Oxidation of single crystal PWA 1483 at 950 °C in flowing air

The oxidation behaviour of single crystal PWA 1483 at 950 °C was investigated by means of XRD, SEM and EDS. The parabolic oxidation behaviour, as defined by mass gain and the respective oxide layer thicknesses, is characterized by a parabolic rate constant of about 4 × 10⁻⁶ mg²/(cm⁴ × s) and the formation of a multi-layered oxide scale. An outer scale contains a Ti-bearing thin film composed of TiO₂ and NiTiO₃ but mostly Cr in Cr₂O₃ and (Ni/Co)Cr₂O₄ besides NiTaO₄. This outer scale is connected to a discontinuous layer of Al₂O₃ and an area of γ′-depletion within the base material.     

Chromia layer growth on a Ni-based superalloy: Sub-parabolic kinetics and the role of titanium

Oxidation of the Ni-based superalloy RR1000 has been undertaken in air over the temperature range 600–900 °C for times up to 5000 h. The surface oxide consisted of a protective Ti-doped chromia layer but with rutile forming on its outer surface. Sub-surface oxidation of Al and Ti also occurred. The thickening kinetics of the chromia layer were sub-parabolic with initial rates around two orders of magnitude higher than expected for Ti-free chromia. This enhancement and the sub-parabolic kinetics are accounted for by Ti-doping of the chromia layer. Over time the enhancement reduced because of Ti-depletion in the alloy.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

Quantification of growth kinetics and adherence of oxide scales formed on Ni-based superalloys at high temperature

Cyclic and isothermal oxidation behaviors of first and fourth-generation superalloys AM1 and MCNG were investigated to evaluate the ability of the scratch test to quantify the adhesion of multi-layered oxide scales. Effects of sulfur content and of scale thickness were studied independently. Available models lead to large discrepancies in the calculated work of adhesion values with the evaluation of the residual stress being the largest source of error. Nevertheless, models can assess the effect of sulfur content and the scratch test can be used to correlate the long-term cyclic oxidation behavior and the adhesion of oxide scales.     

The influence of microstructure on the oxidation of duplex stainless steels in simulated propane combustion products at 1000 °C

A low nickel Type S32101 duplex stainless steel has been oxidised in simulated industrial reheating conditions. The surfaces have been studied using optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Observations show that local breakaway regions (LBRs) form on the austenitic regions whereas thinner oxides are observed on the ferritic regions of the substrate. The reason proposed for these differences is the formation of a continuous oxide layer on the ferrite region and a discontinuous layer on the austenitic region during the early stages of oxidation. The chemical composition of these LBRs have been shown to be oxide islands of iron and manganese and oxide craters of chromium rich oxides. The more protective regions consist of chromium and manganese rich oxides. A silica layer formed below the oxide which may be attributable to a slight enrichment of silicon in the ferritic regions or due to faster rates of diffusion in ferrite.     

Cyclic oxidation of Ti3AlC2 at 1000–1300 °C in air

The cyclic-oxidation behavior of Ti₃AlC₂ was investigated at 1000–1300 °C in air for up 40 cycles. It was revealed that Ti₃AlC₂ had excellent resistance to thermal cycling. The cyclic oxidation of Ti₃AlC₂ basically obeyed a parabolic law. In all cases, the scales were dense, resistant to spalling and highly stratified. The inner continuous α-Al₂O₃ layer was well adhesive, while the outermost layer changed from rutile TiO₂ at temperatures below 1100 °C to Al₂TiO₅ at 1200 and 1300 °C, respectively. At 1300 °C, a mechanical-keying structure of inner Al₂O₃ to the Ti₃AlC₂ substrate formed, which improved the resistance to scale-spallation.     

Pourbaix diagrams for titanium in concentrated aqueous lithium bromide solutions at 25 °C

Pourbaix diagrams (electrode potential-pH diagrams) for Ti–Br⁻–H₂O system at 25 °C in the absence and presence of titanium hydrides were developed in 400, 700, 850, and 992 g/L LiBr solutions. The diagrams were compared with the simple Ti–H₂O system at 25 °C. Comparison of the simple Ti–H₂O system with the diagrams of the Ti–Br⁻–H₂O system at 25 °C showed that the titanium solubility range in the acid, neutral, and weak alkaline areas of the diagrams extended slightly to both higher pH values and potentials with increasing bromide ion activity and decreasing water activity.     

(S)TEM study of different stages of Ti-45Al-8Nb-0.2W-0.2B-0.02Y alloy oxidation at 900 °C

The oxidation process of Ti-45Al-8Nb-0.2W-0.2B-0.02Y (at%) can be divided into three stages during 100 h at 900 °C. The change of ion diffusion path leads to the discontinuity of the oxidation curve between the second and third stage. Different stages of oxidation morphology have been investigated combined with TEM and STEM. AlNb₂ phase forms at the interface of oxide scale/base alloy. Contrary to a continuous formation of ‘x’ phase in γ-TiAl with Nb free at the interface of oxide scale/base alloy. AlNb₂ phase hinder continuous ‘x’ phase formation, which can promote TiAl oxidation protection.     

The oxidation of two ternary Ni-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of the ternary alloys Ni-45Cu-10Al and Ni-30Cu-10Al has been studied at 800-900 °C under 1 atm O₂. The presence of 10 at.% Al reduces significantly the oxidation rate of the corresponding Cu-Ni alloys during the initial oxidation stages, even before the establishment of a complete Al₂O₃ layer. The weight of individual sample of the two ternary Ni-Cu-10Al alloys at 800 °C increases more rapidly than at 900 °C during the initial oxidation stage. As oxidation proceeds, the weight gain at 800 °C slows down to a degree that the total weight gain after 24 h oxidation at 800 °C is less than that at 900 °C. Due to a faster formation of the Al₂O₃ layer, which suppresses earlier the further oxidation of Cu and Ni, the external region of the scales grown on Ni-45Cu-10Al contain much less Cu and Ni oxides than those grown on Ni-30Cu-10Al. The transition from the internal oxidation to the selective external oxidation of the most reactive component Al in Ni-Cu-Al alloys is favored by higher values of the Al content, of temperature and of the Cu/Ni ratio.     

Effect of sand blasting and glass matrix composite coating on oxidation resistance of a nickel-based superalloy at 1000 °C

The oxidation behavior of K38G superalloy, which is mechanically polished or sand blasted and then coated with glass matrix composite coatings, is investigated at 1000 °C. Results indicate that sand blasting pre-treatment enhances the resistance to oxidation and spallation of the glass coatings on superalloys by promoting formation of an alumina interlayer. The most important factors favoring formation of this alumina interlayer are discussed in terms of that sand blasting not only changes the microstructure of alloy, it also modifies the alloy composition at surface by affecting the progressing of interfacial reactions between the superalloy substrates and glass coatings.     

Oxidation of Cr2AlC at 1300 °C in air

High purity, dense Cr₂AlC compounds were synthesized via a powder metallurgical route, and their oxidation behavior was investigated at 1300 °C in air for up to 336 h. A thin external oxide layer formed, which consisted primarily of not Cr₂O₃ but Al₂O₃. Since Al was consumed to produce the Al₂O₃, Al-depletion and Cr-enrichment occurred underneath the Al₂O₃ layer. This led to the formation of a Cr₇C₃ layer containing voids. These grew during oxidation, eventually destroying the Cr₇C₃ layer formed on the unoxidized Cr₂AlC matrix.     

Oxidation behaviour of Al2O3–TiC–Co composites at 800–1000 °C in air

The oxidation behaviour of nanometre and micrometre sized Al₂O₃–TiC–Co composites is investigated at 800–1000 °C in air for 25 h. The oxidation resistance of nanometre sized samples is better than of micrometre sized. Phase compositions and microstructures were studied by XRD and SEM. The values of general rate constant k and oxidation exponent n are dependent on oxidation temperature and composites. The oxidation kinetics followed a rate that is slightly faster than the parabolic-rate law at 800–1000 °C. The activation energy of the nanometre sized is higher than of micrometre sized in the range of 800–1000 °C.     

Preparation and oxidation behaviour of an AlSiY diffusion coating on a Ni-based single crystal superalloy

An AlSiY diffusion coating was developed by a combined approach of arc ion plating and subsequent diffusion treatment. Cyclic and isothermal oxidation behaviour of the coating specimens was tested in a muffle furnace at 1000 °C and 1100 °C. The results confirmed an enhanced performance of the diffusion coating due to the presence of β-NiAl enriched outer layer. The microstructure change and the oxidation behaviour of the diffusion coating are discussed.     

Oxidation kinetics of an Fe-31.8Mn-6.09Al-1.60Si-0.40C alloy at temperatures from 600 to 900 °C

An Fe-31.8Mn-6.09Al-1.60Si-0.40C alloy was subjected to thermogravimetric tests, in atmospheres of pure oxygen and synthetic air. The alloy showed good oxidation resistance, especially at 600 and 700 °C in air and at 600 °C in oxygen. Owing to autocorrelation identified in the results, kinetic data were analyzed by the statistical method of Cochrane and Orcutt. Parabolic kinetic behaviour was observed for the Fe-Mn-Al-Si-C alloy oxidation process in air at 600, 700 and 800 °C and in pure oxygen at 900 °C.     

Grain size effects on the oxidation of two ternary Cu-Ni-20wt.% Cr alloys at 700-800 °C in 1 atm O2

Two nanocrystalline two-phase Cu-Ni-Cr alloys, both prepared by mechanical alloying and containing about 20 at.% Cr but with different Ni contents (40 and 20 wt.%, respectively), have been oxidized in 1 atm O₂ at 700-800 °C. Their oxidation behavior has been compared with that of two cast alloys of the same composition, already studied previously, to examine the effects of a large reduction of the size of the individual phase grains and particles. The nanophase alloy with 40 wt.% Ni formed a flat external layer of chromia of regular thickness, while the corresponding cast alloy produced a very irregular chromia layer, often protruding deeply into the alloy, only after an initial stage of rather fast corrosion involving also copper and nickel, associated with some degree of internal oxidation. By oxidation at 700 °C the nanophase alloy with 20 wt.% Ni formed an irregular chromia layer associated with low corrosion rates. The corresponding cast alloy formed complex scales containing Cu, Ni and Cr oxides, extending into the alloy in the form of large pegs, even though a very irregular and discontinuous innermost chromia layer was still able to produce low corrosion rates. On the contrary, at 800 °C both alloys formed complex scales containing mixtures of the oxides of the three metal components. However, the scales grown on the cast alloy were much more irregular in thickness and formed large protrusions into the alloy. In spite of this, the corrosion kinetics of the nanophase 20 wt.% Ni alloy at 800 °C were more irregular and, except for an initial stage, less protective than that of the cast alloy with the same composition.     

Effect of Dy on oxide scale adhesion of NiAl coatings at 1200 °C

The cyclic oxidation behaviour of β-NiAlDy coatings produced by electron beam physical vapour deposition (EB-PVD) was investigated. For the Dy-free NiAl coating, numerous voids developed at the Al₂O₃ scale/NiAl interface and sulfur was found to segregate at the void surfaces, leading to early spallation of the scale. The addition of Dy prevented sulfur segregation and void formation at the scale/NiAl interface. As a result, the scale grown on the NiAlDy coating remained adherent and no spallation occurred even after 400 h cyclic oxidation. The 0.05 at.% Dy doped coating revealed lower oxidation rate than the more Dy doped coatings.     

Microstructure and oxidation behaviour of an AlSiY/NiCrAlYSi composite coating at 1150 °C

A composite coating consisting of an outer AlSiY layer and an inner NiCrAlYSi layer has been prepared by a two-step arc ion plating method. The isothermal and cyclic oxidation behaviour of the composite coating at 1150 °C, including the growth of oxide scale and the microstructure transformation of the coatings, have been investigated comparing with the reference coating, NiCrAlYSi. The results show enhanced oxidation performance of the composite coating, which is concerned with its abundant possession of β-NiAl phase reservoirs, providing it the long term healing capacity of re-growing the α-Al₂O₃ scale.     

Effect of water vapor on the phase transformation of alumina grown on NiAl at 950 °C

This particular study includes the analysis of the effect of H₂O on promoting the phase transformation in thermally grown aluminum oxides formed on NiAl at 950 °C. Oxidation of NiAl is carried out at 950 °C in O₂ and O₂ + 15 vol.% H₂O. It is observed that transient alumina initially formed on NiAl transforms to stable α-alumina in the presence of water vapor which promotes the subject transformation and eventually results in a compact scale, offering superior oxidation resistance. Present study includes the analysis of θ to α-alumina transformation under the effect of temperature and environment.     

Short-term oxidation resistance and degradation of Cr2AlC coating on M38G superalloy at 900–1100 °C

High temperature oxidation behavior of the Cr₂AlC coating was investigated at 900–1100 °C. During the oxidation, a continuous Al₂O₃ scale formed, resulting in the improvement of the oxidation resistance of the substrate. Meanwhile, the oxidation induced depletion of Al within the Cr₂AlC coating resulted in the transformation of Cr₂AlC to Cr–C phases. Compared with bulk Cr₂AlC, the Cr₂AlC coating possessed similar oxidation behavior, but with higher oxidation rate. This is because a great number of columnar grain boundaries existed in the as-deposited coating, through which oxygen and nitrogen could diffuse inwardly, resulting in the internal oxidation and nitridation.     

Cyclic oxidation and interdiffusion behavior of a NiAlDy/RuNiAl coating on a Ni-based single crystal superalloy

A novel NiAlDy/RuNiAl coating was deposited onto a Ni-based single crystal superalloy DD3. The cyclic oxidation and interdiffusion of the NiAlDy/RuNiAl coating and a NiAlDy coating at 1100 °C were investigated. For the NiAlDy coating, secondary reaction zone (SRZ) consisting of γ′, γ and σ-topologically-closed-packed phase (σ-TCP) needles was formed in the superalloy after 100 h annealing. The NiAlDy/RuNiAl coating effectively suppressed the formation of SRZ as the RuNiAl layer worked as a buffer layer. Premature scale spallation was observed on the NiAlDy coating after about 80 h cyclic oxidation, whereas the NiAlDy/RuNiAl coating exhibited improved oxidation-resistance property.