G. Smoła

Mechanistic aspects of Pt-modified β-NiAl alloy oxidation

Abstract The effects of Pt addition (5, 10 and 15 at.%) on the high temperature oxidation behaviour of the intermetallic compound β-NiAl were studied at 1150°C under both isothermal and thermal cycling conditions. The scale growth mechanism was assessed using two-stage oxidation exposures with 18O2 as a tracer in conjunction with high-resolution secondary ion mass spectrometer analysis of isotopic distributions. The scale morphologies were further characterized using secondary electron microscopy, Pt was found to reduce slightly the scale growth rate and improve considerably its resistance to spallation but it did not affect the sequence of scale development. Instead, it slowed the rate at which the scale developed. The α-Al2O3 scales that were eventually established were duplex in structure, consisting of a compact inner layer and a thinner outer layer. This outer layer consisted of ridges and grew by the coarsening of these and/or by a dislocation climb mechanism.

The effect of alloyed and/or implanted yttrium on the mechanism of the scale development on β-NiAl at 1100°C

Abstract This paper reports on the comparison of scale development on unmodified β-NiAl which contains volume and surface additions of yttrium. This reactive element was incorporated using conventional alloying (0.05 or 0.1 wt%) and ion implantation. The materials containing alloyed yttrium were also implanted with yttrium to study the combined influence of volume and surface additions. A two-stage oxidation approach was applied with the use of 18O2 as a tracer. The reaction temperature was 1100°C and the samples were oxidised for up to 64 h. The scale growth mechanism was followed with the aid of in-depth distributions of the oxygen isotopes across the scale using secondary ion mass spectrometry, the surface morphology of the scales was observed using scanning electron microscopy, while their phase composition was determined using photoluminescence spectroscopy. Similar stages of the scale evolution were observed for the growing scales. Initially the oxide layer consisted of transient oxides, and at its surface more or less developed blade-like surface grains were observed. Subsequently, phase-transformation-related cracks and round patches appeared, and finally ridges which successively covered the outer surface of the scale and constituted the outer layer of a duplex scale developed. The mechanism of scale growth was a mixed outward and inward, and the relative contribution of both mechanisms varied depending on the analysed scale region and on the oxidation stage. The cracks and patches were regions where the transient aluminas were preferentially transformed into α-Al2O3. The ridges formed in cracks essentially consisted of α-Al2O3. For the initial stages of oxidation, the transient aluminas and α-Al2O3 co-existed in the scales, while subsequently the latter prevailed and finally, became the only phase found in the oxide layers. The evolution rate of the scale was affected by alloyed and implanted yttrium additions: the former exhibited minor and content-dependent effect while the latter significantly retarded it.

The mechanism of early oxidation stages of Fe20Cr5Al-type alloys at 1123 K

Abstract Fe20Cr5Al-type alumina forming alloys are known as α alumina-forming materials at relatively high temperatures (≥1273 K). This paper reports on the oxidation mechanism of commercial and synthetic Fe20Cr5Al alloys, at a considerably lower temperature, 1123 K. Some of the alloys were implanted with oxygen or yttrium ions prior to the oxidation exposure. Two-stage-oxidation, secondary ion mass spectrometry, secondary electron microscopy and lowangle X-ray diffraction were used as experimental tools. Results showed that even prolonged exposures are not sufficient to ensure that the scale consists essentially of α-Al2O3. Mechanisms involved from the observed scale morphology, microstructure and phase composition are discussed

On the phase composition changes during high temperature oxidation of Pt-modified β-NiAl at 1150°C

Abstract The phase composition of the scales growing during oxidation at 1150°C on Pt-free and Pt-containing (0, 5, 10 and 15 at.%) β-NiAl intermetallic compounds were studied for exposure periods of up to 6 hours using photoluminescence spectroscopy with spatial resolution down to ca 1 μm. Scanning electron microscopy was used to observe the evolution of the surface morphology of the growing scales. It was found that initially unstable polymorphs of Al2O3 oxide (γ, δ and/or θ) developed, which subsequently transformed into α-Al2O3. The transformation occurred relatively early, and after 1 h oxidation essentially α-Al2O3 was detected on all the materials. After 15 min reaction, different regions were found in all cases, and both types of phases, transient aluminas and α-Al2O3 were detected. However, the rate of the phase composition depended on the region: in some regions, the transient aluminas co-existed with a-phase, while in others only transient or only α-Al2O3 were found. Regions composed of only α-Al2O3 were observed on materials containing higher amounts of Pt-additions, i.e. 10 and 15 at.%. This indicates that Pt additions accelerated the phase transformation of alumina polymorphs into α-Al2O3. In the early scales, comprising different regions, α-Al2O3 development was favoured in patches, and only this phase was always detected in ridges growing in cracks formed in patches. Conclusions are inferred concerning the scale evolution on the materials studied as well as on the effect of Pt additions.

On the early stages of scale development on Ni–22Al–30Pt with and without Hf additions at 1150°C

Abstract Platinum modified Ni –Al-based alloys play an important role in the aeronautical industry as materials applied as Bond Coats (BCs) in thermal barrier coating systems (TBC), which provide protection against high temperature oxidation. Therefore, it is crucial for the assessment of the performance of the TBC to understand the oxidation behaviour of the Bond Coat material and the properties of the thermally grown oxide developing on it. This paper reports on the scale growth mechanism of Pt-modified γ′-Ni3Al-based alloys with and without Hf additions at the early stages of oxidation. Samples were oxidized at 1150°C for up to 50 min. The consecutive stages of the scale evolution were followed by a systematic approach using a two-stage oxidation treatment in atmospheres containing different amounts of 18O2 oxygen isotope. The scale growth mechanism was followed with the aid of the in-depth analysis of elemental distribution, in particular of the oxygen isotopes, across the scale using secondary ion mass spectrometry (SIMS). The surface morphology of the scales was observed using scanning electron microscopy, while its local phase composition was determined using photoluminescence spectroscopy. In addition, the imaging SIMS was used to generate distribution maps of the oxygen isotopes. Because of the limited lateral resolution of the SIMS technique (down to 0.5 μm) they were conclusive only for the most prolonged exposures (50 min). Typical stages of scale evolution were observed for the growing scales: initially flat layers were formed, followed by more or less developed blade-like surface grains, phase-transformation-related cracks and round patches and, finally, ridges. The latter were only found on the Hf-free material. The scale growth mechanism evolved from an initially predominant outward growth mechanism towards an increasing contribution of the inward mechanism. The relative extents of the outward and inward growth mechanisms depended strongly on the surface fraction of patches and through-scale cracks. The phase composition analyses showed that cracks and patches are regions where the transient aluminas were preferentially transformed into the α-Al2O3 phase which is the required protective scale. The ridges formed in cracks essentially consisted of α-Al2O3. For the most of studied exposure periods both types of phases: transient aluminas and α-Al2O3 co-existed in the scales. The results obtained indicate that: (i) phase transformations occur locally and not simultaneously in the entire scale; (ii) Hf additions retard the phase transformation; (iii) the phase transformation is completed only on Hf-free material and not after oxidation for 50 min; (iv) the two-stage oxidation approach should be carefully applied to study the growth mechanism of evolving scales in which regions of different phase compositions develop.

The Interpretation of Marker Experiment Conducted during Formation of Higher Oxide on the Surface of Lower Oxide

The marker method in studying the formation mechanism and defect structure of higher oxide during oxidation of lower oxide has been discussed. The approach to this problem needs specific treatment, both in experimental procedure and in the interpretation of results. It has been shown that the correct results of marker experiments in the case of highly defected substrates can be obtained, if these substrates before the marker deposition process are submitted to homogenization under highest oxidant activity, at which they remain stable at a given temperature. In addition, the nonstoichiometry must be considered in formulating appropriate chemical reactions, being the basis for foreseeing the location of markers in the interior of reaction product. The other very important problem consists in the possibility of the formation of reaction product not only on the surface of oxidized substrate but also inside of this substrate. In such a situation, the formulation of final conclusions concerning the crystalline lattice disorder from marker position should be combined with considerations of chemical reactions and transport processes occurring in a given substrate.

The early Stages of the Scale Growth on FeCrAl(+RE)-Type Alumina Formers

The early stages of the scale growth process were studied of two FeCrAl alloys: one synthetic (Fe23Cr5Al) and one commercial (Kanthal APM alloy). In addition, Yttrium was implanted to the Fe23Cr5Al alloy. Oxidation exposures were carried out at 1000°C using two-stage-oxidation exposures in atmospheres containing significantly different amounts of 18O2-tracer. The scales were analyzed in terms of SIMS, PLS and SEM. The distribution of oxygen isotopes which corresponded to the location of new oxide formation, the scale phase composition, scale morphology and microstructure were determined which enabled description of the scale evolution on all studied alloys. Similar evolution stages were observed, but minor differences were related to the rate of disappearing of the transient aluminium oxides.

The effect of surface finishing and sample thickness on the early oxidation mechanism of Fe20Cr5Al+RE alloys

Abstract Usually, ‘as-received’ thin foils and/or sheets of FeCrAl+RE (RE: mostly Y and Hf) alloys are subjected to oxidation exposures directed towards testing their high-temperature oxidation behaviour at 900 and 1000°C. However, their mechanical treatment during manufacturing has been carried out at elevated temperatures which might affect further oxidation processes by virtue of influencing the initial conditions of oxidation exposures that follow. To verify this possibility, Fe20Cr5Al+(Y+Hf) foils and sheets were prepared in two ways: either left without any further treatment after manufacturing (‘as-received’) or mechanically polished down with 1 μm diamond paste. The samples were subjected to two-stage-oxidation exposures using 18O2-tracer containing atmospheres, at 900°C and 1000°C, and examined subsequently with high-spatial-resolution secondary ion mass spectrometry analyses and secondary electron microscopy/electron diffraction X-ray. The starting materials were characterized with the aid of XPS and SIMS in order to determine their surface composition. A thin layer of Fe- and Al-rich oxides was found on the ‘as-received’ foils and sheets. The oxidation results showed at surface finishing and, to lesser extent, the thickness of the sample, affected both the scale growth mechanism, as well as its morphology and microstructure. An inward growth mechanism was observed on polished samples independent of alloy thickness, while the ‘asreceived’ samples exhibited a complex growth mechanism related to the stage of phase transformation of transient aluminas into α-Al2O3. Hence, polishing was found to accelerate this transformation. Surface roughness was found to be the major factor affecting the oxidation mechanism at relatively early oxidation stages and the influence of surface chemistry was much more significant for thin foils than for thicker materials.

Oxidation Resistance of Austenitic Steels under Thermal Shock Conditions in an Environment Containing Water Vapor

Abstract The oxidation behavior of Super 304 H, Sanicro 25, HR3C and HR6W steels, which are recommended for use in ultra-supercritical power plants, as well as corrosion resistant X2CrNiMo17-12-2 steel was studied in this work. Oxidation tests were carried out under thermal shock conditions in an oxygen-rich environment (containing 50 vol. % water vapor) at a temperature equal to 750 °C. The investigated steels (excluding X2CrNiMo17-12-2 steel) are characterized by good oxidation resistance under thermal shock conditions. A highly protective Cr2O3 layer was formed in the internal part of scales growing on the surfaces of investigated steels. The X2CrNiMo17-12-2 steel has worse oxidation resistant properties than the other grades of steels.

Marker Method in Studying the Defect Structure in Products of the Oxidation of Highly Disordered Substrates

Abstract The interpretation of marker experiments in studying the formation mechanism and defect structure of higher oxides during oxidation of lower oxides, showing rather high deviations from stoichiometry, has been discussed. It has been shown that correct results can be obtained only if the concentration of point defects in the substrate is taken into account. Theoretical considerations presented in this paper have been illustrated by the experimental results obtained during sulphidation of highly disordered Co1-yS to form CoS2, as well as Ni1-yS to form NiS2.