual stress lessened the compressive oxide growth stress to reduce AAO plastic deformation for higher pore density. The findings provide new foundations for realizing AAO films on silicon.

Effect of Specimen Thickness on the Oxidation Rate of High Chromium Ferritic Steels: The Significance of Intrinsic Alloy Creep Strength

Previous studies revealed that initial sample thickness affects the growth rate of oxide scales formed during 800 or 900 °C air exposure. The effect is partially related to differences in depletion of minor alloying additions such as Mn, Ti, La in thick and thin specimens. However, it has previously been proposed that the specimen thickness dependence is partially governed by differences in creep strength of thick and thin substrates. To investigate this hypothesis, discontinuous air oxidation experiments were carried out with the Laves phase strengthened ferritic steel Crofer 22 H at 800 °C. Comparing the data for solution annealed and pre-aged (500 h, 900 °C) materials it could be shown that intrinsic creep strength of the alloy substantially affects oxidation rates. The observations can qualitatively be explained by assuming the relaxation of oxide growth stresses by plastic deformation of the metallic substrate to be an important parameter affecting the kinetics of oxide scale growth.

Oxide growth stress measurements and relaxation mechanisms for alumina scales grown on FeCrAlY

AbstractEarly‐stage tensile stress evolution in α‐Al2O3 scales during oxidation of FeCrAlY at 1000, 1050, 1100, and 1200 °C was monitored in situ by use of synchrotron radiation. Tensile stress development as a function of oxidation temperature indicated a dynamic interplay between stress generation and relaxation. An analysis of the time dependence of the data indicated that the observed relaxation of the initial tensile stress in the oxide scales at 1100 and 1200 °C is dominated by creep in the α‐Al2O3. A thin layer of a (Fe,Cr,Al) oxide was observed at the oxide‐gas interface, consistent with a mechanism whereby the conversion of (Fe,Cr,Al)2O3 to α‐Al2O3 produces an initial tensile stress in the alumina scale.

Thermal Shock Damage and Microstructure Evolution of Thermal Barrier Coatings on Mar-M247 Superalloy in a Combustion Gas Environment

The effect of preoxidation on the thermal shock of air plasma sprayed thermal barrier coatings (TBCs) was completely investigated in a combustion gas environment by burning jet fuel with high speed air. Results show that with increasing cycles, the as-oxidized TBCs lost more weight and enlarged larger spallation area than the as-sprayed ones. Thermally grown oxide (TGO) growth and thermal mismatch stress were proven to play critical roles on the as-oxidized TBC failure. Two types of significant cracks were identified: the type I crack was vertical to the TGO interface and the type II crack was parallel to the TGO interface. The former accelerated the TGO growth to develop the latter as long as the oxidizing gas continuously diffused inward and then oxidized the more bond coat (BC). The preoxidation treatment directly increased the TGO thickness, formed the parallel cracks earlier in the TGO during the thermal shocks, and eventually resulted in the worse thermal shock resistance.

The oxidation behaviour of the 9 % Cr steel P92in CO2- and H2O-rich gases relevant to oxyfuel environments

Abstract In oxyfuel plants metallic heat exchanging components will be subjected to service environments containing high amounts of CO2 and water vapour. In the present paper, the oxidation behaviour of the ferritic/martensitic 9 % Cr steel P92 was studied in a model gas mixture containing 70 % CO2-30 % H2O in the temperature range 550 – 650 °C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. In the CO2- and/or H2O-rich gases, the steel formed iron-rich oxide scales which possess substantially higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich oxide scales are formed as a result of a decreased flux of chromium in the bulk alloy toward the surface. This is the result of enhanced internal oxidation of chromium in the H2O-containing gases and carburisation in the CO2 gases. The oxide scales allow molecular transport of CO2 towards the metallic surface, resulting in carburisation of the alloy. The presence of water vapour induced buckling in the outer haematite layer, apparently as a result of compressive oxide growth stresses. Buckling did not occur in the H2O-free gas. This has been discussed in terms of the potential for H2O to increase growth stresses and accelerate crack propagation. The oxidation rates in CO2–H2O do not seem to be higher than those observed in flue gases of conventional fossil fuel fired power plants.